Java static and this Keywords

static and this are two keywords which every Java developer must understand.

static fields: one copy for the class

A normal (instance) field has a copy in every object. A static field has only one copy shared by all the objects.

class Student {
    static int count = 0;      // shared
    String name;               // one per object

    Student(String name) {
        this.name = name;
        count++;
    }
}

public class Main {
    public static void main(String[] args) {
        new Student("Riya");
        new Student("Aman");
        new Student("Meera");
        System.out.println("Students created: " + Student.count);   // use the class name
    }
}

static methods

A static method belongs to the class. You call it with the class name, and no object is needed. That is why main is static.

class MathUtil {
    static int square(int n) {
        return n * n;
    }

    static final double TAX_RATE = 0.18;      // a constant
}

public class Util {
    public static void main(String[] args) {
        System.out.println(MathUtil.square(7));
        System.out.println(MathUtil.TAX_RATE);
        System.out.println(Math.max(3, 9));        // Math has static methods
    }
}

A static method can not use instance fields or this, because it does not belong to any object:

class Person {
    String name = "Riya";

    static void show() {
        System.out.println(name);
    }
}

public class Bad {
    public static void main(String[] args) {
        Person.show();
    }
}

static block

class Config {
    static final String VERSION;

    static {                                   // runs once, when the class is loaded
        System.out.println("Loading config...");
        VERSION = "1.0";
    }
}

public class Block {
    public static void main(String[] args) {
        System.out.println("Start");
        System.out.println(Config.VERSION);
    }
}

The this keyword

this is a reference to the current object. Its main uses:

• To separate a field from a parameter with the same name: this.name = name;
• To call another constructor: this(...)
• To pass the current object to another method.

class Account {
    private double balance;

    Account(double balance) {
        this.balance = balance;
    }

    Account deposit(double amount) {
        this.balance += amount;
        return this;                           // allows method chaining
    }

    double getBalance() {
        return balance;
    }
}

public class This {
    public static void main(String[] args) {
        Account a = new Account(100);
        a.deposit(50).deposit(25);              // chaining
        System.out.println(a.getBalance());
    }
}

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