set调用add报错:

Map<String, String> goodsStandMap = new HashMap<>();
goodsStandMap.put("key1", "value1");
goodsStandMap.put("key2", "value2");
Set<String> 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<K> implements Set<E>

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<K> keySet() {
    Set<K> ks = keySet;
    if (ks == null) {
        ks = new KeySet();
        keySet = ks;
    }
    return ks;
}

final class KeySet extends AbstractSet<K> {}

public abstract class AbstractSet<E> extends AbstractCollection<E> implements Set<E> {}