This vs This

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layout: default title: this vs. this()

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We have seen how to use the reference this to allow an object to use its own fields and methods.

public class Car {
  private String make, model, color;
  private int numberOfWheels;
  private double purchasePrice;

  public Car(String make, String model, String color, int numberOfWheels) {
    this.setMake(make);
    this.setModel(model);
    this.setColor(color);
    this.setNumberOfWheels(numberOfWheels);
  }

  public Car(String make, String model, String color, int numberOfWheels, double purchasePrice) {
    this.make = make;
    this.model = model;
    this.color = color;
    this.numberOfWheels = numberOfWheels;
    this.purchasePrice = purchasePrice;
  }
  //...
}

There is another way to use this to allow an object to access its own code.

this() in a Constructor

A constructor can call another constructor in the same class by using the keyword this as a method name.

public class Car2 {
  private String make, model, color;
  private int numberOfWheels;
  private double purchasePrice;

  public Car2(String make, String model, String color, int numberOfWheels) {
    this(make, model, color, numberOfWheels, 0.0);
  }

  public Car2(String make, String model, String color, int numberOfWheels, double purchasePrice) {
    this.make = make;
    this.model = model;
    this.color = color;
    this.numberOfWheels = numberOfWheels;
    this.purchasePrice = purchasePrice;
  }
  //...
}

This reduces code duplication.

If you use this() in a constructor, it must be the first statement.

• You would not be able to put a System.out.println before it, for example.

public Car2(String make, String model, String color, int numberOfWheels) {
  System.out.println("Calling this()");  // WILL NOT COMPILE: Constructor call must be the first statement in a constructor
  this(make, model, color, numberOfWheels, 0.0);
}

Practice Exercise

We often have methods call their overloaded counterparts and pass default values for fields, like the four-arg Car2 constructor is doing. The reason, again, is to avoid code duplication.

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