Map的isEmpty()与==null的区别

1 Map map = new HashMap<String ,String>();
2 System.out.println("判断map是否有内容:"+map.isEmpty());//返回true
3 System.out.println("判断map是否为null:"+map==null);//返回false

1  Map map = new HashMap<String ,String>();
2  map=null;
3  System.out.println("判断map是否为null:"+(map==null));//结果为true
4  System.out.println("判断map是否有内容:"+map.isEmpty());//NullPointerException

1 Map map = new HashMap<String ,String>();
2 map.put(null,null);
3 System.out.println("判断map是否为null:"+(map==null));//false
4 System.out.println("判断map是否有内容:"+map.isEmpty());//false