什麼是java集合架構呢？
在java中，集合架構指的是用于盛放與操作集合的通用架構。在這裡面，盛放指的是類似數組一樣儲存資料的功能，而操作則是有關集合的增删改查等操作操作。在集合架構中。主要包括了下面三個部分:
- 接口（定義了一系列抽象的資料類型）
- 實作（接口的詳細實作類）
- 算法（一些經常使用的計算。包括排序，搜尋）
設計目标
- 降低開發人員的工作量
- 加快開發人員的開發速度以及加強代碼的品質
- 同意不相關接口之間的互操作性
- 降低學習與應用新API
- 降低設計新的API
- 促進軟體的複用
核心接口
在集合架構中。系統定義了一系列核心接口，在經常使用操作中，主要包括了：Collection,List,Set,Map,Queue等。詳細的例如以下圖所看到的。
從圖中能夠看出。Collection作為最大的集合接口，Set，List，Queue，Deque繼承Collection，Map則是一類新的接口。以下我們直接從源代碼看看Collection的接口定義。Collection源代碼例如以下：
public interface Collection<E> extends Iterable<E> {
// Query Operations
/**
* Returns the number of elements in this collection. If this collection
* contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* @return the number of elements in this collection
*/
int size();
/**
* Returns <tt>true</tt> if this collection contains no elements.
*
* @return <tt>true</tt> if this collection contains no elements
*/
boolean isEmpty();
/**
* Returns <tt>true</tt> if this collection contains the specified element.
* More formally, returns <tt>true</tt> if and only if this collection
* contains at least one element <tt>e</tt> such that
* <tt>(o==null ? e==null : o.equals(e))</tt>.
*
* @param o element whose presence in this collection is to be tested
* @return <tt>true</tt> if this collection contains the specified
* element
* @throws ClassCastException if the type of the specified element
* is incompatible with this collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* collection does not permit null elements
* (<a href="#optional-restrictions">optional</a>)
*/
boolean contains(Object o);
/**
* Returns an iterator over the elements in this collection. There are no
* guarantees concerning the order in which the elements are returned
* (unless this collection is an instance of some class that provides a
* guarantee).
*
* @return an <tt>Iterator</tt> over the elements in this collection
*/
Iterator<E> iterator();
/**
* Returns an array containing all of the elements in this collection.
* If this collection makes any guarantees as to what order its elements
* are returned by its iterator, this method must return the elements in
* the same order.
*
* <p>The returned array will be "safe" in that no references to it are
* maintained by this collection. (In other words, this method must
* allocate a new array even if this collection is backed by an array).
* The caller is thus free to modify the returned array.
*
* <p>This method acts as bridge between array-based and collection-based
* APIs.
*
* @return an array containing all of the elements in this collection
*/
Object[] toArray();
/**
* Returns an array containing all of the elements in this collection;
* the runtime type of the returned array is that of the specified array.
* If the collection fits in the specified array, it is returned therein.
* Otherwise, a new array is allocated with the runtime type of the
* specified array and the size of this collection.
*
* <p>If this collection fits in the specified array with room to spare
* (i.e., the array has more elements than this collection), the element
* in the array immediately following the end of the collection is set to
* <tt>null</tt>. (This is useful in determining the length of this
* collection <i>only</i> if the caller knows that this collection does
* not contain any <tt>null</tt> elements.)
*
* <p>If this collection makes any guarantees as to what order its elements
* are returned by its iterator, this method must return the elements in
* the same order.
*
* <p>Like the {@link #toArray()} method, this method acts as bridge between
* array-based and collection-based APIs. Further, this method allows
* precise control over the runtime type of the output array, and may,
* under certain circumstances, be used to save allocation costs.
*
* <p>Suppose <tt>x</tt> is a collection known to contain only strings.
* The following code can be used to dump the collection into a newly
* allocated array of <tt>String</tt>:
*
* <pre>
* String[] y = x.toArray(new String[0]);</pre>
*
* Note that <tt>toArray(new Object[0])</tt> is identical in function to
* <tt>toArray()</tt>.
*
* @param a the array into which the elements of this collection are to be
* stored, if it is big enough; otherwise, a new array of the same
* runtime type is allocated for this purpose.
* @return an array containing all of the elements in this collection
* @throws ArrayStoreException if the runtime type of the specified array
* is not a supertype of the runtime type of every element in
* this collection
* @throws NullPointerException if the specified array is null
*/
<T> T[] toArray(T[] a);
// Modification Operations
/**
* Ensures that this collection contains the specified element (optional
* operation). Returns <tt>true</tt> if this collection changed as a
* result of the call. (Returns <tt>false</tt> if this collection does
* not permit duplicates and already contains the specified element.)<p>
*
* Collections that support this operation may place limitations on what
* elements may be added to this collection. In particular, some
* collections will refuse to add <tt>null</tt> elements, and others will
* impose restrictions on the type of elements that may be added.
* Collection classes should clearly specify in their documentation any
* restrictions on what elements may be added.<p>
*
* If a collection refuses to add a particular element for any reason
* other than that it already contains the element, it <i>must</i> throw
* an exception (rather than returning <tt>false</tt>). This preserves
* the invariant that a collection always contains the specified element
* after this call returns.
*
* @param e element whose presence in this collection is to be ensured
* @return <tt>true</tt> if this collection changed as a result of the
* call
* @throws UnsupportedOperationException if the <tt>add</tt> operation
* is not supported by this collection
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this collection
* @throws NullPointerException if the specified element is null and this
* collection does not permit null elements
* @throws IllegalArgumentException if some property of the element
* prevents it from being added to this collection
* @throws IllegalStateException if the element cannot be added at this
* time due to insertion restrictions
*/
boolean add(E e);
/**
* Removes a single instance of the specified element from this
* collection, if it is present (optional operation). More formally,
* removes an element <tt>e</tt> such that
* <tt>(o==null ?
e==null : o.equals(e))</tt>, if
* this collection contains one or more such elements. Returns
* <tt>true</tt> if this collection contained the specified element (or
* equivalently, if this collection changed as a result of the call).
*
* @param o element to be removed from this collection, if present
* @return <tt>true</tt> if an element was removed as a result of this call
* @throws ClassCastException if the type of the specified element
* is incompatible with this collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified element is null and this
* collection does not permit null elements
* (<a href="#optional-restrictions">optional</a>)
* @throws UnsupportedOperationException if the <tt>remove</tt> operation
* is not supported by this collection
*/
boolean remove(Object o);
// Bulk Operations
/**
* Returns <tt>true</tt> if this collection contains all of the elements
* in the specified collection.
*
* @param c collection to be checked for containment in this collection
* @return <tt>true</tt> if this collection contains all of the elements
* in the specified collection
* @throws ClassCastException if the types of one or more elements
* in the specified collection are incompatible with this
* collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified collection contains one
* or more null elements and this collection does not permit null
* elements
* (<a href="#optional-restrictions">optional</a>),
* or if the specified collection is null.
* @see #contains(Object)
*/
boolean containsAll(Collection<?
> c);
/**
* Adds all of the elements in the specified collection to this collection
* (optional operation). The behavior of this operation is undefined if
* the specified collection is modified while the operation is in progress.
* (This implies that the behavior of this call is undefined if the
* specified collection is this collection, and this collection is
* nonempty.)
*
* @param c collection containing elements to be added to this collection
* @return <tt>true</tt> if this collection changed as a result of the call
* @throws UnsupportedOperationException if the <tt>addAll</tt> operation
* is not supported by this collection
* @throws ClassCastException if the class of an element of the specified
* collection prevents it from being added to this collection
* @throws NullPointerException if the specified collection contains a
* null element and this collection does not permit null elements,
* or if the specified collection is null
* @throws IllegalArgumentException if some property of an element of the
* specified collection prevents it from being added to this
* collection
* @throws IllegalStateException if not all the elements can be added at
* this time due to insertion restrictions
* @see #add(Object)
*/
boolean addAll(Collection<? extends E> c);
/**
* Removes all of this collection's elements that are also contained in the
* specified collection (optional operation). After this call returns,
* this collection will contain no elements in common with the specified
* collection.
*
* @param c collection containing elements to be removed from this collection
* @return <tt>true</tt> if this collection changed as a result of the
* call
* @throws UnsupportedOperationException if the <tt>removeAll</tt> method
* is not supported by this collection
* @throws ClassCastException if the types of one or more elements
* in this collection are incompatible with the specified
* collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if this collection contains one or more
* null elements and the specified collection does not support
* null elements
* (<a href="#optional-restrictions">optional</a>),
* or if the specified collection is null
* @see #remove(Object)
* @see #contains(Object)
*/
boolean removeAll(Collection<?> c);
/**
* Retains only the elements in this collection that are contained in the
* specified collection (optional operation). In other words, removes from
* this collection all of its elements that are not contained in the
* specified collection.
*
* @param c collection containing elements to be retained in this collection
* @return <tt>true</tt> if this collection changed as a result of the call
* @throws UnsupportedOperationException if the <tt>retainAll</tt> operation
* is not supported by this collection
* @throws ClassCastException if the types of one or more elements
* in this collection are incompatible with the specified
* collection
* (<a href="#optional-restrictions">optional</a>)
* @throws NullPointerException if this collection contains one or more
* null elements and the specified collection does not permit null
* elements
* (<a href="#optional-restrictions">optional</a>),
* or if the specified collection is null
* @see #remove(Object)
* @see #contains(Object)
*/
boolean retainAll(Collection<?> c);
/**
* Removes all of the elements from this collection (optional operation).
* The collection will be empty after this method returns.
*
* @throws UnsupportedOperationException if the <tt>clear</tt> operation
* is not supported by this collection
*/
void clear();
// Comparison and hashing
/**
* Compares the specified object with this collection for equality. <p>
*
* While the <tt>Collection</tt> interface adds no stipulations to the
* general contract for the <tt>Object.equals</tt>, programmers who
* implement the <tt>Collection</tt> interface "directly" (in other words,
* create a class that is a <tt>Collection</tt> but is not a <tt>Set</tt>
* or a <tt>List</tt>) must exercise care if they choose to override the
* <tt>Object.equals</tt>. It is not necessary to do so, and the simplest
* course of action is to rely on <tt>Object</tt>'s implementation, but
* the implementor may wish to implement a "value comparison" in place of
* the default "reference comparison." (The <tt>List</tt> and
* <tt>Set</tt> interfaces mandate such value comparisons.)<p>
*
* The general contract for the <tt>Object.equals</tt> method states that
* equals must be symmetric (in other words, <tt>a.equals(b)</tt> if and
* only if <tt>b.equals(a)</tt>). The contracts for <tt>List.equals</tt>
* and <tt>Set.equals</tt> state that lists are only equal to other lists,
* and sets to other sets. Thus, a custom <tt>equals</tt> method for a
* collection class that implements neither the <tt>List</tt> nor
* <tt>Set</tt> interface must return <tt>false</tt> when this collection
* is compared to any list or set. (By the same logic, it is not possible
* to write a class that correctly implements both the <tt>Set</tt> and
* <tt>List</tt> interfaces.)
*
* @param o object to be compared for equality with this collection
* @return <tt>true</tt> if the specified object is equal to this
* collection
*
* @see Object#equals(Object)
* @see Set#equals(Object)
* @see List#equals(Object)
*/
boolean equals(Object o);
/**
* Returns the hash code value for this collection. While the
* <tt>Collection</tt> interface adds no stipulations to the general
* contract for the <tt>Object.hashCode</tt> method, programmers should
* take note that any class that overrides the <tt>Object.equals</tt>
* method must also override the <tt>Object.hashCode</tt> method in order
* to satisfy the general contract for the <tt>Object.hashCode</tt> method.
* In particular, <tt>c1.equals(c2)</tt> implies that
* <tt>c1.hashCode()==c2.hashCode()</tt>.
*
* @return the hash code value for this collection
*
* @see Object#hashCode()
* @see Object#equals(Object)
*/
int hashCode();
} 從源代碼中能夠看出，Collection定義了一系列關于集合的基本操作，比方檢視集合的大小size()，加入新的元素add()。清空集合clear()等。
從集合接口圖中，我們能夠發現，Set，List，Queue。Deque繼承了Collection。
說明這四個接口也相同是屬于Collection，可是卻又各不相同。
- Set表示一個不能包括同樣元素的集合。
- List表示一個能夠包括同樣元素的有序集合。
- Queue表示隊列，資料的進出遵循First-In-First-Out原則。
- Deque表示一個雙向連結清單，既能夠從表頭操作連結清單，也能夠從末端來操作連結清單。經常使用的詳細實作有：LinkedList。
Map是一個與Collection不一樣的資料結構，主要是key->value資料結構。在程式中。該類資料結構常常會被用到。比方HttpServletRequest中擷取參數，redis緩存，enchance緩存等，以下是Map接口的源代碼：
public interface Map<K,V> {
// Query Operations
/**
* Returns the number of key-value mappings in this map. If the
* map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
* <tt>Integer.MAX_VALUE</tt>.
*
* @return the number of key-value mappings in this map
*/
int size();
/**
* Returns <tt>true</tt> if this map contains no key-value mappings.
*
* @return <tt>true</tt> if this map contains no key-value mappings
*/
boolean isEmpty();
/**
* Returns <tt>true</tt> if this map contains a mapping for the specified
* key. More formally, returns <tt>true</tt> if and only if
* this map contains a mapping for a key <tt>k</tt> such that
* <tt>(key==null ? k==null : key.equals(k))</tt>. (There can be
* at most one such mapping.)
*
* @param key key whose presence in this map is to be tested
* @return <tt>true</tt> if this map contains a mapping for the specified
* key
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean containsKey(Object key);
/**
* Returns <tt>true</tt> if this map maps one or more keys to the
* specified value. More formally, returns <tt>true</tt> if and only if
* this map contains at least one mapping to a value <tt>v</tt> such that
* <tt>(value==null ?
v==null : value.equals(v))</tt>. This operation
* will probably require time linear in the map size for most
* implementations of the <tt>Map</tt> interface.
*
* @param value value whose presence in this map is to be tested
* @return <tt>true</tt> if this map maps one or more keys to the
* specified value
* @throws ClassCastException if the value is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified value is null and this
* map does not permit null values
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
boolean containsValue(Object value);
/**
* Returns the value to which the specified key is mapped,
* or {@code null} if this map contains no mapping for the key.
*
* <p>More formally, if this map contains a mapping from a key
* {@code k} to a value {@code v} such that {@code (key==null ? k==null :
* key.equals(k))}, then this method returns {@code v}; otherwise
* it returns {@code null}. (There can be at most one such mapping.)
*
* <p>If this map permits null values, then a return value of
* {@code null} does not <i>necessarily</i> indicate that the map
* contains no mapping for the key; it's also possible that the map
* explicitly maps the key to {@code null}. The {@link #containsKey
* containsKey} operation may be used to distinguish these two cases.
*
* @param key the key whose associated value is to be returned
* @return the value to which the specified key is mapped, or
* {@code null} if this map contains no mapping for the key
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this map
* does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
V get(Object key);
// Modification Operations
/**
* Associates the specified value with the specified key in this map
* (optional operation). If the map previously contained a mapping for
* the key, the old value is replaced by the specified value. (A map
* <tt>m</tt> is said to contain a mapping for a key <tt>k</tt> if and only
* if {@link #containsKey(Object) m.containsKey(k)} would return
* <tt>true</tt>.)
*
* @param key key with which the specified value is to be associated
* @param value value to be associated with the specified key
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* (A <tt>null</tt> return can also indicate that the map
* previously associated <tt>null</tt> with <tt>key</tt>,
* if the implementation supports <tt>null</tt> values.)
* @throws UnsupportedOperationException if the <tt>put</tt> operation
* is not supported by this map
* @throws ClassCastException if the class of the specified key or value
* prevents it from being stored in this map
* @throws NullPointerException if the specified key or value is null
* and this map does not permit null keys or values
* @throws IllegalArgumentException if some property of the specified key
* or value prevents it from being stored in this map
*/
V put(K key, V value);
/**
* Removes the mapping for a key from this map if it is present
* (optional operation). More formally, if this map contains a mapping
* from key <tt>k</tt> to value <tt>v</tt> such that
* <code>(key==null ? k==null : key.equals(k))</code>, that mapping
* is removed. (The map can contain at most one such mapping.)
*
* <p>Returns the value to which this map previously associated the key,
* or <tt>null</tt> if the map contained no mapping for the key.
*
* <p>If this map permits null values, then a return value of
* <tt>null</tt> does not <i>necessarily</i> indicate that the map
* contained no mapping for the key; it's also possible that the map
* explicitly mapped the key to <tt>null</tt>.
*
* <p>The map will not contain a mapping for the specified key once the
* call returns.
*
* @param key key whose mapping is to be removed from the map
* @return the previous value associated with <tt>key</tt>, or
* <tt>null</tt> if there was no mapping for <tt>key</tt>.
* @throws UnsupportedOperationException if the <tt>remove</tt> operation
* is not supported by this map
* @throws ClassCastException if the key is of an inappropriate type for
* this map
* (<a href="Collection.html#optional-restrictions">optional</a>)
* @throws NullPointerException if the specified key is null and this
* map does not permit null keys
* (<a href="Collection.html#optional-restrictions">optional</a>)
*/
V remove(Object key);
// Bulk Operations
/**
* Copies all of the mappings from the specified map to this map
* (optional operation). The effect of this call is equivalent to that
* of calling {@link #put(Object,Object) put(k, v)} on this map once
* for each mapping from key <tt>k</tt> to value <tt>v</tt> in the
* specified map. The behavior of this operation is undefined if the
* specified map is modified while the operation is in progress.
*
* @param m mappings to be stored in this map
* @throws UnsupportedOperationException if the <tt>putAll</tt> operation
* is not supported by this map
* @throws ClassCastException if the class of a key or value in the
* specified map prevents it from being stored in this map
* @throws NullPointerException if the specified map is null, or if
* this map does not permit null keys or values, and the
* specified map contains null keys or values
* @throws IllegalArgumentException if some property of a key or value in
* the specified map prevents it from being stored in this map
*/
void putAll(Map<? extends K, ?
extends V> m);
/**
* Removes all of the mappings from this map (optional operation).
* The map will be empty after this call returns.
*
* @throws UnsupportedOperationException if the <tt>clear</tt> operation
* is not supported by this map
*/
void clear();
// Views
/**
* Returns a {@link Set} view of the keys contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation), the results of
* the iteration are undefined. The set supports element removal,
* which removes the corresponding mapping from the map, via the
* <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
* <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
* operations. It does not support the <tt>add</tt> or <tt>addAll</tt>
* operations.
*
* @return a set view of the keys contained in this map
*/
Set<K> keySet();
/**
* Returns a {@link Collection} view of the values contained in this map.
* The collection is backed by the map, so changes to the map are
* reflected in the collection, and vice-versa. If the map is
* modified while an iteration over the collection is in progress
* (except through the iterator's own <tt>remove</tt> operation),
* the results of the iteration are undefined. The collection
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Collection.remove</tt>, <tt>removeAll</tt>,
* <tt>retainAll</tt> and <tt>clear</tt> operations. It does not
* support the <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a collection view of the values contained in this map
*/
Collection<V> values();
/**
* Returns a {@link Set} view of the mappings contained in this map.
* The set is backed by the map, so changes to the map are
* reflected in the set, and vice-versa. If the map is modified
* while an iteration over the set is in progress (except through
* the iterator's own <tt>remove</tt> operation, or through the
* <tt>setValue</tt> operation on a map entry returned by the
* iterator) the results of the iteration are undefined. The set
* supports element removal, which removes the corresponding
* mapping from the map, via the <tt>Iterator.remove</tt>,
* <tt>Set.remove</tt>, <tt>removeAll</tt>, <tt>retainAll</tt> and
* <tt>clear</tt> operations. It does not support the
* <tt>add</tt> or <tt>addAll</tt> operations.
*
* @return a set view of the mappings contained in this map
*/
Set<Map.Entry<K, V>> entrySet();
/**
* A map entry (key-value pair). The <tt>Map.entrySet</tt> method returns
* a collection-view of the map, whose elements are of this class. The
* <i>only</i> way to obtain a reference to a map entry is from the
* iterator of this collection-view. These <tt>Map.Entry</tt> objects are
* valid <i>only</i> for the duration of the iteration; more formally,
* the behavior of a map entry is undefined if the backing map has been
* modified after the entry was returned by the iterator, except through
* the <tt>setValue</tt> operation on the map entry.
*
* @see Map#entrySet()
* @since 1.2
*/
interface Entry<K,V> {
/**
* Returns the key corresponding to this entry.
*
* @return the key corresponding to this entry
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
K getKey();
/**
* Returns the value corresponding to this entry. If the mapping
* has been removed from the backing map (by the iterator's
* <tt>remove</tt> operation), the results of this call are undefined.
*
* @return the value corresponding to this entry
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
V getValue();
/**
* Replaces the value corresponding to this entry with the specified
* value (optional operation). (Writes through to the map.) The
* behavior of this call is undefined if the mapping has already been
* removed from the map (by the iterator's <tt>remove</tt> operation).
*
* @param value new value to be stored in this entry
* @return old value corresponding to the entry
* @throws UnsupportedOperationException if the <tt>put</tt> operation
* is not supported by the backing map
* @throws ClassCastException if the class of the specified value
* prevents it from being stored in the backing map
* @throws NullPointerException if the backing map does not permit
* null values, and the specified value is null
* @throws IllegalArgumentException if some property of this value
* prevents it from being stored in the backing map
* @throws IllegalStateException implementations may, but are not
* required to, throw this exception if the entry has been
* removed from the backing map.
*/
V setValue(V value);
/**
* Compares the specified object with this entry for equality.
* Returns <tt>true</tt> if the given object is also a map entry and
* the two entries represent the same mapping. More formally, two
* entries <tt>e1</tt> and <tt>e2</tt> represent the same mapping
* if<pre>
* (e1.getKey()==null ?
* e2.getKey()==null : e1.getKey().equals(e2.getKey())) &&
* (e1.getValue()==null ?
* e2.getValue()==null : e1.getValue().equals(e2.getValue()))
* </pre>
* This ensures that the <tt>equals</tt> method works properly across
* different implementations of the <tt>Map.Entry</tt> interface.
*
* @param o object to be compared for equality with this map entry
* @return <tt>true</tt> if the specified object is equal to this map
* entry
*/
boolean equals(Object o);
/**
* Returns the hash code value for this map entry. The hash code
* of a map entry <tt>e</tt> is defined to be: <pre>
* (e.getKey()==null ? 0 : e.getKey().hashCode()) ^
* (e.getValue()==null ? 0 : e.getValue().hashCode())
* </pre>
* This ensures that <tt>e1.equals(e2)</tt> implies that
* <tt>e1.hashCode()==e2.hashCode()</tt> for any two Entries
* <tt>e1</tt> and <tt>e2</tt>, as required by the general
* contract of <tt>Object.hashCode</tt>.
*
* @return the hash code value for this map entry
* @see Object#hashCode()
* @see Object#equals(Object)
* @see #equals(Object)
*/
int hashCode();
}
// Comparison and hashing
/**
* Compares the specified object with this map for equality. Returns
* <tt>true</tt> if the given object is also a map and the two maps
* represent the same mappings. More formally, two maps <tt>m1</tt> and
* <tt>m2</tt> represent the same mappings if
* <tt>m1.entrySet().equals(m2.entrySet())</tt>. This ensures that the
* <tt>equals</tt> method works properly across different implementations
* of the <tt>Map</tt> interface.
*
* @param o object to be compared for equality with this map
* @return <tt>true</tt> if the specified object is equal to this map
*/
boolean equals(Object o);
/**
* Returns the hash code value for this map. The hash code of a map is
* defined to be the sum of the hash codes of each entry in the map's
* <tt>entrySet()</tt> view. This ensures that <tt>m1.equals(m2)</tt>
* implies that <tt>m1.hashCode()==m2.hashCode()</tt> for any two maps
* <tt>m1</tt> and <tt>m2</tt>, as required by the general contract of
* {@link Object#hashCode}.
*
* @return the hash code value for this map
* @see Map.Entry#hashCode()
* @see Object#equals(Object)
* @see #equals(Object)
*/
int hashCode();
} 在Map接口中，相同也定義了有關于Map的經常使用操作，比方插入資料put()，清空資料clear()，拿到key的Set集合keySet()。在這裡面使用了一個Entry<K,V>接口來表示一個鍵值對。一個Map能夠傳回三種類型的集合，一種是Key組成的keySet()。一種是Value組成的values()。第二種則是由key,value組成的entrySet()。
泛型技術與集合架構
在集合架構中，使用了大量的泛型技術。比方Collection<V>則代表了Collection使用了泛型，開發人員能夠依據須要來選擇V的詳細内容。
比方Collection<String>,Collection<Integer>等。在泛型中，須要注意一下幾點：
- 泛型不能為基本資料類型（byte，char。short，int，float，double，boolean），假設須要基本資料類型，能夠使用基本使用類型相應的包裝類型來替代。（Byte。Character，Short，Integer，Float，Double，Boolean）
- 泛型能夠是集合架構的随意類型。初看java集合架構，會感覺該架構僅僅定義了幾個接口，對于複雜資料結構的表達能力有所不夠，可是通過泛型中嵌套集合架構。則能夠實作複雜的資料。比方Map<List,Map<List,Map<String,List<String>>>>
詳細實作
有了總體上對java集合架構的基本認識。再了解整個集合架構中的設計便變得簡單起來。下圖是java集合架構的類圖結構：
通過類型能夠發現，這裡面大量使用了AbstractXxx的類。這樣子做的目的是什麼呢？以下我們觀察下AbstractList<E>的源代碼：
public abstract class AbstractList<E> extends AbstractCollection<E> implements List<E> {
/**
* Sole constructor. (For invocation by subclass constructors, typically
* implicit.)
*/
protected AbstractList() {
}
/**
* Appends the specified element to the end of this list (optional
* operation).
*
* <p>Lists that support this operation may place limitations on what
* elements may be added to this list. In particular, some
* lists will refuse to add null elements, and others will impose
* restrictions on the type of elements that may be added. List
* classes should clearly specify in their documentation any restrictions
* on what elements may be added.
*
* <p>This implementation calls {@code add(size(), e)}.
*
* <p>Note that this implementation throws an
* {@code UnsupportedOperationException} unless
* {@link #add(int, Object) add(int, E)} is overridden.
*
* @param e element to be appended to this list
* @return {@code true} (as specified by {@link Collection#add})
* @throws UnsupportedOperationException if the {@code add} operation
* is not supported by this list
* @throws ClassCastException if the class of the specified element
* prevents it from being added to this list
* @throws NullPointerException if the specified element is null and this
* list does not permit null elements
* @throws IllegalArgumentException if some property of this element
* prevents it from being added to this list
*/
public boolean add(E e) {
add(size(), e);
return true;
}
/**
* {@inheritDoc}
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
abstract public E get(int index);
/**
* {@inheritDoc}
*
* <p>This implementation always throws an
* {@code UnsupportedOperationException}.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E set(int index, E element) {
throw new UnsupportedOperationException();
}
/**
* {@inheritDoc}
*
* <p>This implementation always throws an
* {@code UnsupportedOperationException}.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public void add(int index, E element) {
throw new UnsupportedOperationException();
}
/**
* {@inheritDoc}
*
* <p>This implementation always throws an
* {@code UnsupportedOperationException}.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public E remove(int index) {
throw new UnsupportedOperationException();
}
// Search Operations
/**
* {@inheritDoc}
*
* <p>This implementation first gets a list iterator (with
* {@code listIterator()}). Then, it iterates over the list until the
* specified element is found or the end of the list is reached.
*
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public int indexOf(Object o) {
ListIterator<E> it = listIterator();
if (o==null) {
while (it.hasNext())
if (it.next()==null)
return it.previousIndex();
} else {
while (it.hasNext())
if (o.equals(it.next()))
return it.previousIndex();
}
return -1;
}
/**
* {@inheritDoc}
*
* <p>This implementation first gets a list iterator that points to the end
* of the list (with {@code listIterator(size())}). Then, it iterates
* backwards over the list until the specified element is found, or the
* beginning of the list is reached.
*
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
*/
public int lastIndexOf(Object o) {
ListIterator<E> it = listIterator(size());
if (o==null) {
while (it.hasPrevious())
if (it.previous()==null)
return it.nextIndex();
} else {
while (it.hasPrevious())
if (o.equals(it.previous()))
return it.nextIndex();
}
return -1;
}
// Bulk Operations
/**
* Removes all of the elements from this list (optional operation).
* The list will be empty after this call returns.
*
* <p>This implementation calls {@code removeRange(0, size())}.
*
* <p>Note that this implementation throws an
* {@code UnsupportedOperationException} unless {@code remove(int
* index)} or {@code removeRange(int fromIndex, int toIndex)} is
* overridden.
*
* @throws UnsupportedOperationException if the {@code clear} operation
* is not supported by this list
*/
public void clear() {
removeRange(0, size());
}
/**
* {@inheritDoc}
*
* <p>This implementation gets an iterator over the specified collection
* and iterates over it, inserting the elements obtained from the
* iterator into this list at the appropriate position, one at a time,
* using {@code add(int, E)}.
* Many implementations will override this method for efficiency.
*
* <p>Note that this implementation throws an
* {@code UnsupportedOperationException} unless
* {@link #add(int, Object) add(int, E)} is overridden.
*
* @throws UnsupportedOperationException {@inheritDoc}
* @throws ClassCastException {@inheritDoc}
* @throws NullPointerException {@inheritDoc}
* @throws IllegalArgumentException {@inheritDoc}
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
boolean modified = false;
for (E e : c) {
add(index++, e);
modified = true;
}
return modified;
}
// Iterators
/**
* Returns an iterator over the elements in this list in proper sequence.
*
* <p>This implementation returns a straightforward implementation of the
* iterator interface, relying on the backing list's {@code size()},
* {@code get(int)}, and {@code remove(int)} methods.
*
* <p>Note that the iterator returned by this method will throw an
* {@link UnsupportedOperationException} in response to its
* {@code remove} method unless the list's {@code remove(int)} method is
* overridden.
*
* <p>This implementation can be made to throw runtime exceptions in the
* face of concurrent modification, as described in the specification
* for the (protected) {@link #modCount} field.
*
* @return an iterator over the elements in this list in proper sequence
*/
public Iterator<E> iterator() {
return new Itr();
}
/**
* {@inheritDoc}
*
* <p>This implementation returns {@code listIterator(0)}.
*
* @see #listIterator(int)
*/
public ListIterator<E> listIterator() {
return listIterator(0);
}
/**
* {@inheritDoc}
*
* <p>This implementation returns a straightforward implementation of the
* {@code ListIterator} interface that extends the implementation of the
* {@code Iterator} interface returned by the {@code iterator()} method.
* The {@code ListIterator} implementation relies on the backing list's
* {@code get(int)}, {@code set(int, E)}, {@code add(int, E)}
* and {@code remove(int)} methods.
*
* <p>Note that the list iterator returned by this implementation will
* throw an {@link UnsupportedOperationException} in response to its
* {@code remove}, {@code set} and {@code add} methods unless the
* list's {@code remove(int)}, {@code set(int, E)}, and
* {@code add(int, E)} methods are overridden.
*
* <p>This implementation can be made to throw runtime exceptions in the
* face of concurrent modification, as described in the specification for
* the (protected) {@link #modCount} field.
*
* @throws IndexOutOfBoundsException {@inheritDoc}
*/
public ListIterator<E> listIterator(final int index) {
rangeCheckForAdd(index);
return new ListItr(index);
}
private class Itr implements Iterator<E> {
/**
* Index of element to be returned by subsequent call to next.
*/
int cursor = 0;
/**
* Index of element returned by most recent call to next or
* previous. Reset to -1 if this element is deleted by a call
* to remove.
*/
int lastRet = -1;
/**
* The modCount value that the iterator believes that the backing
* List should have. If this expectation is violated, the iterator
* has detected concurrent modification.
*/
int expectedModCount = modCount;
public boolean hasNext() {
return cursor != size();
}
public E next() {
checkForComodification();
try {
int i = cursor;
E next = get(i);
lastRet = i;
cursor = i + 1;
return next;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}
public void remove() {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
AbstractList.this.remove(lastRet);
if (lastRet < cursor)
cursor--;
lastRet = -1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException e) {
throw new ConcurrentModificationException();
}
}
final void checkForComodification() {
if (modCount != expectedModCount)
throw new ConcurrentModificationException();
}
}
private class ListItr extends Itr implements ListIterator<E> {
ListItr(int index) {
cursor = index;
}
public boolean hasPrevious() {
return cursor != 0;
}
public E previous() {
checkForComodification();
try {
int i = cursor - 1;
E previous = get(i);
lastRet = cursor = i;
return previous;
} catch (IndexOutOfBoundsException e) {
checkForComodification();
throw new NoSuchElementException();
}
}
public int nextIndex() {
return cursor;
}
public int previousIndex() {
return cursor-1;
}
public void set(E e) {
if (lastRet < 0)
throw new IllegalStateException();
checkForComodification();
try {
AbstractList.this.set(lastRet, e);
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
public void add(E e) {
checkForComodification();
try {
int i = cursor;
AbstractList.this.add(i, e);
lastRet = -1;
cursor = i + 1;
expectedModCount = modCount;
} catch (IndexOutOfBoundsException ex) {
throw new ConcurrentModificationException();
}
}
}
/**
* {@inheritDoc}
*
* <p>This implementation returns a list that subclasses
* {@code AbstractList}. The subclass stores, in private fields, the
* offset of the subList within the backing list, the size of the subList
* (which can change over its lifetime), and the expected
* {@code modCount} value of the backing list. There are two variants
* of the subclass, one of which implements {@code RandomAccess}.
* If this list implements {@code RandomAccess} the returned list will
* be an instance of the subclass that implements {@code RandomAccess}.
*
* <p>The subclass's {@code set(int, E)}, {@code get(int)},
* {@code add(int, E)}, {@code remove(int)}, {@code addAll(int,
* Collection)} and {@code removeRange(int, int)} methods all
* delegate to the corresponding methods on the backing abstract list,
* after bounds-checking the index and adjusting for the offset. The
* {@code addAll(Collection c)} method merely returns {@code addAll(size,
* c)}.
*
* <p>The {@code listIterator(int)} method returns a "wrapper object"
* over a list iterator on the backing list, which is created with the
* corresponding method on the backing list. The {@code iterator} method
* merely returns {@code listIterator()}, and the {@code size} method
* merely returns the subclass's {@code size} field.
*
* <p>All methods first check to see if the actual {@code modCount} of
* the backing list is equal to its expected value, and throw a
* {@code ConcurrentModificationException} if it is not.
*
* @throws IndexOutOfBoundsException if an endpoint index value is out of range
* {@code (fromIndex < 0 || toIndex > size)}
* @throws IllegalArgumentException if the endpoint indices are out of order
* {@code (fromIndex > toIndex)}
*/
public List<E> subList(int fromIndex, int toIndex) {
return (this instanceof RandomAccess ?
new RandomAccessSubList<>(this, fromIndex, toIndex) :
new SubList<>(this, fromIndex, toIndex));
}
// Comparison and hashing
/**
* Compares the specified object with this list for equality. Returns
* {@code true} if and only if the specified object is also a list, both
* lists have the same size, and all corresponding pairs of elements in
* the two lists are <i>equal</i>. (Two elements {@code e1} and
* {@code e2} are <i>equal</i> if {@code (e1==null ? e2==null :
* e1.equals(e2))}.) In other words, two lists are defined to be
* equal if they contain the same elements in the same order.<p>
*
* This implementation first checks if the specified object is this
* list. If so, it returns {@code true}; if not, it checks if the
* specified object is a list. If not, it returns {@code false}; if so,
* it iterates over both lists, comparing corresponding pairs of elements.
* If any comparison returns {@code false}, this method returns
* {@code false}. If either iterator runs out of elements before the
* other it returns {@code false} (as the lists are of unequal length);
* otherwise it returns {@code true} when the iterations complete.
*
* @param o the object to be compared for equality with this list
* @return {@code true} if the specified object is equal to this list
*/
public boolean equals(Object o) {
if (o == this)
return true;
if (!(o instanceof List))
return false;
ListIterator<E> e1 = listIterator();
ListIterator e2 = ((List) o).listIterator();
while (e1.hasNext() && e2.hasNext()) {
E o1 = e1.next();
Object o2 = e2.next();
if (!(o1==null ?
o2==null : o1.equals(o2)))
return false;
}
return !(e1.hasNext() || e2.hasNext());
}
/**
* Returns the hash code value for this list.
*
* <p>This implementation uses exactly the code that is used to define the
* list hash function in the documentation for the {@link List#hashCode}
* method.
*
* @return the hash code value for this list
*/
public int hashCode() {
int hashCode = 1;
for (E e : this)
hashCode = 31*hashCode + (e==null ?
0 : e.hashCode());
return hashCode;
}
/**
* Removes from this list all of the elements whose index is between
* {@code fromIndex}, inclusive, and {@code toIndex}, exclusive.
* Shifts any succeeding elements to the left (reduces their index).
* This call shortens the list by {@code (toIndex - fromIndex)} elements.
* (If {@code toIndex==fromIndex}, this operation has no effect.)
*
* <p>This method is called by the {@code clear} operation on this list
* and its subLists. Overriding this method to take advantage of
* the internals of the list implementation can <i>substantially</i>
* improve the performance of the {@code clear} operation on this list
* and its subLists.
*
* <p>This implementation gets a list iterator positioned before
* {@code fromIndex}, and repeatedly calls {@code ListIterator.next}
* followed by {@code ListIterator.remove} until the entire range has
* been removed. <b>Note: if {@code ListIterator.remove} requires linear
* time, this implementation requires quadratic time.</b>
*
* @param fromIndex index of first element to be removed
* @param toIndex index after last element to be removed
*/
protected void removeRange(int fromIndex, int toIndex) {
ListIterator<E> it = listIterator(fromIndex);
for (int i=0, n=toIndex-fromIndex; i<n; i++) {
it.next();
it.remove();
}
}
/**
* The number of times this list has been <i>structurally modified</i>.
* Structural modifications are those that change the size of the
* list, or otherwise perturb it in such a fashion that iterations in
* progress may yield incorrect results.
*
* <p>This field is used by the iterator and list iterator implementation
* returned by the {@code iterator} and {@code listIterator} methods.
* If the value of this field changes unexpectedly, the iterator (or list
* iterator) will throw a {@code ConcurrentModificationException} in
* response to the {@code next}, {@code remove}, {@code previous},
* {@code set} or {@code add} operations. This provides
* <i>fail-fast</i> behavior, rather than non-deterministic behavior in
* the face of concurrent modification during iteration.
*
* <p><b>Use of this field by subclasses is optional.</b> If a subclass
* wishes to provide fail-fast iterators (and list iterators), then it
* merely has to increment this field in its {@code add(int, E)} and
* {@code remove(int)} methods (and any other methods that it overrides
* that result in structural modifications to the list). A single call to
* {@code add(int, E)} or {@code remove(int)} must add no more than
* one to this field, or the iterators (and list iterators) will throw
* bogus {@code ConcurrentModificationExceptions}. If an implementation
* does not wish to provide fail-fast iterators, this field may be
* ignored.
*/
protected transient int modCount = 0;
private void rangeCheckForAdd(int index) {
if (index < 0 || index > size())
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size();
}
}
class SubList<E> extends AbstractList<E> {
private final AbstractList<E> l;
private final int offset;
private int size;
SubList(AbstractList<E> list, int fromIndex, int toIndex) {
if (fromIndex < 0)
throw new IndexOutOfBoundsException("fromIndex = " + fromIndex);
if (toIndex > list.size())
throw new IndexOutOfBoundsException("toIndex = " + toIndex);
if (fromIndex > toIndex)
throw new IllegalArgumentException("fromIndex(" + fromIndex +
") > toIndex(" + toIndex + ")");
l = list;
offset = fromIndex;
size = toIndex - fromIndex;
this.modCount = l.modCount;
}
public E set(int index, E element) {
rangeCheck(index);
checkForComodification();
return l.set(index+offset, element);
}
public E get(int index) {
rangeCheck(index);
checkForComodification();
return l.get(index+offset);
}
public int size() {
checkForComodification();
return size;
}
public void add(int index, E element) {
rangeCheckForAdd(index);
checkForComodification();
l.add(index+offset, element);
this.modCount = l.modCount;
size++;
}
public E remove(int index) {
rangeCheck(index);
checkForComodification();
E result = l.remove(index+offset);
this.modCount = l.modCount;
size--;
return result;
}
protected void removeRange(int fromIndex, int toIndex) {
checkForComodification();
l.removeRange(fromIndex+offset, toIndex+offset);
this.modCount = l.modCount;
size -= (toIndex-fromIndex);
}
public boolean addAll(Collection<? extends E> c) {
return addAll(size, c);
}
public boolean addAll(int index, Collection<? extends E> c) {
rangeCheckForAdd(index);
int cSize = c.size();
if (cSize==0)
return false;
checkForComodification();
l.addAll(offset+index, c);
this.modCount = l.modCount;
size += cSize;
return true;
}
public Iterator<E> iterator() {
return listIterator();
}
public ListIterator<E> listIterator(final int index) {
checkForComodification();
rangeCheckForAdd(index);
return new ListIterator<E>() {
private final ListIterator<E> i = l.listIterator(index+offset);
public boolean hasNext() {
return nextIndex() < size;
}
public E next() {
if (hasNext())
return i.next();
else
throw new NoSuchElementException();
}
public boolean hasPrevious() {
return previousIndex() >= 0;
}
public E previous() {
if (hasPrevious())
return i.previous();
else
throw new NoSuchElementException();
}
public int nextIndex() {
return i.nextIndex() - offset;
}
public int previousIndex() {
return i.previousIndex() - offset;
}
public void remove() {
i.remove();
SubList.this.modCount = l.modCount;
size--;
}
public void set(E e) {
i.set(e);
}
public void add(E e) {
i.add(e);
SubList.this.modCount = l.modCount;
size++;
}
};
}
public List<E> subList(int fromIndex, int toIndex) {
return new SubList<>(this, fromIndex, toIndex);
}
private void rangeCheck(int index) {
if (index < 0 || index >= size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private void rangeCheckForAdd(int index) {
if (index < 0 || index > size)
throw new IndexOutOfBoundsException(outOfBoundsMsg(index));
}
private String outOfBoundsMsg(int index) {
return "Index: "+index+", Size: "+size;
}
private void checkForComodification() {
if (this.modCount != l.modCount)
throw new ConcurrentModificationException();
}
}
class RandomAccessSubList<E> extends SubList<E> implements RandomAccess {
RandomAccessSubList(AbstractList<E> list, int fromIndex, int toIndex) {
super(list, fromIndex, toIndex);
}
public List<E> subList(int fromIndex, int toIndex) {
return new RandomAccessSubList<>(this, fromIndex, toIndex);
}
} 通過源代碼能夠發現。該類提供了List<E>的基本實作。對于不支援的操作，則直接抛出UnSupportedOperationException。其它之類直接繼承該類。然後提供各自的實作。就可以實作同樣子產品的代碼重用。
如今相信通過類圖中的結構，以及類的名字就可以猜到某個類的詳細實作方式了吧。比方ArrayList能夠猜到裡面的資料結構是使用數組來實作的。LinkedList能夠猜到裡面是使用連結清單實作的。既然兩者都能夠作為List來使用。那兩者之間就必定存在着相似點與不同點。
相似點就是大家都實作了List，提供了List應該具備的操作，不同點則是實作方式不同，對于有些操作的性能是有差異的。詳細的差異。相比熟悉資料結構的人都明确。數組查詢效率高，而連結清單則在插入資料時。更加高速。詳細的，大家能夠參考下資料結構的介紹。
總結
初看java集合架構。肯定會被龐大的類體系結構給弄的暈乎暈乎的。
可是，作為一個java開發人員。集合架構不可缺少。在開發中，心中應該時刻明确。什麼情況下，應該選擇使用什麼樣的類來盛放資料。為了程式的課擴充性，也盡量保持對接口程式設計的風格。