/**
* forEach(BiConsumer<? super K, ? super V> action)
* 循环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)
* 如果根据key得到value为null并且不包含这个key，则返回默认设置的值
*/
@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);
*
* 只要新value不为null就会插入新key或者覆盖原来value,如果新value为null,就会把这个键值对直接删除
*/
@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);//原来为空，测试现在不为空
map.put("orange","女");//原来不为空，测试现在为空
map.put("apple",null);//原来为空，测试现在为空
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);
* 返回值就是Function里面的V，所以computeIfAbsent方法执行后还可以继续做一系列的操作,
* 但是value已经放到集合中了，所以只有引用类型才可以继续操作，String不行
*
* 只要是原来的value为null,就会插入或者更新，如果不为null，不变
* 如果新的value为null,则不做操作,原来的数据不会删除
*
*/
@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);//原来为空，测试现在不为空
map.put("orange","女");//原来不为空，测试现在不为空
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没有达到效果
}
@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达到效果了
}
/**
* computeIfPresent(K key,BiFunction<? super K, ? super V, ? extends V> remappingFunction)
*
* 不会插入新key-value，
* 根据key获取value,
* 如果oldValue不为null, newValue为null则删除，newValue不为null则更新，
* 如果oldValue为null，当不存在这个键值对处理，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);//原来为空，测试现在不为空
map.put("orange","女");//原来不为空，测试现在为空
map.put("apple","男");//原来不为空，测试现在不为空
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)
* 根据key获取value为null,则用新value，
* 若不为null, oldvalue与newValue会执行这个函数remappingFunction，赋值给当前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)
*
* 如果获取的value为null,则用赋予新的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}
}
map方法compute/computeIfAbsent/computeIfPresent/merge/putIfAbsent/forEach/getOrDefault
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转载自blog.csdn.net/u012326462/article/details/81155342
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