Java Language Tutorial => Pitfall of Auto-Unboxing Null Objects...

Example

public class Foobar {
    public static void main(String[] args) {

        // example: 
        Boolean ignore = null;
        if (ignore == false) {
            System.out.println("Do not ignore!");
        }
    }
}

The pitfall here is that null is compared to false. Since we're comparing a primitive boolean against a Boolean, Java attempts to unbox the the Boolean Object into a primitive equivalent, ready for comparison. However, since that value is null, a NullPointerException is thrown.

Java is incapable of comparing primitive types against null values, which causes a NullPointerException at runtime. Consider the primitive case of the condition false == null; this would generate a compile time error incomparable types: int and <null>.

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Java Language Java Pitfalls - Language syntax Pitfall of Auto-Unboxing Null Objects into Primitives

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