Static and Non-Static Methods

  • I sometimes stumble between the definition and usage of the static keyword in java for classes and methods. Hence, these are my notes on this topic.

  • A good way to look at it is through an example with the code below:

MyPublicInt.java

 1public class MyPublicInt {
 2    int mValue = 0;
 3
 4    static MyPublicInt createInt() {
 5        return new MyPublicInt();
 6    }
 7
 8    MyPublicInt factoryInt() {
 9        return new MyPublicInt();
10    }
11}

  • The non-static class MyPublicInt can define either static or non-static methods as shown above.

sol_102.java

 1public class sol_102 {
 2    class MyPrivateInt {
 3        int mPrivateValue;
 4
 5        MyPrivateInt createInt() {
 6            return new MyPrivateInt();
 7        }
 8    }
 9
10    // This method can be called from static void main()
11    public static MyPublicInt Solution102a() {
12        MyPublicInt myPublicInt = MyPublicInt.createInt();
13        return myPublicInt;
14    }
15
16    // This method can be called from static void main()
17    public static MyPrivateInt Solution102b() {
18        // This following statement is not allowed
19        MyPrivateInt myPrivateInt = new MyPrivateInt();   // NOT ALLOWED!
20        return myPrivateInt;
21    }
22
23    public static MyPublicInt Solution102c() {
24        // This following statement is not allowed
25        MyPublicInt myPublicInt = MyPublicInt.factoryInt();  // NOT ALLOWED!
26        return myPublicInt;
27    }
28
29    public MyPrivateInt Solution102d() {
30        MyPrivateInt myPrivateInt = MyPrivateInt.createInt();   // NOT ALLOWED!
31        return myPrivateInt;
32    }
33
34    public MyPrivateInt Solution102e() {
35        MyPrivateInt myPrivateInt = new MyPrivateInt();
36        return myPrivateInt;
37    }
38}

  • We have two MyInt classes created. One as a public class MyPublicInt and another as an inner class MyPrivateInt.

  • The idea is to create an instance of the class using a static factory method from either of the MyInt class.

  • The method Solution102a which is static, can create an instance of the MyPublicInt by calling its static method createInt(). This is valid because MyPublicInt is not a static class, as it is residing outside of the sol_102 static class.

  • A static method can only access static data members and static methods of another class. It is not able to access non-static methods and variables. A static method is able to rewrite the values of any static data member. This is because the memory of a static method is fixed in ram. We do not need to the instance of its class.

  • The only way to call a non-static method from a static method, is to have an instance of the class containing the non-static method. Hence, from the static method Solution_102c(), we are not able to call the non-static method factoryInt() directly.