Java8新特性之-Map

Map一些新方法的具体使用案例

1、getOrDefault:default V getOrDefault(Object key, V defaultValue)

package com.qbb.threadpool;

import java.util.HashMap;
import java.util.Map;

/**
 * @author QiuQiu&LL (个人博客:https://www.cnblogs.com/qbbit)
 * @version 1.0
 * @date 2022-07-23  11:49
 * @Description:
 */
public class Java8NewFeature {

    public static void main(String[] args) {
        Map<String,String> map = new HashMap<>();
        map.put("name", "qiuqiu");
        map.put("age", "22");
        map.put("hobby", "haha");
        map.put("interest", "no");

        /**
         * getOrDefault
         */
        String orDefaultValue = getOrDefaultTest(map);
        System.out.println("orDefaultValue = " + orDefaultValue);


    }

    /**
     * getOrDefault
     * 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
     */
    public static String getOrDefaultTest(Map<String,String> map){
        String orDefault = map.getOrDefault("qiu", "baoabo");
        return orDefault;
    }
}

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

/**
     * forEach
     * 遍历map
     */
    public static String forEachTest(Map<String, String> map) {
        /**
         * 以前的写法
         */
        for (Map.Entry<String, String> entry : map.entrySet()) {
            String key = entry.getKey();
            String value = entry.getValue();
            System.out.println(key + "===" + value);
        }

        /**
         * 现在的写法
         */
        map.forEach((k, v) -> System.out.println(k + "===" + v));

        /**
         * 还可以做一些中间操作
         */
        map.forEach((k, v) -> {
            System.out.println(k + "===" + v);
            v = v + "you";
            map.put(k, v);
        });
        map.forEach((k, v) -> System.out.println(k + "===" + v));


        return null;
    }

3、putIfAbsent:default V putIfAbsent(K key, V value)

/**
     * putIfAbsent
     * 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。
     */
    public static String putIfAbsentTest(Map<String, String> map) {
        String result1 = map.put("name", "ll");
        System.out.println("result1 = " + result1);
        String result2 = map.putIfAbsent("name", "qiuqiu");
        System.out.println("result2 = " + result2);

        String name = map.get("name");
        System.out.println("name = " + name);


        String absent = map.putIfAbsent("hello", "qiuqiu");

        System.out.println("absent = " + absent);

        String hello = map.get("hello");
        System.out.println("hello = " + hello);

        return null;
    }

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

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;
    }
}

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

public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("name", "qiuqiu");
        map.put("age", "22");
        map.put("hobby", "haha");
        map.put("interest", "no");

       /* 
        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。
        */
        String ifAbsent = map.computeIfAbsent("qiuqiu", k -> {
            return "hello:" + k;
        });
        System.out.println("ifAbsent = " + ifAbsent);
    }

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

public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("name", "qiuqiu");
        map.put("age", "22");
        map.put("hobby", "haha");
        map.put("interest", "no");

        /**
         * 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。
         */

        // 给存在的key进行映射处理
        String merge = map.merge("name", "ll", (k, v) -> {
            System.out.println(k + "===" + v);
            return k + v;
        });
        System.out.println("merge = " + merge);

        // 给不存在的key,不会执行
        String result = map.merge("haha", "ll", (k, v) -> {
            System.out.println(k + "===" + v);
            return k + v;
        });
        System.out.println("map.get(haha) = " + map.get("haha"));
        System.out.println("result = " + result);
    }

7、remove(key,value):default boolean remove(Object key, Object value)

/**
     * remove
     * 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。
     */
    public static String removeTest(Map<String, String> map) {
        boolean flag = map.remove("name", "qiuqiu");
        System.out.println("flag = " + flag);

        boolean flag2 = map.remove("hobby", "ll");
        System.out.println("flag2 = " + flag2);

        return null;
    }

8、replace:default boolean replace(K key, V oldValue, V newValue)

 /**
     * replace
     */
    public static String replaceTest(Map<String, String> map){
        String one = map.replace("name", "haha");
        System.out.println("one = " + one);

        String two = map.replace("haha", "hehe");
        System.out.println("two = " + two);

        boolean flag = map.replace("name", "haha", "hehe");
        System.out.println("flag = " + flag);
        return null;
    }

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

public static void main(String[] args) {
        Map<String, String> map = new HashMap<>();
        map.put("name", "qiuqiu");
        map.put("age", "22");
        map.put("hobby", "haha");
        map.put("interest", "no");

        map.replaceAll((k, v) -> {
            // 对 k v 进行处理
            if ("hobby".equals(k)) {
                return v.toUpperCase();
            }
            return v;
        });
        // 打印
        map.forEach((k, v) -> System.out.println(k + "=" + v));
    }

当然还有其他的方法就不一一列举了，大家可以去试一下