掌握JDK1.8版本中为HashMap添加的骚操作

getOrDefault

方法

default V getOrDefault(Object key, V defaultValue)

含义

根据Key获取Value，如果获取不到，则返回指定的defaultValue

示例

正常我们要先使用containsKey方法判断Key是否存在，如果存在才可根据Key获取Value，再返回。

java 复制代码

public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();
    map.put("name", "lisi");

    String value = "zhangsan";
    if (map.containsKey("name")) {
        value = map.get("name");
    }

    System.out.println(value);
}

使用getOrDefault方法，一步到位。

java 复制代码

public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();
    String value = map.getOrDefault("name", "zhangsan");
    System.out.println(value);
}

统计每个字符出现的次数，使用getOrDefault非常方便。

java 复制代码

public static void main(String[] args) {
    List<String> chars = Arrays.asList("a", "b", "a", "c", "a", "d", "a", "e");
    Map<String, Integer> map = new HashMap<>();
    for (String str : chars) {
        map.put(str, map.getOrDefault(str, 0) + 1);
    }
    System.out.println(map);
}

computeIfAbsent

方法

default V computeIfAbsent(K key, Function<? super K, ? extends V> mappingFunction)

含义

根据Key查找，如果Key存在，则什么也不做，如果Key不存在，则将Value添加进去。

示例

以下返回的是{hobby=[dance, sing, draw]}，也就是说在使用computeIfAbsent时，如果Key已经存在则不会操作对应的Value。

java 复制代码

public static void main(String[] args) {
    Map<String, List<String>> map = new HashMap<>();
    List<String> hobbyValues = map.computeIfAbsent("hobby", v -> Lists.newArrayList("dance", "sing", "draw"));
    System.out.println(hobbyValues);
    map.computeIfAbsent("hobby", v -> Lists.newArrayList("basketball"));
    System.out.println(map);
}

输出

java 复制代码

[dance, sing, draw]
{hobby=[dance, sing, draw]}

computeIfPresent

方法

default V computeIfPresent(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)

含义

computeIfPresent与computeIfAbsent刚好相反，computeIfPresent是如果Key存在就用新Value替换老Value，如果不存在则什么也不做，而computeIfAbsent是如果Key存在就什么也不做，如果Key不存在，则使用新Value。

示例

java 复制代码

public static void main(String[] args) {
    Map<String, List<String>> map = new HashMap<>();
    // 因为key不存在，所以什么也不做
    List<String> hobbyValue = map.computeIfPresent("hobby", (k, v) -> Lists.newArrayList("basketball"));
    System.out.println(hobbyValue);
    System.out.println(map);
    System.out.println();
    
    // 因为key不存在，所以将value添加到map中
    List<String> hobbyValues = map.computeIfAbsent("hobby", v -> Lists.newArrayList("dance", "sing", "draw"));
    System.out.println(hobbyValues);
    System.out.println(map);
    System.out.println();
    
    // 因为key存在，所以用新value替换老value
    hobbyValue = map.computeIfPresent("hobby", (k, v) -> Lists.newArrayList("basketball"));
    System.out.println(hobbyValue);
    System.out.println(map);
}

输出

java 复制代码

null
{}

[dance, sing, draw]
{hobby=[dance, sing, draw]}

[basketball]
{hobby=[basketball]}

compute

方法

default V compute(K key, BiFunction<? super K, ? super V, ? extends V> remappingFunction)

含义

不考虑Key存不存在，直接替换为新Value

示例

java 复制代码

public static void main(String[] args) {
    Map<String, List<String>> map = new HashMap<>();

    List<String> hobbyValue = map.compute("hobby", (k, v) -> Lists.newArrayList("basketball"));
    System.out.println(hobbyValue);
    System.out.println(map);
    System.out.println();

}

输出

java 复制代码

[basketball]
{hobby=[basketball]}

replace

方法

default V replace(K key, V value)

boolean replace(K key, V oldValue, V newValue)

含义

replace方法，实现的功能和computeIfPresent类似，只不过computeIfPresent入参是BiFunction函数接口，更灵活一些，而replace入参则是具体的Value类型。

示例

存在hobby就替换，否则什么也不做。

java 复制代码

map.replace("hobby", new ArrayList<>());

java 复制代码

public static void main(String[] args) {
    Map<String, String> map = new HashMap<>();
    map.put("name", "zhangsan");
    map.replace("name", "zhangsan", "lisi");
    System.out.println(map);
}

replaceAll

方法

default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function)

含义

该方法可以一次性替换所有Value，这通常用于需要对Value进行统一处理时。

示例

java 复制代码

public static void main(String[] args) {
    Map<String, List<String>> map = new HashMap<>();
    map.put("hobby_1", Lists.newArrayList("dance", "sing", "draw"));
    map.put("hobby_2", Lists.newArrayList("swim", "football", "basketball"));
    System.out.println(map);
    map.replaceAll((k, v) -> v.stream().sorted().collect(Collectors.toList()));
    System.out.println(map);
}

输出

java 复制代码

排序前：{hobby_2=[swim, football, basketball], hobby_1=[dance, sing, draw]}
排序后：{hobby_2=[basketball, football, swim], hobby_1=[dance, draw, sing]}

putIfAbsent

方法

default V putIfAbsent(K key, V value)

含义

putIfAbsent方法，实现功能又和computeIfAbsent类似，区别同样是computeIfAbsent入参是函数接口。如果Key存在，则什么也不做，返回Key对应的Value，如果Key不存在，则将Value添加进去，并返回null。

示例

java 复制代码

public static void main(String[] args) {
    Map<String, List<String>> map = new HashMap<>();
    map.put("hobby_1", Lists.newArrayList("dance", "sing", "draw"));
    List<String> hobby_1 = map.putIfAbsent("hobby_1", Lists.newArrayList("swim", "football", "basketball"));
    System.out.println(hobby_1);
    List<String> hobby_2 = map.putIfAbsent("hobby_2", Lists.newArrayList("swim", "football", "basketball"));
    System.out.println(hobby_2);
    System.out.println(map);
}

输出

java 复制代码

[dance, sing, draw]
null
{hobby_2=[swim, football, basketball], hobby_1=[dance, sing, draw]}

merge

方法

default V merge(K key, V value, BiFunction<? super V, ? super V, ? extends V> remappingFunction)

含义

merge方法可以在原Value的基础上继续操作。

示例

java 复制代码

public static void main(String[] args) {
    Map<String, List<String>> map = new HashMap<>();
    map.put("hobby_1", Lists.newArrayList("dance", "sing", "draw"));
    map.merge("hobby_1", Lists.newArrayList("swim", "football", "basketball", "dance"), MapUtils::apply);
    System.out.println(map);
}
private static List<String> apply(List<String> oldValue, List<String> newValue) {
    oldValue.addAll(newValue);
    return oldValue.stream().distinct().collect(Collectors.toList());
}

输出

java 复制代码

{hobby_1=[dance, sing, draw, swim, football, basketball]}

forEach

方法

default void forEach(BiConsumer<? super K, ? super V> action)

含义

forEach方法提供了便捷的遍历map中Key和Value的方式。

示例

java 复制代码

public static void main(String[] args) {
    Map<String, List<String>> map = new HashMap<>();
    map.put("hobby_1", Lists.newArrayList("dance", "sing", "draw"));
    map.put("hobby_2", Lists.newArrayList("swim", "football", "basketball"));
    map.put("hobby_3", Lists.newArrayList("run"));
    map.forEach((k, v) -> System.out.println(k + ": " + v.size()));
}

输出

java 复制代码

hobby_2: 3
hobby_1: 3
hobby_3: 1