Null check in map gets null pointer exception

问题

I am trying to use a Java HashMap. Using map.get("A") to get a value for a key from the map resulted in a NullPointerException. Then I used if(map.get("A")) to avoid a NullPointerException being thrown, but it gets thrown anyway.

What am I doing wrong?

回答1:

I have answering my own question. I have used to check

         if(map.containsKey("A"))
                 String b =  map.get("A")

rather than

    if(map.get("A") != null)
            map.get("A")

It will help me to avoid null pointer exception

回答2:

Well, you probably didn't instantiate the map object itself.

Try this before accessing the map:

Map map = new HashMap();

And later:

map.put("A", "someValue");
if(map.get("A") != null){/*your code here*/}

回答3:

There are two possible problems:

map itself is null because you never initialized it.
map is declared as Map<String, Boolean> map. If this is the case, then your if statement doesn't do what you think it does. It actually gets the Boolean object from the map and tries to unbox it to a primitive bool. If there is no value with the key "A", then you will get a NullPointerException. The way to fix this is to change your if statement:
```
Boolean b = map.get("A");
if (b != null && b)
    ...
```
Note that you have to explicitly compare with null.

回答4:

First of all check whether the map is null or not by

map != null

Then check map contains that key by,

map.containsKey("A")

Totally you can check as follows,

if(map!=null && map.containsKey("A")){
   if(map.get("A") != null){
       System.out.println(map.get("A"));
       \\ value corresponding to key A
   }
}

来源：https://stackoverflow.com/questions/26559768/null-check-in-map-gets-null-pointer-exception

标签

java

nullpointerexception

hashmap