java.util.HashMap map=new java.util.HashMap();
map.put("name",null);      
map.put("name","Jack");
System.out.println(map.size());

A.0
B.null
C.1
D.2

• 0
• null
• 1
• 2
  

java.util.HashMap map=new java.util.HashMap();
map.put("name",null);      
map.put("name","Jack");
System.out.println(map.size());

A.0
B.null
C.1
D.2

• 0
• null
• 1
• 2