java - 怎么判断map key对应的值是不是空的

Question

System.out.println("a**"+map.get(a).getClass());
System.out.println("a**"+map.get(a));

-------------分割线------------
a**class java.util.ArrayList
a**[]

以上是控制台打印出来的东西。

map.get(a).toString().equals("")；一直是false
map.get(a)！=null；一直是true

Answer 1

if(map.get(a)==null)Can’t you tell whether it is empty?

Answer 2

In Java, null and empty array are not the same concept. If you want to determine whether the array is empty, you can write it like this.

`
if (map.get(a) == null || map.get(a).size() == 0)
`

If it is a production environment, it is recommended to use tool classes, such as apache common或者spring都提供CollectionUtils.isEmpty(map.get(a))

Answer 3

If an ArrayList has been instantiated, it is definitely not equal to null.
Since the toString method can be executed without reporting an error, it must have been instantiated. Because toString is a method of the top-level parent class Object, how can an uninstantiated object have this method.

ArrayList list = new ArrayList();
System.out.println(list == null);  // false

ArrayList list1 = null;
System.out.println(list1.toString()); // 空指针

Answer 4

map.get(a).toString().equals(""); is always false

The result you print outa**[]可以看出来map.get(a).toString()不是空字符串，而是[]，因此"[]".equals("")is always false

map.get(a)! =null; always true

If map.get(a).toString()没抛空指针异常，那么map.get(a) must be non-empty

Judgment empty

If the value corresponding to the key is always ArrayList, then you can use the following method:

map.get(a) == null || map.get(a).isEmpty()