Java中Map的computeIfAbsent方法详解

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;
    }

/**
 * 如果key对应的value值为null,则在map中放入该key和设置相应的value
 */
public class Test {
    public static void main(String[] args) {
        Map<Integer,Integer> map = new HashMap<>();
        /**
         * 方法1:使用方法引用
         */
        map.computeIfAbsent(1,Test::compute);
        /**
         * 方法2:使用匿名内部类
         */
        map.computeIfAbsent(1, new Function<Integer, Integer>() {
            @Override
            public Integer apply(Integer integer) {
                return integer;
            }
        });
        /**
         * 方法3:使用lambda
         */
        map.computeIfAbsent(1,key -> key);
        
        System.out.println(map.get(1));
    }

    static public Integer compute(Integer integer){
        return integer;
    }
}

/**
 * 统计List出现相同字符串的个数
 */
public class Test {
    public static void main(String[] args) {
        Map<String,AtomicInteger> map = new HashMap<>();
        List<String> list = Arrays.asList(new String[]{"1","2","2","3","3","4","4","4"});
        list.forEach(str->map.computeIfAbsent(str,k->new AtomicInteger()).incrementAndGet());
        System.out.println(map);
    }
}
/**
 * {1=1, 2=2, 3=2, 4=3}
 */

/**
 * 统计List出现相同字符串的个数
 */
public class Test {
    public static void main(String[] args) {
        Map<String,AtomicInteger> map = new HashMap<>();
        List<String> list = Arrays.asList(new String[]{"1","2","2","3","3","4","4","4"});
        list.forEach(new Consumer<String>() {
            @Override
            public void accept(String s) {
                //寻找map中key对应的value,如果map不存在指定key的值则new 一个AtomicInteger对象并加入缓存map中，如果存在则取到对应的AtomicInteger对象并加一
                map.computeIfAbsent(s, new Function<String, AtomicInteger>() {
                    @Override
                    public AtomicInteger apply(String s) {
                        return new AtomicInteger();
                    }
                }).incrementAndGet();
            }
        });
        System.out.println(map);
    }
}
/**
 * {1=1, 2=2, 3=2, 4=3}
 */