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?
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
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*/}
if statement will only compile if the OP has Map<String, Boolean> map.if(map.get("A")) will cause a compiler error. This isn't C++ or Python.
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.
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
}
}
