ACSA Exam Prep
Unit 3 · Class Creation · Topic 3.9

this Keyword

Skill 2.B

Big idea: Inside an instance method or constructor, this means "the object this code is running on."

Learning objectives
  • Develop code for expressions that are self-referencing and determine the result of these expressions.

Fixing shadowing with this

this.width always means the instance variable, even when a parameter is also called width.

public class Rectangle {
  private int width;
  private int height;

  public Rectangle(int width, int height) {
    this.width = width;      // this.width is the instance variable
    this.height = height;    // height alone is the parameter
  }

  public void badSetWidth(int width) {
    width = width;           // assigns the parameter to itself!
  }

  public void setWidth(int width) {
    this.width = width;
  }

  public int area() { return width * height; }
}

Passing the current object

this can be passed as an argument so another method receives the current object.

Code
Player p = new Player("Zed", 30);
Player q = new Player("Ann", 45);
System.out.println(p.beats(q) + " " + q.beats(p));
Output
false true
Key points
  • In an instance method or constructor, this refers to the current object.
  • this can be passed as an argument.
  • Static methods have no this.
Watch out
  • this.name = name; fixes the shadowing problem from 3.8.
Quick check
Can you use this inside public static void main?
Show answer
No. main is static, so there is no current object.
Practice 3.9 (10 questions)