Question

I was just discussing about calling static methods using class name with my friend and tried out this code and expected it to throw NPE at runtime.but as it turn out it dint. i just want to understand the execution order.

public class One {

    public static void method() {
        System.out.println("in static one");
    }
}

public class Two {

    static One o;

    public static void main(String[] args) {
        o.method(); // expected NPE here, as o is null
    }
}

I know that static methods should be invoked with their class name, I even know that IDE’s would give a compiler warning when we call static methods with an instance. but we could also call them by creating an instance, however, i never created an instance here, o should get its default value null, thus calling o.method() should throw an NPE at run time, but it doesn’t. can you guys please shed some light on how the execution order is in this code.

Answer 1

It works because what matters is the compile-time type of the o field. The compiler will compile o.method() into the same byte code as One.method().

In particular, if you had a class Two that extends One, and both declare a static void method(), then

One x = new Two();
x.method(); // calls One.method(), not Two.method()

Good for obfuscation purposes, less good for maintainability…