Understanding significance of static in Java

Static modifier in Java spans its influence on

Key Take Aways

Type :

• Static modifier when attached to inner class means, the inner class doesn’t hold ant reference of the outer class.
• To instantiate the inner static class , outer class object instantiation is not needed.

     StaticDemo.InnerClass innerClass = new StaticDemo.InnerClass();

Field:

• Static field first take default values and then are overwritten by static initialize.
• Static fields are initialized when the class is loaded by the class loader and not at the time of Object instantiation.
• Static field of a class means it represents attribute of the class and not the object.
• Static field is not stored as part of the Object.
• Static field is saved along with the class and there is only one copy(memory allocation).
• Static field can be referenced in both static and not static methods.
• Static field is shared by all the objects of the class in the JVM . change by one will be reflected in all others as there is only one copy.
• Static field can be reference directly by Classname. Reference is not needed .

     StaticDemo.getStaticMethod();

Methods:

• All methods are part of the class and none is part of the object.
• Static methods signify they represent functionality of Class and not of the object.
• Static modifier in a method signifies that the method is not accessing any instance field of the class directly.
• Static modifier in a method signifies that the method is not dependant on the object.
• Static method can only access static fields , as “this” pointer is not available with the method.

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Code:

package com.big.data.java.modifiers;

public class StaticDemo {

    // Default value if int is 0 , objects is null both for static and instance fields
    private static int staticState;

    static {
        // Executed when class is loaded by JVM
        System.out.println("Inside Static Initializer block staticState value before is " + staticState);
        staticState = 10;
        System.out.println("Inside Static Initializer block staticState value before is " + staticState);
        // state = 20  not possible as state is part of object not class
    }

    private int state;

    public StaticDemo() {
        state = 10;
        System.out.println("Inside Constructor staticState value before is " + staticState);
        // Static field is accessible from non static methods and constructors
        staticState = 30;
    }

    // all methods are part of class and not of object .
    // the significance of static methods is it is not accessing any instance members directly
    // Static method refer class behaviour and not object behaviour
    private static int getStaticMethod() {
        return staticState;
        //state = 30;  non static field is not accessible withing static methods
    }

    public static void main(String[] args) {
        StaticDemo reference = new StaticDemo();
        // Static method is accessible via class Name .
        StaticDemo.getStaticMethod();

        StaticDemo.InnerClass innerClass = new StaticDemo.InnerClass();
    }

    // Static innerClass signifies it will not hold the reference of the outer class.
    // A non static innerClass holds the reference of the outer class
    public static class InnerClass {
        private int innerState;

        public InnerClass() {
            innerState = 20;
            //state = 10; cannot access member of the outer class. as it dosent hold refrence of the outer class
            System.out.println("Inside Constructor of InnerClass , innerState value is  " + innerState);
        }
    }

}

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Output :

Inside Static Initializer block staticState value before is 0
Inside Static Initializer block staticState value before is 10
Inside Constructor staticState value before is 10
Inside Constructor of InnerClass , innerState value is  20