Time estimate: 45 min.

5.9. this Keyword¶

The keyword this can be used in a class to refer to the current calling object.

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. Run the code below and also check it out in the Java visualizer with the Show CodeLens button which shows how this refers to different objects when the code is run.

Observe the use of the keyword this in the code below. Click on the CodeLens button and then forward to see the memory in action.

public class Person
{
    // instance variables
    private String name;
    private String email;
    private String phoneNumber;

// constructor
    public Person(String theName)
    {
        this.name = theName;
    }

// accessor methods - getters
    public String getName()
    {
        return this.name;
    }

public String getEmail()
    {
        return this.email;
    }

public String getPhoneNumber()
    {
        return this.phoneNumber;
    }

// mutator methods - setters
    public void setName(String theName)
    {
        this.name = theName;
    }

public void setEmail(String theEmail)
    {
        this.email = theEmail;
    }

public void setPhoneNumber(String thePhoneNumber)
    {
        this.phoneNumber = thePhoneNumber;
    }

public String toString()
    {
        return this.name + " " + this.email + " " + this.phoneNumber;
    }

// main method for testing
    public static void main(String[] args)
    {
        Person p1 = new Person("Sana");
        System.out.println(p1);
        Person p2 = new Person("Jean");
        p2.setEmail("jean@gmail.com");
        p2.setPhoneNumber("404 899-9955");
        System.out.println(p2);
    }
}

====
 import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
 {
     public RunestoneTests()
     {
         super("Person");
     }

@Test
     public void testMain() throws IOException
     {
         String output = getMethodOutput("main");
         String expect = "Sana null null\nJean jean@gmail.com 404 899-9955";

boolean passed = getResults(expect, output, "Expected output from main", true);
         assertTrue(passed);
     }
 }

Note

Note that in the code above, this.name, this.email, and this.phoneNumber are equivalent to writing just name, email, and phoneNumber, but this.variable is a way to indicate that we are referring to the instance variables of this object instead of a local variable.

Static methods cannot refer to this or instance variables because they are called with the classname, not an object, so there is no this object.

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.

// 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;
}

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. Try this code in the active code exercise below with the Show CodeLens button to trace through it step by step. Here is an image that shows how this, myPay and p all refer to the same object in memory.

What does this code print out? Trace through the code with the Show CodeLens button. Notice how the this Pay object is passed to the Overtime constructor.

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 static void main(String[] args)
    {
        Pay myPay = new Pay(100.0);
        myPay.calculatePayWithOvertime();
        System.out.println(myPay.getPay());
    }
}

class Overtime
{
    private double payWithOvertime;

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

public double getOvertimePay()
    {
        return payWithOvertime;
    }
}

====
 import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
 {
     @Test
     public void testMain() throws IOException
     {
         String output = getMethodOutput("main");
         String expect = "150.0";

boolean passed = getResults(expect, output, "Expected output from main", true);
         assertTrue(passed);
     }
 }

Check Your Understanding

5-9-3: 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?

10.0
The pay starts at 20 and then increases with overtime.
15.0
If the pay started at 10, this would be the result.
20.0
The pay starts at 20 and then increases with overtime.
30.0
Correct! The pay starts at 20 and then increases with overtime by multiplying by 1.5.
Nothing is printed because the code will not compile.
Incorrect. The code will compile.

5.9.1. Programming Challenge : Bank Account¶

A bank account can be used to store your money. The bank keeps track of the account holder’s name, the acount balance which is the amount of money in the account, and assigns 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.

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, a toString() method, and withdraw(amount) and deposit(amount) methods. Use the this keyword in the constructor and methods. Test your class in a main method.

public class BankAccount {}

====
 import static org.junit.Assert.*;

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
 {
     public RunestoneTests()
     {
         super("BankAccount");
     }

@Test
     public void test0()
     {
         String output = getMethodOutput("main");
         String expect = "Something like:\nName 101 100.0\nName 101 200.0\nName 101 100.0";

boolean passed = !output.contains("Method main does not exist");

getResults(expect, output, "Expected output from main", passed);
         assertTrue(passed);
     }

@Test
     public void test1()
     {
         String output = checkConstructor(2);
         String expect = "pass";

boolean passed = getResults(expect, output, "Checking 2-parameter constructor");
         assertTrue(passed);
     }

@Test
     public void test2()
     {
         String output = checkConstructor(3);
         String expect = "pass";

boolean passed = getResults(expect, output, "Checking 3-parameter constructor");
         assertTrue(passed);
     }

@Test
     public void test01()
     {
         String expect = "3 Private";
         String output = testPrivateInstanceVariables();

boolean passed = getResults(expect, output, "Checking Private Instance Variable(s)");
         assertTrue(passed);
     }

@Test
     public void test3()
     {
         String target = "public String toString()";

boolean passed = checkCodeContainsRegex("toString method", target);
         assertTrue(passed);
     }

@Test
     public void test41()
     {
         String target = "public void withdraw(*)";

boolean passed = checkCodeContainsRegex("withdraw method", target);
         assertTrue(passed);
     }

@Test
     public void test42()
     {
         String target = "public void deposit(*)";

boolean passed = checkCodeContainsRegex("deposit method", target);
         assertTrue(passed);
     }

@Test
     public void test5()
     {
         String target = "this.";
         String code = getCode();

int num = countOccurences(code, target);

boolean passed = num >= 6;

getResults("6+", "" + num, "use of this.*", passed);
         assertTrue(passed);
     }
 }

5.9.2. Summary¶

Within a non-static method or a constructor, the keyword this is 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.
Static methods do not have a this reference.
The this variable can be used anywhere you would use an object variable, even to pass it to another method as an argument.

5.9.3. AP Practice¶

5-9-5: 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?

50
The liquid water has a temperature of 50 but more is added to the jar.
15
The liquid milk has a temperature of 15 but more is added to the jar.
65
Correct! The liquid water with a temperature of 50 and then the liquid milk with a temperature of 15 are added to the jar.
Nothing, the code segment attempts to access the private variable currentTemp outside of its scope.
Incorrect. The currentTemp is never used outside its scope.
Nothing, the code segment attempts to access the private variable totalTemp outside of its scope.
Incorrect. The totalTemp is never used outside its scope.

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