Java Classes and Objects

Java is an object oriented language. A class is a blueprint, and an object is a real thing made from that blueprint. A class Car is the idea of a car, and your red Swift is an object of it.

Create a class

A class has fields (the data) and methods (the actions).

class Student {
    String name;          // fields
    int marks;

    void show() {         // method
        System.out.println(name + " scored " + marks);
    }
}

public class Main {
    public static void main(String[] args) {
        Student s1 = new Student();    // create an object
        s1.name = "Riya";
        s1.marks = 88;

        Student s2 = new Student();
        s2.name = "Aman";
        s2.marks = 72;

        s1.show();
        s2.show();
    }
}

new Student() creates an object in memory. The dot . lets you use the fields and methods of the object. Every object has its own copy of the fields.

Default values of fields

class Box {
    int number;
    boolean open;
    String label;
    double size;
}

public class Defaults {
    public static void main(String[] args) {
        Box b = new Box();
        System.out.println(b.number + " " + b.open + " " + b.label + " " + b.size);
    }
}

References

A variable of a class type does not keep the object. It keeps a reference (the address). Two variables can point to the same object:

class Counter {
    int count;
}

public class Ref {
    public static void main(String[] args) {
        Counter a = new Counter();
        Counter b = a;               // same object, not a copy
        b.count = 10;
        System.out.println(a.count);

        Counter c = new Counter();   // a new object
        System.out.println(c.count);
        System.out.println(a == b);  // same object
        System.out.println(a == c);
    }
}

null

A reference can be null, which means “no object”. Using it gives a NullPointerException.

class Pet {
    String name = "Tommy";
}

public class Null {
    static Pet p = null;

    public static void main(String[] args) {
        System.out.println(p == null);
        System.out.println(p.name);
    }
}

The toString method

Printing an object shows its class name and a code. You can write a toString() method to print something useful:

class Point {
    int x, y;

    Point(int x, int y) {
        this.x = x;
        this.y = y;
    }

    @Override
    public String toString() {
        return "Point(" + x + ", " + y + ")";
    }
}

public class Str {
    public static void main(String[] args) {
        Point p = new Point(3, 4);
        System.out.println(p);
    }
}

Garbage collection

You do not delete objects. When no variable points to an object anymore, the garbage collector of the JVM frees its memory by itself.

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