Map
    
      goodsStandMap = new HashMap<>(); goodsStandMap.put("key1", "value1"); goodsStandMap.put("key2", "value2"); Set
     
       unitKeySet = goodsStandMap.keySet();
     
    

报错:"exceptionTypeName":"java.lang.UnsupportedOperationException","message":null}}} 原因: map.keySet方法返回的是 keySet不是HashSet   KeySet extends AbstractSet
    
      implements Set
     
    

keySet中没有add方法,也好理解,如果map的keyset中添加新的元素,那map中也添加key,那么value呢?

public Set
    
      keySet() {
      Set
     
       ks = keySet;     if (ks == null) {
          ks = new KeySet();         keySet = ks;     }     return ks; } final class KeySet extends AbstractSet
      
        {}
      
     
    

public abstract class AbstractSet
    
      extends AbstractCollection
     
       implements Set
      
        {}