关于Map集合中方法说明

map集合中提供的方法较多,整理一下常见且使用的方法

1 getOrDefault

1 Map中源码:

    /**
     * Returns the value to which the specified key is mapped, or
     * {@code defaultValue} if this map contains no mapping for the key.
     *
     * @implSpec
     * The default implementation makes no guarantees about synchronization
     * or atomicity properties of this method. Any implementation providing
     * atomicity guarantees must override this method and document its
     * concurrency properties.
     *
     * @param key the key whose associated value is to be returned
     * @param defaultValue the default mapping of the key
     * @return the value to which the specified key is mapped, or
     * {@code defaultValue} if this map contains no mapping for the key
     * @throws ClassCastException if the key is of an inappropriate type for
     * this map
     * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @throws NullPointerException if the specified key is null and this map
     * does not permit null keys
     * (<a href="{@docRoot}/java/util/Collection.html#optional-restrictions">optional</a>)
     * @since 1.8
     */
    default V getOrDefault(Object key, V defaultValue) {
    
    
        V v;
        return (((v = get(key)) != null) || containsKey(key))
            ? v
            : defaultValue;
    }

Map中getOrDefault方法表示当map集合中有这个这个key时,就返回key对应的值,否则就使用默认值defaultValue.

2 案列

public class TestDemo {
    
    

    public static void main(String[] args) {
    
    
        Map<String, String> map = new HashMap<>();
        map.put("name", "李白");
        map.put("age", "48");
    
        String name = map.getOrDefault("name", "杜甫");
        // map中存在,则直接返回对应的value值
        System.out.println(name);
        String score = map.getOrDefault("score", "80");
        // map中不存在,则直接返回默认值
        System.out.println(score);
    }
}    

// 输出结果
// 李白
// 80