Why can I not have an abstract and static method in the same class in Java

Question

I have the following class:

public abstract class A { public abstract String doSomething(String X, String Y); public static String doSomething(String X, String Y){return X + Y;} ... }

The issue I have is that the static and abstract doSomething() methods seem to clash as duplicates. I thought this should be fine because the static method belongs to the class, not an instance of the class, so I was going to use the abstract method to enforce the method on all subclasses and the static method as a helper so that I have nicely factored code.

I know I could probably add an interface into the mix, but I don't really understand what's wrong with my abstract and static methods existing on the same class. What's wrong with this?

maerics · Accepted Answer · 2012-01-09 20:33:59Z

8

In Java it is valid (despite being misleading and confusing) to call a static method from an object instance rather than the class name (despite warnings generated by many compilers).

System.out.println(String.valueOf(true)); // Prints "true". System.out.println("".valueOf(true)); // Prints "true", unfortunately.

So the following seemingly valid code wouldn't know which of those methods to call:

A a = getInstanceOfConcreteSubclassOfA(); a.doSomething(null, null); // Compiler can't decide which method to call...

Unfortunately, it's just one of the few ugly corners of the Java language.

edited Jan 9, 2012 at 20:33

answered Jan 9, 2012 at 20:14

maerics

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5 Comments

ruakh Over a year ago

To clarify slightly: it's the compiler, not the runtime, that would have to figure out which method to call.

user982733 Over a year ago

You can't override static methods. Can you?

maerics Over a year ago

@srikanthradix: nope, static methods cannot be overridden.

Philip Couling Over a year ago

Personally I wouldn't call it ugly. The joy of Java is that it blocks you from doing things which are misleading. This is a great example. If you want a "helper" method, then it is better to give it a different name so that someone reading your code sees the difference at a glance between the method itself and its helper.

ruakh Over a year ago

@couling: It's ugly because it is misleading. a.staticMethod() looks like it's calling a non-static virtual method, but it's actually calling a static non-virtual method. And ((A)null).staticMethod() looks like it should throw a NullPointerException, but it doesn't.

ruakh · Accepted Answer · 2012-01-09 20:15:38Z

It's not specific to abstract methods; in general, Java doesn't let you have two methods with the same parameter-types but one being static and one not. Something like this:

public String doSomething(String X, String Y){return X + Y;} public static String doSomething(String X, String Y){return X + Y;}

would also be illegal.

(This makes sense when you consider that you're allowed to call a static method "on" an actual instance, or for that matter, on any expression of the appropriate type. The compiler translates ((A)null).staticMethod() to A.staticMethod().)

Adel Boutros · Accepted Answer · 2012-01-09 20:15:02Z

Each method has a signature composed of:

method name parameter type Return type

If 2 methods have the same signature, this will cause an error.

the word static does not interfere in the signature of the method just like const.

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