3.9 this Keyword

3.9 this Keyword

Each coding exercise has its own folder. Open its .java file and press Run to try it. Run Tests checks every graded exercise in this lesson at once.

Within an instance method or a constructor, the keyword this acts as a special variable that holds a reference to the current object—the object whose method or constructor is being called.

For example, in the following Class Person, when we create an object p1 and call the constructor or p1.setEmail(), the word “this” refers to p1. And when we make the same method calls with object p2, “this” refers to p2.

The this variable can only be used in instance methods and constructors. Class methods cannot refer to this or instance variables because they are called with the classname, not an object, so there is no this object.

Exercise 1

Observe the use of the keyword this in the code.

Open the 01-PersonClassThis folder and run Person.java. It is an example, so it has no tests.

this.instanceVariable

The keyword this is sometimes used by programmers to distinguish between variables. Programmers can give the parameter variables the same names as the instance variables and this can distinguish them and avoid a naming conflict. For example, both the instance variable and the parameter variable are called name in the code below, but name on its own looks for the closest local variable, the parameter variable, whereas this.name refers to this object's instance variable.

this.instanceVariable can be used to distinguish between this object’s instance variables and local parameter variables that may have the same variable names.

// instance variables
private String name;

// constructor
public Person(String name)
{
   // Set this object's instance variable name to the parameter variable name
   this.name = name;
}

this as an Argument

The this variable can be used anywhere you would use an object variable. You can even pass it to another method as an argument. Consider the classes below, Pay and Overtime. The Pay class declares an Overtime object and passes in this (the current Pay object) to its constructor which computes the overtime with respect to that Pay object. Here is an image that shows how this, myPay and p all refer to the same object in memory.

Exercise 2

What does this code print out? Notice how the this Pay object is passed to the Overtime constructor.

Open the 02-PayClassThis folder and run Pay.java. It is an example, so it has no tests.

Check your understanding: Consider the following class definitions.

public class Pay
{
    private double pay;

    public Pay(double p)
    {
        pay = p;
    }

    public double getPay()
    {
        return pay;
    }

    public void calculatePayWithOvertime()
    {
        // this Pay object is passed to the Overtime constructor
        Overtime ot = new Overtime(this);
        pay = ot.getOvertimePay();
    }
}

public class Overtime
{
    private double payWithOvertime;

    public Overtime(Pay p)
    {
        payWithOvertime = p.getPay() * 1.5;
    }

    public double getOvertimePay()
    {
        return payWithOvertime;
    }
}

The following code segment appears in a class other than Pay or Overtime.

Pay one = new Pay(20.0);
one.calculatePayWithOvertime();
System.out.println(one.getPay());

What, if anything, is printed as a result of executing the code segment?

Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-3-9-this.html#AP-this-arg

Coding Challenge: Bank Account

Many people keep their money in a bank account. The bank may keep track of the account holder’s name, the acount balance which is the amount of money in the account, and assign an account number to each account. At the bank or an ATM (automatic teller machine) or on a phone app, the account holder can deposit (add) or withdraw (subtract) an amount from their account. Here’s a video that shows the steps to use an ATM to withdraw money from a bank acount. Phone apps like Venmo and Paypal connect to your bank account or credit card to send and get money from businesses or friends.

For this challenge, you can work in pairs to:

• Create a class called BankAccount below that keeps track of the account holder’s name, the account number, and the balance in the account. Make sure you use the appropriate data types for these.
• Write 2 constructors for the class: one that initializes all the instance variables and one that only has 2 parameters for the name and account number and initializes the balance to 0. For the parameters, use the same variable names as your instance variables. Use the this keyword to distinguish between the instance variables and the parameter variables.
• Write a toString method for the class. Use the this keyword to return the instance variables.
• Write withdraw(amount) and deposit(amount) methods for the class. The withdraw method should subtract the amount from the balance as long as there is enough money in the account (the balance is larger than the amount). And deposit should add the amount to the balance. Use the this keyword to refer to the balance.
• Test your class below with a main method that creates a BankAccount object and calls its deposit and withdraw methods and prints out the object to test its toString method.

Exercise 3

Create a class called BankAccount that keeps track of the account holder’s name, the account number, and the balance in the account. Create 2 constructors using this (one constructor to initialize all 3 instance variables and one that only has 2 parameters for the name and account number and initializes the balance to 0), a toString() method, and withdraw(amount) and deposit(amount) methods. Test your class in a main method.

Open the 03-challenge-BankAccount folder and edit BankAccount.java. Press Run to try it; Run Tests checks it (8 tests).

Summary

• (AP 3.9.A.1) Within an instance method or a constructor, the keyword this acts as a special variable that holds a reference to the current object—the object whose method or constructor is being called.
• this.instanceVariable can be used to distinguish between this object’s instance variables and local parameter variables that may have the same variable names.
• (AP 3.9.A.2) The keyword this can be used to pass the current object as an argument in a method call.
• (AP 3.9.A.3) Class methods do not have a this reference.

AP Practice

Check your understanding: Consider the following class definitions.

public class Liquid
{
    private int currentTemp;

    public Liquid(int ct)
    {
        currentTemp = ct;
    }

    public int getCurrentTemp()
    {
        return currentTemp;
    }

    public void addToJar(LiquidJar j)
    {
        j.addLiquid(this);
    }
}

public class LiquidJar
{
    private int totalTemp;

    public LiquidJar()
    {
        totalTemp = 0;
    }

    public void addLiquid(Liquid l)
    {
        totalTemp += l.getCurrentTemp();
    }

    public int getTotalTemp()
    {
        return totalTemp;
    }
    // Constructor not shown.
}

Consider the following code segment, which appears in a class other than Liquid or LiquidJar.

Liquid water = new Liquid(50);
Liquid milk = new Liquid(15);

LiquidJar j = new LiquidJar();
water.addToJar(j);
milk.addToJar(j);
System.out.println(j.getTotalTemp());

What, if anything, is printed out after the execution of the code segment?

Answer it on CS Awesome: https://runestone.academy/ns/books/published/csawesome2/topic-3-9-this.html#AP-this