java.lang.NullPointerException with boolean

Question

I wrote a realy simple code based on another question and here it is:

It throws me an error

java.lang.NullPointerException line 5 and 17

I don't know what I'm doing wrong.

 public class Main {

    public static String bool(Boolean param){
        if(param == true){    (line 5)
            return "a";
        }else if(param == false){
            return "b";
        }
        return "c";

    }

    public static void main(String[] args){

        System.out.println(bool(true));
        System.out.println(bool(null)); (line 17)
        System.out.println(bool(false));


    }
}

One reason to avoid using Boolean (or other wrapper class types) if at all possible, or if you must use it, test it for null before using it. — Hovercraft Full Of Eels, Sep 13 '14 at 14:43
@HovercraftFullOfEels This is our friend from a few minutes ago, testing out the solution to his assignment question :) — Marko Topolnik, Sep 13 '14 at 14:44
possible duplicate of [Why comparing Integer with int can throw NullPointerException in Java?](http://stackoverflow.com/questions/3352791/why-comparing-integer-with-int-can-throw-nullpointerexception-in-java) — Joe, Sep 14 '14 at 06:13

Marko Topolnik · Accepted Answer · 2014-09-13T14:55:00.423

44

null cannot be auto-unboxed to a primitive boolean value, which is what happens when you try to compare it with true. In

param == true

The type of true is boolean, therefore the left-hand operand must also be a boolean. You are passing in a Boolean, which is an object, but can be auto-unboxed to boolean.

Therefore this is equivalent to

param.booleanValue() == true

Clearly, if param is null, the above throws NullPointerException.

To avoid the hidden pitfalls of auto-unboxing, you could instead work with the Boolean objects:

if (Boolean.TRUE.equals(param))
  return "a";
if (Boolean.FALSE.equals(param))
  return "b";
return "c";

edited Sep 13 '14 at 14:55

answered Sep 13 '14 at 14:41

Marko Topolnik

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Great trick with the Boolean statics and methods, it simplifies code so much! +1 – Japheth Ongeri - inkalimeva Nov 01 '16 at 07:37
This also works: `import static java.lang.Boolean.TRUE; if (param == TRUE) {} ` – sorin.silaghi Sep 01 '17 at 11:48
3

@sorin7486 It doesn't work in general: `new Boolean(true) != Boolean.TRUE`. – Marko Topolnik Sep 01 '17 at 15:01
1

@MarkoTopolnik you are correct.. have I mentioned I don't like java? :) – sorin.silaghi Sep 04 '17 at 11:56

Mureinik · Answer 2 · 2019-05-26T15:39:56.873

1

Your code compares a java.lang.Boolean instance with a primitive boolean, which means unboxing the java.lang.Boolean. Since null can't be unboxed, a NullPointerException is thrown.

You could work around this by using the built in constants Boolean.TRUE and Boolean.FALSE:

public static String bool(Boolean param) {
    if (Boolean.TRUE.equals(param)) {
        return "a";
    } else if (Boolean.FALSE.equals(param)) {
        return "b";
    }
    return "c";
}

edited May 26 '19 at 15:39

answered Sep 13 '14 at 14:47

Mureinik

297,002
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Great trick with the Boolean statics and methods, it simplifies code so much! +1 – Japheth Ongeri - inkalimeva Nov 01 '16 at 07:42

score 0 · Answer 3 · answered Sep 13 '14 at 14:46

You used the Boolean instead of boolean. Boolean is a class, which means you can assign objects to it. In your case, you passed in a null, which is then assigned to param. You then tried to use param, which of course resulted in a NullPointerException.

You can:

Get rid of the line bool(null)
Change Boolean to boolean in the parameters for bool()
Add an else if for when param is null

score 0 · Answer 4 · answered Sep 13 '14 at 15:07

So your program must be something like this.

public class BooleanBug {

    public static String bool(Boolean param) {
        if ((null != param) && param.booleanValue() == true) {
            return "a";
        } else if ((null != param) && param.booleanValue() == false) {
            return "b";
        }
        return "c";

    }

    public static void main(String[] args) {

        System.out.println(bool(true));
        System.out.println(bool(null));
        System.out.println(bool(false));

    }
}

score 0 · Answer 5 · answered Jul 21 '21 at 06:09

Clearly, as mentioned by others if param is null, the below throws NullPointerException.

param == true

According to the Java language specification, unboxing happens via calling Boolean.booleanValue(), Number.longValue(), Number.intValue() etc. Its like calling those methods manually and NullPointerException is the result of unboxing a null.

We can also detect these Unboxing vulnerabilities, if not handled properly using static code analysis tools. Kindly refer this for more information Boolean Violation

java.lang.NullPointerException with boolean

5 Answers5

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