NullPointerException from Boolean
This is one for the java purists I think. I recently had an issue with a method to perform a custom parsing of String values to a Boolean. A simple enough task, but for some reason the method below was throwing a NullPointerException in the null case...
static Boolean parseBoolean(String s)
{
return ("1".equals(s) ? true : ("0".equals(s) ? false : null));
}
The return type for the method is Boolean so why or how can a NullPointerException be thrown? From debugging through it seems the exception is being thrown at the point where the nested in-line conditional statement evaluates to null and returns null to the outer in-line conditional, but again I can't explain why.
Eventually I gave up and rewrote the method as follows, which works as expected:
static Boolean parseBoolean(String s)
{
if ("1".equals(s)) return true;
if ("0".equals(s)) return false;
return null;
}
The following code is half way between the two and also works as expected:
static Boolean parseBoolean(String s)
{
if ("1".equals(s)) return true;
return "0".equals(s) ? false : null;
}
This also works:
static Boolean parseBoolean(String s)
{
return ("1".equals(s) ? Boolean.TRUE : ("0".equals(s) ? Boolean.FALSE : null));
}
So the reason you get an NPE is due to autoboxing because using boolean in the ternary operator causes the result of the expression to be treated as a boolean . And un-boxing of null causes an NPE.
My suggestion? Don't return Boolean , return boolean and throw an exception:
static boolean parseBoolean(String s)
{
if ("1".equals(s)) return true;
if ("0".equals(s)) return false;
throw new IllegalArgumentException(s + " is not a boolean value.");
}
Adopting an approach like the above will help avoid you accidentally referencing a null Boolean object.
See the excellent answer from NilsH to see why your original method is throwing an exception.
Interesting but no answer tells you why this happens in the first place.
This has to do with the ternary expression.
The compiler interprets null as a null reference to Boolean, applies the autoboxing/unboxing rules for Boolean (on a null) => you get a NullPointerException at runtime.
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