Java Objects from Scratch: A Step‑by‑Step Guide

Creating Objects

Object creation in Java consists of three steps: declaration, instantiation, and referencing.

1.1 Declare an object ClassName objectName; Example:

Car c1, c2; // declare two Car objects
Float f1, f2; // declare two Float objects

After declaration, no memory is allocated and the object cannot be used.

1.2 Instantiate an object

objectName = new ClassConstructor([actual parameters]);

Key points:

The new operator allocates memory and invokes the constructor, returning a reference.

The constructor name is the same as the class name.

Actual parameters (arguments) must match the constructor’s signature; parentheses cannot be omitted even if there are no arguments.

Example:

c1 = new Car("Red", "Black");
 f1 = new Float(10f);

Declaration and instantiation can be combined:

ClassName objectName = new ClassConstructor([actual parameters]);

Example:

Car c1 = new Car("Red", "Black");
Float f1 = new Float(10f);

1.3 Reference an object

After an object is created, its fields and methods are accessed via the reference using the dot operator ( .).

objectName.fieldName;
objectName.methodName(actualParameters);

Example of correcting a field after creation:

Car c2 = new Car("Great Wall", "Rainbow");
// Fix the color typo
c2.color = "Red";

Simple HelloTest Example

public class HelloTest {
    public static void main(String[] args) {
        // Declare a Hello instance
        Hello hello = new Hello();
        // Call the sayHello method three times
        hello.sayHello();
        hello.sayHello();
        hello.sayHello();
    }
}

class Hello {
    // Member method
    void sayHello() {
        System.out.println("Hello!");
    }
}

Program output:

Hello!
Hello!
Hello!

Object Usage with Point and Rectangle

The following demo defines two classes, Point and Rectangle, and shows how to create and manipulate their instances.

public class Point {
    public int x = 0; // x coordinate
    public int y = 0; // y coordinate
    public Point(int x, int y) {
        this.x = x;
        this.y = y;
    }
}

public class Rectangle {
    public int width = 0;
    public int length = 0;
    public Point origin;
    // Default constructor
    public Rectangle() {
        origin = new Point(0, 0);
    }
    // Constructor with a Point
    public Rectangle(Point p) {
        origin = p;
    }
    // Constructor with width and length
    public Rectangle(int w, int l) {
        origin = new Point(0, 0);
        width = w;
        length = l;
    }
    // Constructor with Point, width, and length
    public Rectangle(Point p, int w, int l) {
        origin = p;
        width = w;
        length = l;
    }
    public void move(int newX, int newY) {
        origin.x = newX;
        origin.y = newY;
    }
    public int getArea() {
        return width * length;
    }
}

public class ObjectOperationTest {
    public static void main(String[] args) {
        Point p = new Point(11, 22);
        Rectangle rect1 = new Rectangle(p, 20, 10);
        Rectangle rect2 = new Rectangle(20, 10);
        System.out.println("rect1 width: " + rect1.width);
        System.out.println("rect1 length: " + rect1.length);
        System.out.println("rect1 area: " + rect1.getArea());
        rect2.origin = p;
        System.out.println("rect2 X: " + rect2.origin.x);
        System.out.println("rect2 Y: " + rect2.origin.y);
        rect2.move(30, 40);
        System.out.println("rect2 new X: " + rect2.origin.x);
        System.out.println("rect2 new Y: " + rect2.origin.y);
    }
}

Program output:

rect1 width: 20
rect1 length: 10
rect1 area: 200
rect2 X: 11
rect2 Y: 22
rect2 new X: 30
rect2 new Y: 40

Object Cleanup

Java automatically reclaims memory of unreachable objects via its garbage collector. Developers can optionally request a collection with System.gc(), use finalize() (though it is discouraged), or nullify references to make objects eligible for collection.