簡介
Optional
是一個沒有子類的工具類，是一個可以為null的容器對象，它的主要作用就是用來避免null的檢查，防止出現
NPE
。
Optional
是個容器：它可以儲存類型T的值，或者僅僅儲存null。
Optional
提供很多有用的方法，這樣我們就不用顯式進行空值檢測。Optional 類的引入很好的解決空指針異常。
public final class Optional<T> {} 複制
Optional
Opitonal
類就是Java提供的為了解決大家平時判斷對象是否為空用 會用
null != obj
這樣的方式存在的判斷，進而令人頭疼導緻
NPE
（Null Pointer Exception 空指針異常），同時
Optional
的存在可以讓代碼更加簡單，可讀性跟高，代碼寫起來更高效。
源碼
package java.util;
import java.util.function.Consumer;
import java.util.function.Function;
import java.util.function.Predicate;
import java.util.function.Supplier;
/**
* @since 1.8
*/
public final class Optional<T> {
/**
* Common instance for {@code empty()}.
*/
private static final Optional<?> EMPTY = new Optional<>();
/**
* If non-null, the value; if null, indicates no value is present
*/
private final T value;
/**
* 私有構造方法
*/
private Optional() {
this.value = null;
}
/**
* Returns an empty {@code Optional} instance. No value is present for this
* Optional.
*
* @apiNote Though it may be tempting to do so, avoid testing if an object
* is empty by comparing with {@code ==} against instances returned by
* {@code Option.empty()}. There is no guarantee that it is a singleton.
* Instead, use {@link #isPresent()}.
*
* @param <T> Type of the non-existent value
* @return an empty {@code Optional}
*/
// 傳回一個空的 Optional執行個體
public static<T> Optional<T> empty() {
@SuppressWarnings("unchecked")
Optional<T> t = (Optional<T>) EMPTY;
return t;
}
/**
* 私有構造方法
*/
private Optional(T value) {
this.value = Objects.requireNonNull(value);
}
/**
* Returns an {@code Optional} with the specified present non-null value.
*
* @param <T> the class of the value
* @param value the value to be present, which must be non-null
* @return an {@code Optional} with the value present
* @throws NullPointerException if value is null
*/
// 傳回具有 Optional的目前非空值的Optional
public static <T> Optional<T> of(T value) {
return new Optional<>(value);
}
/**
* Returns an {@code Optional} describing the specified value, if non-null,
* otherwise returns an empty {@code Optional}.
*
* @param <T> the class of the value
* @param value the possibly-null value to describe
* @return an {@code Optional} with a present value if the specified value
* is non-null, otherwise an empty {@code Optional}
*/
// 傳回一個 Optional指定值的Optional，如果非空，則傳回一個空的 Optional
public static <T> Optional<T> ofNullable(T value) {
return value == null ? empty() : of(value);
}
/**
* If a value is present in this {@code Optional}, returns the value,
* otherwise throws {@code NoSuchElementException}.
*
* @return the non-null value held by this {@code Optional}
* @throws NoSuchElementException if there is no value present
*
* @see Optional#isPresent()
*/
// 如果Optional中有一個值，傳回值，否則抛出 NoSuchElementException 。
public T get() {
if (value == null) {
throw new NoSuchElementException("No value present");
}
return value;
}
/**
* Return {@code true} if there is a value present, otherwise {@code false}.
*
* @return {@code true} if there is a value present, otherwise {@code false}
*/
// 傳回true如果存在值，否則為 false
public boolean isPresent() {
return value != null;
}
/**
* If a value is present, invoke the specified consumer with the value,
* otherwise do nothing.
*
* @param consumer block to be executed if a value is present
* @throws NullPointerException if value is present and {@code consumer} is
* null
*/
// 如果存在值，則使用該值調用指定的消費者，否則不執行任何操作。
public void ifPresent(Consumer<? super T> consumer) {
if (value != null)
consumer.accept(value);
}
/**
* If a value is present, and the value matches the given predicate,
* return an {@code Optional} describing the value, otherwise return an
* empty {@code Optional}.
*
* @param predicate a predicate to apply to the value, if present
* @return an {@code Optional} describing the value of this {@code Optional}
* if a value is present and the value matches the given predicate,
* otherwise an empty {@code Optional}
* @throws NullPointerException if the predicate is null
*/
// 如果一個值存在，并且該值給定的謂詞相比對時，傳回一個 Optional描述的值，否則傳回一個空的 Optional
public Optional<T> filter(Predicate<? super T> predicate) {
Objects.requireNonNull(predicate);
if (!isPresent())
return this;
else
return predicate.test(value) ? this : empty();
}
/**
* If a value is present, apply the provided mapping function to it,
* and if the result is non-null, return an {@code Optional} describing the
* result. Otherwise return an empty {@code Optional}.
*
* @apiNote This method supports post-processing on optional values, without
* the need to explicitly check for a return status. For example, the
* following code traverses a stream of file names, selects one that has
* not yet been processed, and then opens that file, returning an
* {@code Optional<FileInputStream>}:
*
* <pre>{@code
* Optional<FileInputStream> fis =
* names.stream().filter(name -> !isProcessedYet(name))
* .findFirst()
* .map(name -> new FileInputStream(name));
* }</pre>
*
* Here, {@code findFirst} returns an {@code Optional<String>}, and then
* {@code map} returns an {@code Optional<FileInputStream>} for the desired
* file if one exists.
*
* @param <U> The type of the result of the mapping function
* @param mapper a mapping function to apply to the value, if present
* @return an {@code Optional} describing the result of applying a mapping
* function to the value of this {@code Optional}, if a value is present,
* otherwise an empty {@code Optional}
* @throws NullPointerException if the mapping function is null
*/
// 如果存在一個值，則應用提供的映射函數，如果結果不為空，則傳回一個 Optional結果的 Optional 。
public<U> Optional<U> map(Function<? super T, ? extends U> mapper) {
Objects.requireNonNull(mapper);
if (!isPresent())
return empty();
else {
return Optional.ofNullable(mapper.apply(value));
}
}
/**
* If a value is present, apply the provided {@code Optional}-bearing
* mapping function to it, return that result, otherwise return an empty
* {@code Optional}. This method is similar to {@link #map(Function)},
* but the provided mapper is one whose result is already an {@code Optional},
* and if invoked, {@code flatMap} does not wrap it with an additional
* {@code Optional}.
*
* @param <U> The type parameter to the {@code Optional} returned by
* @param mapper a mapping function to apply to the value, if present
* the mapping function
* @return the result of applying an {@code Optional}-bearing mapping
* function to the value of this {@code Optional}, if a value is present,
* otherwise an empty {@code Optional}
* @throws NullPointerException if the mapping function is null or returns
* a null result
*/
// 如果一個值存在，應用提供的 Optional映射函數給它，傳回該結果，否則傳回一個空的 Optional 。
public<U> Optional<U> flatMap(Function<? super T, Optional<U>> mapper) {
Objects.requireNonNull(mapper);
if (!isPresent())
return empty();
else {
return Objects.requireNonNull(mapper.apply(value));
}
}
/**
* Return the value if present, otherwise return {@code other}.
*
* @param other the value to be returned if there is no value present, may
* be null
* @return the value, if present, otherwise {@code other}
*/
// 如果值存在，就傳回值，不存在就傳回指定的其他值
public T orElse(T other) {
return value != null ? value : other;
}
/**
* Return the value if present, otherwise invoke {@code other} and return
* the result of that invocation.
*
* @param other a {@code Supplier} whose result is returned if no value
* is present
* @return the value if present otherwise the result of {@code other.get()}
* @throws NullPointerException if value is not present and {@code other} is
* null
*/
public T orElseGet(Supplier<? extends T> other) {
return value != null ? value : other.get();
}
/**
* Return the contained value, if present, otherwise throw an exception
* to be created by the provided supplier.
*
* @apiNote A method reference to the exception constructor with an empty
* argument list can be used as the supplier. For example,
* {@code IllegalStateException::new}
*
* @param <X> Type of the exception to be thrown
* @param exceptionSupplier The supplier which will return the exception to
* be thrown
* @return the present value
* @throws X if there is no value present
* @throws NullPointerException if no value is present and
* {@code exceptionSupplier} is null
*/
public <X extends Throwable> T orElseThrow(Supplier<? extends X> exceptionSupplier) throws X {
if (value != null) {
return value;
} else {
throw exceptionSupplier.get();
}
}
/**
* Indicates whether some other object is "equal to" this Optional. The
* other object is considered equal if:
* <ul>
* <li>it is also an {@code Optional} and;
* <li>both instances have no value present or;
* <li>the present values are "equal to" each other via {@code equals()}.
* </ul>
*
* @param obj an object to be tested for equality
* @return {code true} if the other object is "equal to" this object
* otherwise {@code false}
*/
@Override
public boolean equals(Object obj) {
if (this == obj) {
return true;
}
if (!(obj instanceof Optional)) {
return false;
}
Optional<?> other = (Optional<?>) obj;
return Objects.equals(value, other.value);
}
/**
* Returns the hash code value of the present value, if any, or 0 (zero) if
* no value is present.
*
* @return hash code value of the present value or 0 if no value is present
*/
@Override
public int hashCode() {
return Objects.hashCode(value);
}
/**
* Returns a non-empty string representation of this Optional suitable for
* debugging. The exact presentation format is unspecified and may vary
* between implementations and versions.
*
* @implSpec If a value is present the result must include its string
* representation in the result. Empty and present Optionals must be
* unambiguously differentiable.
*
* @return the string representation of this instance
*/
@Override
public String toString() {
return value != null
? String.format("Optional[%s]", value)
: "Optional.empty";
}
} 複制
構造
建立
Optional
對象有三種方法：
empty()
、
of()
、
ofNullable()
，均為靜态方法。
如果
Optional
對象沒有值則用
empty()
方法。
//建立一個包裝對象值為空的Optional對象
Optional optional = Optional.empty(); 複制
如果确定
Optional
對象的值不為null，則可用
of()
方法。
//建立包裝對象值非空的Optional對象
Optional optional = Optional.of("Hello Java");
//傳入null，抛出NullPointerException
Optional<Object> o = Optional.of(null); 複制
如果不确定
Optional
對象的值是否為null，則可用
ofNullable()
。
//建立包裝對象值允許為空也可以不為空的Optional對象
Optional optional = Optional.ofNullable("Hello Java");
通過調用其isPresent方法可以檢視該Optional中是否值為null。isPresent()方法就是會傳回一個boolean類型值，如果對象不為空則為真，如果為空則false
boolean flag = optional.isPresent();
System.out.println(flag); 複制
get
描述：傳回一個option的執行個體值。
Optional<String> op1 = Optional.of("Tom");
String s1 = op1.get();
System.out.println(s1);
備注：如果Optional為null，此時會抛出空指針 複制
isPresent&ifPresent
描述：值存在傳回true，否則傳回false
Optional<String> optiona2 = Optional.of("Hello Java");
System.out.println(optiona2.isPresent());
Optional<String> op1 = Optional.of("Tom");
Optional<String> op2 = Optional.empty();
op1.ifPresent(s -> System.out.println("有值：" + s));
op1.ifPresent(System.out::println); 複制
orElse
描述：如果有值就傳回，否則傳回一個給定的值作為預設值
Optional<String> op1 = Optional.of("Tom");
Optional<String> op2 = Optional.empty();
String s3 = op1.orElse("Bob");
String s4 = op2.orElse("Bob");
System.out.println(s3);
System.out.println(s4);
Tom
Bob 複制
orElseGet
描述：orElseGet()方法與orElse()方法作用類似，但生成預設值的方式不同。該方法接受一個Supplier函數式接口參數，用于生成預設值。
Optional<String> op2 = Optional.empty();
String s5 = op2.orElseGet(() -> {
return "Hello";
});
System.out.println(s5); 複制
orElseThrow
描述：orElseThrow()方法與get()方法類似，當值為null時調用會抛出NullPointerException異常，但該方法可以指定抛出的異常類型。
Optional.empty().orElseThrow(() -> new RuntimeException("抛出異常了"));
Exception in thread "main" java.lang.RuntimeException: 抛出異常了 複制
map
描述：将對應Funcation函數式接口中的對象，進行二次運算，封裝成新的對象然後傳回在Optional中。
Person person = new Person();
Optional optional = Optional.ofNullable(person);
System.out.println(optional.map(Person::getName));
Optional<String> optional = Optional.of("xiaoming");
String s = optional.map(e -> e.toUpperCase()).orElse("shiyilingfeng");
System.out.println(s); //輸出: XIAOMING 複制
flatMap
描述：如果有值，為其執行mapping函數傳回Optional類型傳回值，否則傳回空Optional。與map不同的是，flatMap 的傳回值必須是Optional，而map的傳回值可以是任意的類型T。
Optional<String> optional = Optional.of("xiaoming");
Optional<String> s = optional.flatMap(e -> Optional.of(e.toUpperCase()));
System.out.println(s.get()); //輸出：XIAOMING 複制
filter
描述：用于判斷Optional對象是否滿足給定條件，一般用于條件過濾。
List<String> strings = Arrays.asList("rmb", "doller", "ou");
for (String s : strings) {
Optional<String> o = Optional.of(s).filter(s1 -> !s1.contains("o"));
System.out.println(o.orElse("沒有不包含o的"));
}
//輸出：
rmb
沒有不包含o的
沒有不包含o的 複制