What is the difference between a static method and a non-static method?

Question

See the code snippets below:

Code 1

public class A {
    static int add(int i, int j) {
        return(i + j);
    }
}

public class B extends A {
    public static void main(String args[]) {
        short s = 9;
        System.out.println(add(s, 6));
    }
}

Code 2

public class A {
    int add(int i, int j) {
        return(i + j);
    }
}

public class B extends A {
    public static void main(String args[]) {
    A a = new A();
        short s = 9;
        System.out.println(a.add(s, 6));
    }
}

What is the difference between these code snippets? Both output 15 as an answer.

Answer 1

A static method belongs to the class itself and a non-static (aka instance) method belongs to each object that is generated from that class. If your method does something that doesn't depend on the individual characteristics of its class, make it static (it will make the program's footprint smaller). Otherwise, it should be non-static.

Example:

class Foo {
    int i;

    public Foo(int i) { 
       this.i = i;
    }

    public static String method1() {
       return "An example string that doesn't depend on i (an instance variable)";
    }

    public int method2() {
       return this.i + 1; // Depends on i
    }
}

You can call static methods like this: Foo.method1(). If you try that with method2, it will fail. But this will work: Foo bar = new Foo(1); bar.method2();

Answer 2

Static methods are useful if you have only one instance (situation, circumstance) where you're going to use the method, and you don't need multiple copies (objects). For example, if you're writing a method that logs onto one and only one web site, downloads the weather data, and then returns the values, you could write it as static because you can hard code all the necessary data within the method and you're not going to have multiple instances or copies. You can then access the method statically using one of the following:

MyClass.myMethod();
this.myMethod();
myMethod();

Non-static methods are used if you're going to use your method to create multiple copies. For example, if you want to download the weather data from Boston, Miami, and Los Angeles, and if you can do so from within your method without having to individually customize the code for each separate location, you then access the method non-statically:

MyClass boston = new MyClassConstructor(); 
boston.myMethod("bostonURL");

MyClass miami = new MyClassConstructor(); 
miami.myMethod("miamiURL");

MyClass losAngeles = new MyClassConstructor();
losAngeles.myMethod("losAngelesURL");

In the above example, Java creates three separate objects and memory locations from the same method that you can individually access with the "boston", "miami", or "losAngeles" reference. You can't access any of the above statically, because MyClass.myMethod(); is a generic reference to the method, not to the individual objects that the non-static reference created.

If you run into a situation where the way you access each location, or the way the data is returned, is sufficiently different that you can't write a "one size fits all" method without jumping through a lot of hoops, you can better accomplish your goal by writing three separate static methods, one for each location.