java8中map新增方法详解

作者: 兴国First | 来源:发表于2019-10-21 09:33 被阅读0次

map新增的方法：

getOrDefault
forEach
putIfAbsent
compute
computeIfAbsent
computeIfPresent
merge
remove(key,value)
replace
replaceAll

getOrDefault

default V getOrDefault(Object key, V defaultValue) {
    V v;
    return (((v = get(key)) != null) || containsKey(key))
        ? v
        : defaultValue;
}

如果Map中不存在该key，可以提供一个默认值，方法会返回改默认值。如果存在该key，返回键对应的值。

java8之前的写法：

Map<String, String> map = new HashMap<>();
String value = "D";
if(map.containsKey("d")) {
    value = map.get("d");
}

java8：

String value = map.getOrDefault("d", "D");

forEach

default void forEach(BiConsumer<? super K, ? super V> action) {
    Objects.requireNonNull(action);
    for (Map.Entry<K, V> entry : entrySet()) {
        K k;
        V v;
        try {
            k = entry.getKey();
            v = entry.getValue();
        } catch(IllegalStateException ise) {
            // this usually means the entry is no longer in the map.
            throw new ConcurrentModificationException(ise);
        }
        action.accept(k, v);
    }
}

forEach遍历map，对map中的每个映射执行action指定的操作。
比如：

Map<String, String> map = new HashMap<>();
map.put("a", "A");
map.put("b", "B");

// 遍历
map.forEach((k, v)-> {
    System.out.println(k + "=" + v);
    map.put(k, k + v);
});
// 输出
// a=A
// b=B

map.forEach((k, v)-> {
    System.out.println(k + "=" + v);
});
// 输出
// a=aA
// b=bB

putIfAbsent

default V putIfAbsent(K key, V value) {
    V v = get(key);
    if (v == null) {
        v = put(key, value);
    }

    return v;
}

V putIfAbsent(K key, V value)只有在不存在key值的映射或者映射值为null，才将value值赋值给key。否则不做修改。该方法将条件判断和赋值合二为一。

比如：

Map<String, String> map = new HashMap<>();
map.put("a", "A");
map.put("b", "B");
        
String e = map.putIfAbsent("e", "E");
String b = map.putIfAbsent("b", "E");
System.out.println("返回e="+e); //返回e=null
System.out.println("键e="+map.get("e"));//键e=E
System.out.println("返回b="+b);//返回b=B
System.out.println("键b="+map.get("b"));//键b=B

remove(key,value)

default boolean remove(Object key, Object value) {
    Object curValue = get(key);
    if (!Objects.equals(curValue, value) ||
        (curValue == null && !containsKey(key))) {
        return false;
    }
    remove(key);
    return true;
}

只有在当前Map中key映射的值等于value时才删除该映射，否则什么也不做。

Map<String, String> map = new HashMap<>();
map.put("a", "A");
map.put("b", "B");

map.remove("a", "B");
map.remove("b", "B");
System.out.println(map.get("a")); // A
System.out.println(map.get("b")); // null

replace(K key, V value)

default V replace(K key, V value) {
    V curValue;
    if (((curValue = get(key)) != null) || containsKey(key)) {
        curValue = put(key, value);
    }
    return curValue;
}

只有在当前Map中包含key,才用value去替换原来的值，否则什么也不做。

Map<String, String> map = new HashMap<>();
map.put("a", "A");
map.put("b", "B");

map.replace("c", "1");
map.replace("b", "1");
System.out.println(map.get("c")); // null
System.out.println(map.get("b")); // 1

replace(K key, V oldValue, V newValue)

default boolean replace(K key, V oldValue, V newValue) {
    Object curValue = get(key);
    if (!Objects.equals(curValue, oldValue) ||
        (curValue == null && !containsKey(key))) {
        return false;
    }
    put(key, newValue);
    return true;
}

只有在当前Map中key的映射存在且映射的值等于oldValue时才用newValue去替换原来的值，否则什么也不做。

Map<String, String> map = new HashMap<>();
map.put("a", "A");
map.put("b", "B");

map.replace("a", "1", "2");
map.replace("b", "B", "2");
System.out.println(map.get("a")); // A
System.out.println(map.get("b")); // 2

replaceAll

default void replaceAll(BiFunction<? super K, ? super V, ? extends V> function) {
    Objects.requireNonNull(function);
    for (Map.Entry<K, V> entry : entrySet()) {
        K k;
        V v;
        try {
            k = entry.getKey();
            v = entry.getValue();
        } catch(IllegalStateException ise) {
            // this usually means the entry is no longer in the map.
            throw new ConcurrentModificationException(ise);
        }

        // ise thrown from function is not a cme.
        v = function.apply(k, v);

        try {
            entry.setValue(v);
        } catch(IllegalStateException ise) {
            // this usually means the entry is no longer in the map.
            throw new ConcurrentModificationException(ise);
        }
    }
}

该方法签名为replaceAll(BiFunction<? super K,? super V,? extends V> function)，作用是对Map中的每个映射执行function指定的操作，并用function的执行结果替换原来的value

比如：

Map<String, String> map = new HashMap<>();
map.put("a", "A");
map.put("b", "B");

map.replaceAll((k, v) -> v.toLowerCase());
map.forEach((k, v)-> {
    System.out.println(k + "=" + v);
});

// a=a
// b=b

merge

default V merge(K key, V value,
        BiFunction<? super V, ? super V, ? extends V> remappingFunction) {
    Objects.requireNonNull(remappingFunction);
    Objects.requireNonNull(value);
    V oldValue = get(key);
    V newValue = (oldValue == null) ? value :
               remappingFunction.apply(oldValue, value);
    if(newValue == null) {
        remove(key);
    } else {
        put(key, newValue);
    }
    return newValue;
}

value和remappingFunction不能为null
如果Map中key对应的映射不存在或者为null，则将value关联到key上；否则执行remappingFunction，如果执行结果为null则删除key的映射，否则用该结果跟key关联。

比如：

Map<String, String> map = new HashMap<>();
map.put("e", "E");
map.merge("f", "F", String::concat);
map.merge("e", "F", String::concat);
System.out.println("map.get(\"f\")="+map.get("f")); // map.get("f")=F
System.out.println("map.get(\"e\")="+map.get("e")); // map.get("e")=EF

compute

default V compute(K key,
        BiFunction<? super K, ? super V, ? extends V> remappingFunction) {
    Objects.requireNonNull(remappingFunction);
    // 获取key对应的值
    V oldValue = get(key);

    //获取新值
    V newValue = remappingFunction.apply(key, oldValue);
    // 如果新值为null，并且key存在，则删除key；否则把新值赋值给key
    if (newValue == null) {
        // delete mapping
        if (oldValue != null || containsKey(key)) {
            // something to remove
            remove(key);
            return null;
        } else {
            // nothing to do. Leave things as they were.
            return null;
        }
    } else {
        // add or replace old mapping
        put(key, newValue);
        return newValue;
    }
}

比如：

Map<String, String> map = new HashMap<>();
map.put("b", "B");
String val = map.compute("b", (k, v) -> null); 
String val2 = map.compute("c", (k, v) -> v + "+v");
System.out.println(val); // null
System.out.println(val2); // null+v

computeIfAbsent

 default V computeIfAbsent(K key,
            Function<? super K, ? extends V> mappingFunction) {
    Objects.requireNonNull(mappingFunction);
    V v;
    if ((v = get(key)) == null) {
        V newValue;
        if ((newValue = mappingFunction.apply(key)) != null) {
            put(key, newValue);
            return newValue;
        }
    }

    return v;
}

当Map中不存在key值的映射或映射值为null时，调用mappingFunction，并在mappingFunction执行结果非null时，将结果赋值给key。

比如(输入每个字母的位置)：

List<String> list = Lists.newArrayList("a", "b", "b", "c", "c", "c", "d", "d", "d", "f", "f", "g");
Map<String, List<Integer>> positionsMap = new HashMap<>();
for (int i = 0; i < list.size(); i++) {
    positionsMap.computeIfAbsent(list.get(i), k -> Lists.newArrayListWithCapacity(1)).add(i);
}

System.out.println(positionsMap);
// {a=[0], b=[1, 2], c=[3, 4, 5], d=[6, 7, 8], f=[9, 10], g=[11]}

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