Object Creation & “this” in java

In this blog, we will take a look inside JVM object creation and “this” object reference.

Everyone knows how to create a class:

public class Rectangle {
    int width;
    int height;

    public Rectangle(int width, int height) {
        this.width = width;
        this.height = height;
    }

    public int getHeight() {
        Rectangle rectangle = new Rectangle(4, 5);
        return height;
    }

    public int getWidth() {
        return width;
    }
}

How you create an object:

Rectangle rectangle = new Rectangle(4,5);

How JVM understands the object creation:

During the compilation phase, JVM creates a method name <init> with the same parameters as your constructor. So Object creation for Rectangle class will look like this.

Rectangle rectangle = new Rectangle; 
Rectangle.<init>(rectangle,4,5);

This looks strange let's expand <init>(rectangle,4,5):

So the first parameter of every instance method is always reference of its object. Internally this first parameter is referred to as “this”.

Rectangle.<init>(rectangle,4,5);

<init>(Rectangle this, int width, int height)
{
        this.width = width;
        this.height = height
 }

JVM’s instruction set for object creation and assignment:

Rectangle rectangle = new Rectangle(4,5);

translates to

new           #2                  // class Rectangle
dup
iconst_4
iconst_5
invokespecial #3                  // Method Rectangle."<init>":(II)V
astore_1

JVM’s instruction set for object creation:

new Rectangle(4,5);

translates to

new           #2                  // class Rectangle
dup
iconst_4
iconst_5
invokespecial #3                  // Method Rectangle."<init>":(II)V
pop