/**
* forEach(BiConsumer<? super K, ? super V> action)
* 循環key,value
*/
@Test
public void testHashMapForEach(){
Map<String,String> map = new HashMap<String, String>();
map.put("a","aaa");
map.put("b","bbb");
map.put("c","ccc");
map.put("d","ddd");
map.forEach((k,v)->{
System.out.println(k+" "+v);
});
/**
* a aaa
* b bbb
* c ccc
* d ddd
*/
}
/**
* V getOrDefault(Object key, V defaultValue)
* 如果根據key得到value為null并且不包含這個key，則傳回預設設定的值
*/
@Test
public void testHashMapGetOrDefault(){
Map<String,String> map = new HashMap<String, String>();
map.put("a","aaa");
map.put("b",null);
map.put("c","ccc");
map.put("d","ddd");
String orDefault1 = map.getOrDefault("a", "eeee");
String orDefault2 = map.getOrDefault("b", "eeee");
String orDefault3 = map.getOrDefault("m", "eeee");
System.out.println(orDefault1+" "+orDefault2+" "+orDefault3);
/**
* aaa null eeee
*/
}
/**
* compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
* Objects.requireNonNull(remappingFunction);
* V oldValue = get(key);
* V newValue = remappingFunction.apply(key, oldValue);
*
* 隻要新value不為null就會插入新key或者覆寫原來value,如果新value為null,就會把這個鍵值對直接删除
*/
@Test
public void testHashMapCompute(){
List<Student> studentList = Arrays.asList(
new Student("apple",null,10),
//new Student("orange","女",30),
// new Student("pink",null,60),
new Student("pink","男",60),
new Student("black","男",60),
new Student("orange",null,150)
);
Map<String,String> map = new HashMap<String, String>();
map.put("pink",null);//原來為空，測試現在不為空
map.put("orange","女");//原來不為空，測試現在為空
map.put("apple",null);//原來為空，測試現在為空
studentList.stream().forEachOrdered(student->map.compute(student.getName(),(k,v)->{
return student.getSex();}));
System.out.println(map);
// {pink=男, black=男}
}
/**
* computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction) {
* Objects.requireNonNull(mappingFunction);
* 傳回值就是Function裡面的V，是以computeIfAbsent方法執行後還可以繼續做一系列的操作,
* 但是value已經放到集合中了，是以隻有引用類型才可以繼續操作，String不行
*
* 隻要是原來的value為null,就會插入或者更新，如果不為null，不變
* 如果新的value為null,則不做操作,原來的資料不會删除
*
*/
@Test
public void testHashMapComputeIfAbsent(){
List<Student> studentList = Arrays.asList(
new Student("apple",null,10),
//new Student("orange","女",30),
// new Student("pink",null,60),
new Student("pink","男",60),
new Student("black","男",60),
new Student("orange",null,150)
);
Map<String,String> map = new HashMap<String, String>();
map.put("pink",null);//原來為空，測試現在不為空
map.put("orange","女");//原來不為空，測試現在不為空
studentList.stream().forEachOrdered(student->map.computeIfAbsent(student.getName(),k->k).toUpperCase());
// studentList.stream().forEachOrdered(student->map.computeIfAbsent(student.getName(),k->null));
System.out.println(map);
// {{orange=女, apple=apple, pink=pink, black=black} toUpperCase沒有達到效果
}
@Test
public void testHashMapComputeIfAbsent2(){
List<Student> studentList = Arrays.asList(
new Student("apple",null,10),
new Student("pink","男",60),
new Student("black","男",60),
new Student("black","女",80)
);
Map<String,List<Student>> map = new HashMap<>();
studentList.stream().forEachOrdered(
student->map.computeIfAbsent(student.getName(),k->new ArrayList<Student>()).add(student)
);
System.out.println(map);
// {
// apple=[Student{name='apple', sex='null', money=10.0}],
// pink=[Student{name='pink', sex='男', money=60.0}],
// black=[Student{name='black', sex='男', money=60.0}, Student{name='black', sex='女', money=80.0}]
// } add達到效果了
}
/**
* computeIfPresent(K key,BiFunction<? super K, ? super V, ? extends V> remappingFunction)
*
* 不會插入新key-value，
* 根據key擷取value,
* 如果oldValue不為null, newValue為null則删除，newValue不為null則更新，
* 如果oldValue為null，當不存在這個鍵值對處理，不做任何操作
*/
@Test
public void testHashMapComputeIfPresent(){
List<Student> studentList = Arrays.asList(
new Student("apple","女",10),
new Student("pink","男",60),
new Student("black","男",60),
new Student("red","女",60),
new Student("orange",null,150)
);
Map<String,String> map = new HashMap<String, String>();
map.put("pink",null);//原來為空，測試現在不為空
map.put("orange","女");//原來不為空，測試現在為空
map.put("apple","男");//原來不為空，測試現在不為空
studentList.stream().forEachOrdered(student->map.computeIfPresent(student.getName(),(k,v)->{
return student.getSex();}));
System.out.println(map);
// {apple=女, pink=null}
}
/**
* V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)
* 根據key擷取value為null,則用新value，
* 若不為null, oldvalue與newValue會執行這個函數remappingFunction，指派給目前key
*/
@Test
public void testHashMapMerge(){
Map<String,String> map = new HashMap<String, String>();
map.put("a","aaa");
map.put("b",null);
map.merge("a","hahaha",String::concat);
map.merge("b","hahaha",String::concat);
System.out.println(map);
//{a=aaahahaha, b=hahaha}
}
/**
* default V putIfAbsent(K key, V value)
*
* 如果擷取的value為null,則用賦予新的value（插入或者更新）
*/
@Test
public void testHashMapPutIfAbsent(){
Map<String,String> map = new HashMap<String, String>();
map.put("a","aaa");
map.put("b",null);
map.putIfAbsent("a","ddd");
map.putIfAbsent("b","ddd");
map.putIfAbsent("c","ddd");
System.out.println(map);
//{a=aaa, b=ddd, c=ddd}
}
![layout 中include和merge标簽的使用[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)