Boolean.valueOf() produces NullPointerException sometimes

Question

I have this code:

package tests;

import java.util.Hashtable;

public class Tests {

    public static void main(String[] args) {

        Hashtable<String, Boolean> modifiedItems = new Hashtable<String, Boolean>();

        System.out.println("TEST 1");
        System.out.println(modifiedItems.get("item1")); // Prints null
        System.out.println("TEST 2");
        System.out.println(modifiedItems.get("item1") == null); // Prints true
        System.out.println("TEST 3");
        System.out.println(Boolean.valueOf(null)); // Prints false
        System.out.println("TEST 4");
        System.out.println(Boolean.valueOf(modifiedItems.get("item1"))); // Produces NullPointerException
        System.out.println("FINISHED!"); // Never executed
    }
}

My problem is that I don't understand why Test 3 works fine (it prints false and doesn't produce NullPointerException) meanwhile Test 4 throws a NullPointerException. As you can see in tests 1 and 2, null and modifiedItems.get("item1") are equals and null.

The behavior is the same in Java 7 and 8.

Answer 1

You've got to look carefully at which overload is being invoked:

• Boolean.valueOf(null) is invoking Boolean.valueOf(String). This doesn't throw an NPE even if supplied with a null parameter.
• Boolean.valueOf(modifiedItems.get("item1")) is invoking Boolean.valueOf(boolean), because modifiedItems's values are of type Boolean, which requires an unboxing conversion. Since modifiedItems.get("item1") is null, it is the unboxing of that value - not the Boolean.valueOf(...) - which throws the NPE.

---

The rules for determining which overload is invoked are pretty hairy, but they roughly go like this:

• In a first pass, a method match is searched for without allowing boxing/unboxing (nor variable arity methods).

  • Because null is an acceptable value for a String but not boolean, Boolean.valueOf(null) is matched to Boolean.valueOf(String) in this pass;
  • Boolean isn't an acceptable for either Boolean.valueOf(String) or Boolean.valueOf(boolean), so no method is matched in this pass for Boolean.valueOf(modifiedItems.get("item1")).

• In a second pass, a method match is searched for, allowing boxing/unboxing (but still not variable arity methods).

  • A Boolean can be unboxed to boolean, so Boolean.valueOf(boolean) is matched for Boolean.valueOf(modifiedItems.get("item1")) in this pass; but an unboxing conversion has to be inserted by the compiler to invoke it: Boolean.valueOf(modifiedItems.get("item1").booleanValue())

• (There's a third pass allowing for variable arity methods, but that's not relevant here, as the first two passes matched these cases)