字典 – Groovy Map.get(key,default)会改变地图

mymap = ['key': 'value']
println mymap

v = mymap.getOrDefault('notexistingkey','default')

println v
println mymap

输出：

[key:value]
default
[key:value]

Groovy SDK通过DefaultGroovyMethods.get(map,key,default)添加了Map.get(key,default),如果你看看Javadoc说你会理解这种行为是预期的：

Looks up an item in a Map for the given key and returns the value – unless there is no entry for the given key in which case add the default value to the map and return that.

这就是这个方法的实现：

/**
 * Looks up an item in a Map for the given key and returns the value - unless
 * there is no entry for the given key in which case add the default value
 * to the map and return that.
 * <pre class="groovyTestCase">def map=[:]
 * map.get("a",[]) &lt;&lt; 5
 * assert map == [a:[5]]</pre>
 *
 * @param map          a Map
 * @param key          the key to lookup the value of
 * @param defaultValue the value to return and add to the map for this key if
 *                     there is no entry for the given key
 * @return the value of the given key or the default value,added to the map if the
 *         key did not exist
 * @since 1.0
 */
public static <K,V> V get(Map<K,V> map,K key,V defaultValue) {
    if (!map.containsKey(key)) {
        map.put(key,defaultValue);
    }
    return map.get(key);
}

这是一个非常古老的概念(自Groovy 1.0以来).但是我建议不要使用它 – 这个.get(key,default)操作既不是原子操作也不是同步操作.当你在ConcurrentMap上使用它时会出现问题,这是为并发访问而设计的 – 这个方法打破了它的契约,因为containsKey,put和final get调用之间没有同步.