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.
You've got to look carefully at which overload is being invoked:
Boolean.valueOf(null)is invokingBoolean.valueOf(String). This doesn't throw anNPEeven if supplied with a null parameter.Boolean.valueOf(modifiedItems.get("item1"))is invokingBoolean.valueOf(boolean), becausemodifiedItems's values are of typeBoolean, which requires an unboxing conversion. SincemodifiedItems.get("item1")isnull, it is the unboxing of that value - not theBoolean.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
nullis an acceptable value for aStringbut notboolean,Boolean.valueOf(null)is matched toBoolean.valueOf(String)in this pass; Booleanisn't an acceptable for eitherBoolean.valueOf(String)orBoolean.valueOf(boolean), so no method is matched in this pass forBoolean.valueOf(modifiedItems.get("item1")).
- Because
In a second pass, a method match is searched for, allowing boxing/unboxing (but still not variable arity methods).
- A
Booleancan be unboxed toboolean, soBoolean.valueOf(boolean)is matched forBoolean.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())
- A
(There's a third pass allowing for variable arity methods, but that's not relevant here, as the first two passes matched these cases)