java8 中 map新增的 default 方法 示例

1，getOrDefault

只要 map.keySet() 中含有该 key, 就会返回对应的 value, 即使 value == null

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

使用示例：

public class Java8MapTest {
    public static void main(String[] args){
        Map<String, String> map = new HashMap<>();
        map.put("AAA", "111");
        map.put("BBB", "222");
        map.put("NNN", null);
        System.out.println(map);//{AAA=111, BBB=222, NNN=null}
        /**
         * public V getOrDefault(Object key, V defaultValue)
         * 使用getOrDefault的时候要注意，当map中包含对应的 key, 即使 value == null, 也会直接返回 null, 而不会使用默认值
         */
        System.out.println(map.getOrDefault("AAA", "default-value"));//111
        System.out.println(map.getOrDefault("CCC", "default-value"));//default-value
        System.out.println(map.getOrDefault("NNN", "default-value"));//null
    }
}

2 , putIfAbsent

当 map.keySet() 不含有该 key, 或者包含有该 key ,但对应的 value == null 时，才会进行添加操作

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

使用示例：

public class Java8MapTest {
    public static void main(String[] args){
        Map<String, String> map = new HashMap<>();
        map.put("AAA", "111");
        map.put("BBB", "222");
        map.put("NNN", null);
        System.out.println(map);//{AAA=111, BBB=222, NNN=null}
        /**
         * public V putIfAbsent(K key, V value)
         * 使用putIfAbsent的时候要注意，当map中包含对应的 key, 如果 value == null, 则也会进行 put() 操作
         */
        map.putIfAbsent("AAA", "333");
        System.out.println(map); //{AAA=111, BBB=222, NNN=null}
        map.putIfAbsent("DDD", "444");
        System.out.println(map); //{AAA=111, BBB=222, DDD=444, NNN=null}
        map.putIfAbsent("NNN", "putIfAbsent");
        System.out.println(map); //{AAA=111, BBB=222, DDD=444, NNN=putIfAbsent}

    }
}

3 ， remove

使用removw(k,v)的时候要注意，只有 key:value 与 map 中的 key:value 完全对应时，才会被移除

    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;
    }

使用示例：

public class Java8MapTest {
    public static void main(String[] args){
        Map<String, String> map = new HashMap<>();
        map.put("AAA", "111");
        map.put("BBB", "222");
        map.put("NNN", null);
        System.out.println(map);//{AAA=111, BBB=222, NNN=null}
        /**
         * public boolean remove(Object key, Object value)
         * 使用removw(k,v)的时候要注意，只有 key:value 与 map 中的 key:value 完全对应时，才会被移除
         */
        map.remove("AAA", "555");
        System.out.println(map);//{AAA=111, BBB=222, NNN=null}
        map.remove("AAA", "111");
        System.out.println(map);//{BBB=222, NNN=null}
        map.remove("NNN", "666");
        System.out.println(map);//{BBB=222, NNN=null}
        map.remove("NNN", null);
        System.out.println(map);//{BBB=222}
    }
}

4 ， replace

replace有两个方法，replace(k,v)直接替换，返回当前值；replace(k,v1,v2)只在当key,value都对应的时候，才会进行替换

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

    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;
    }

使用示例：

public class Java8MapTest {
    public static void main(String[] args){
        Map<String, String> map = new HashMap<>();
        map.put("AAA", "111");
        map.put("BBB", "222");
        map.put("NNN", null);
        System.out.println(map);//{AAA=111, BBB=222, NNN=null}
        /**
         * public V replace(K key, V value)
         * public V replace(K key, V oldValue, V newValue)
         * replace(k,v),当containsKey(k)时，进行替换，返回当前值
         * replace(k,v1,v2), 当containsKey(k)时, curValue.equals(v1) == true 时, 才进行替换
         */
        map.replace("NNN", "new null");
        System.out.println(map);//{AAA=111, BBB=222, NNN=new null}
        map.replace("AAA","333", "new 111");
        System.out.println(map);//{AAA=111, BBB=222, NNN=new null}
        map.replace("AAA","111", "new 111");
        System.out.println(map);//{AAA=new 111, BBB=222, NNN=new null}
    }
}

5 , compute

利用函数计算的结果，作为value替换或者新增 key:value

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

        V newValue = remappingFunction.apply(key, oldValue);
        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;
        }
    }

使用示例：

public class Java8MapTest {
    public static void main(String[] args){
        Map<String, String> map = new HashMap<>();
        map.put("AAA", "111");
        map.put("BBB", "222");
        map.put("NNN", null);
        System.out.println(map);//{AAA=111, BBB=222, NNN=null}

        //BiFunction
        BiFunction<String, String, String> BiF = new BiFunction<String, String, String>() {
            @Override
            public String apply(String s, String s2) {
                if(s2 != null){
                    return "new "+ s2;
                }else{
                    return "the value is null";
                }
            }
        };

        /**
         * map.compute(K key, BiFunction)
         * 利用Bi函数计算的结果，作为value替换或者新增 key:value 对
         */

        map.compute("AAA", BiF);
        System.out.println(map);//{AAA=new 111, BBB=222, NNN=null}
        map.compute("NNN", BiF);
        System.out.println(map);//{AAA=new 111, BBB=222, NNN=the value is null}
    }
}

6 , computeIfPresent 和 computeIfAbsent

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

    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;
    }

使用示例：

public class Java8MapTest {
    public static void main(String[] args){
        Map<String, String> map = new HashMap<>();
        map.put("AAA", "111");
        map.put("BBB", "222");
        map.put("NNN", null);
        System.out.println(map);//{AAA=111, BBB=222, NNN=null}

        //BiFunction
        BiFunction<String, String, String> BiF = new BiFunction<String, String, String>() {
            @Override
            public String apply(String s, String s2) {
                if(s2 != null){
                    return "new "+ s2;
                }else{
                    return "the value is null";
                }
            }
        };

        /**
         * map.computeIfPresent(K key, BiFunction)
         * 当get(key)不为null时，利用Bi函数计算的结果，作为value替换旧的value, 返回旧值; 反之，直接返回 null
         */

        map.computeIfPresent("AAA", BiF);
        System.out.println(map);//{AAA=new 111, BBB=222, NNN=null}
        map.computeIfPresent("NNN", BiF);
        System.out.println(map);//{AAA=new 111, BBB=222, NNN=null}

        /**
         * map.computeIfAbsent(K key, function(k))
         * 当get(key)为null时，利用函数计算的结果，作为value新增或者替换旧的map.entry(k,v)
         */
        map.computeIfAbsent("AAA",  k -> {return k+"value";});
        System.out.println(map);//{AAA=new 111, BBB=222, NNN=null}
        map.computeIfAbsent("CCC",  k -> {return k+"value";});
        System.out.println(map);//{AAA=new 111, CCC=CCCvalue, BBB=222, NNN=null}
        map.computeIfAbsent("NNN",  k -> {return k+"value";});
        System.out.println(map);//{AAA=new 111, CCC=CCCvalue, BBB=222, NNN=NNNvalue}
    }
}

7 , 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;
    }

使用示例：

public class Java8MapTest {
    public static void main(String[] args){
        Map<String, String> map = new HashMap<>();
        map.put("AAA", "111");
        map.put("BBB", "222");
        map.put("NNN", null);
        System.out.println(map);//{AAA=111, BBB=222, NNN=null}

        //BiFunction
        BiFunction<String, String, String> BiF = new BiFunction<String, String, String>() {
            @Override
            public String apply(String s, String s2) {
                if(s2 != null){
                    return s +"-"+ s2;
                }else{
                    return "the value is null";
                }
            }
        };

        /**
         * map.merge(K key, V V, BiFunction BiF)
         * 当get(key)不为null时，以get(key), v 作为参数计算返回结果，如果结果为null,则remove(k),反之则替换旧值
         * 当get(key)为 null 时，直接用 v 进行替换
         */

        map.merge("AAA", "999", BiF);
        System.out.println(map);//{AAA=111-999, BBB=222, NNN=null}
        map.merge("NNN", "nnn999", BiF);
        System.out.println(map);//{AAA=111-999, BBB=222, NNN=nnn999}
    }
}