Writing Constructors

Section 3.4 Writing Constructors

45 minutes

In Unit 1, we learned how to create objects by calling constructors. To review, a call to a constructor consists of the word new followed by the name of the class being constructed, and then an argument list in parentheses. For example, here is how we create World, Turtle, and Person objects.

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// To create a new object, write:
// ClassName variableName = new ConstructorName(arguments);
World world = new World();
Turtle t = new Turtle(world);
Person p = new Person("Pat", "[email protected]", "123-456-7890");

Now it’s time to learn to write our own constructors.

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Subsection 3.4.1 Constructor Signature

In the source code for a class, constructors are usually written after the instance variables and before any methods.

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The signature of a constructor is similar to the signature of a method except there is no return type, not even void, and instead of a method name, the name of the constructor is always the same as the name of the class. The constructors you write will be marked public. Like methods, constructors also have a parameter list specified in parenthesis that declare the variables that will be used to hold the arguments passed when the constructor is called.

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public class ClassName
{
    /* Instance variable declarations go here, before constructors */

    /* Constructor - same name as Class, no return type */
    public ClassName()
    {
        /* Implementation not shown */
    }

    /* Method definitions go here, after constructors */
}

Note 3.4.1.

Constructors must have the same name as the class! Constructors have no return type!

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Subsection 3.4.2 The Job of a Constructor

The job of a constructor is to set the initial values for the object’s instance variables to useful values. But what does “useful” mean? Sometimes we describe the values of all an object’s instance variables at a given time as the object’s state. And we say an object is in a valid state when all its instance variables have values that let us use the object by invoking its public methods. So another way to describe the job of a constructor is to set the object’s instance values so it’s in a valid state and ready to be used. Classes can have zero or more constructors but they should all produce an object in a valid state.

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An object’s state refers to its attributes and their values at a given time and is defined by instance variables belonging to the object. This defines a has-a relationship between the object and its instance variables. For example, a Person object has a name, email, and phone number attributes. A constructor is used to set the initial state of an object, which should include initial values for all instance variables. For example, new Person("Pat", "[email protected]", "123-456-7890") calls the Person constructor with the values to fill in for its name (“Pat”), email (”[email protected]”), and phone number (“123-456-7890”). When a constructor is called, memory is allocated for the object and the associated object reference is returned. Constructor parameters, if specified, provide data to initialize instance variables.

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The easiest way to write a constructor is to not write one. If you do not write a constructor your class will automatically get what is called the default constructor with no parameters. This constructor will initialize all your instance variables to the default value for their type:

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0 for int
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0.0 for double
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false for boolean
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null for all reference types
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If those default values are sufficient to put your object into a valid state you may not need to write a constructor at all.

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Usually, however, if you are writing a class that has instance variables, you need to initialize your instance values to some other values. In that case you probably need to write a constructor that takes arguments and uses them to initialize your instance variables.

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For example, consider the constructor from the Person class from the last lesson.

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public Person(String initName, String initEmail, String initPhone)
{
    name = initName;
    email = initEmail;
    phoneNumber = initPhone;
}

This constructor ensures that all three of the instance variables (name, email, and phoneNumber) in Person are initialized to the values provided by whatever code called the constructor. For example, in the constructor call new Person("Pat", "[email protected]", "123-456-7890"), the argument “Pat” is passed into the parameter variable initName, which the constructor then assigns to the instance variable name.

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One important note: if you do write a constructor, Java will not generate the default constructor for you. This is a good thing because it lets you make sure that instances of your class are always properly initialized. With this constructor in place, for instance, there’s no way to construct a Person object without providing the three required String values.

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Sometimes you will want to write more than one constructor so that there are different ways of making an instance of your class. One reason to do that is to make it convenient to create instances from different kinds of arguments. This is called overloading. You could write a no-argument constructor for Person that provides fill-in values like the following, if those make sense for your program. It’s up to you to decide if this is actually a useful value to have or if it would be better to force the users of the Person class to choose the values themselves using the constructor with arguments.

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public Person()
{
    name = "Anonymous";
    email = "unknown";
    phoneNumber = "unknown";
}

Activity 3.4.1.

Click on all the lines of code that are part of constructors in the following class.

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Constructors are public and have the same name as the class.

public class Name
{
    private String first;
    private String last;

    public Name(String theFirst, String theLast)
    {
        first = theFirst;
        last = theLast;
    }

    public void setFirst(String theFirst)
    {
        first = theFirst;
    }

    public void setLast(String theLast)
    {
        last = theLast;
    }

}

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Activity 3.4.2.

What best describes the purpose of a class’s constructor?

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Determines the amount of space needed for an object and creates the object
The object is already created before the constructor is called but the constructor initializes the instance variables.
Names the new object
Constructors do not name the object.
Return to free storage all the memory used by this instance of the class.
Constructors do not free any memory. In Java the freeing of memory is done when the object is no longer referenced.
Initialize the instance variables in the object
A constructor initializes the instance variables to their default values or in the case of a parameterized constructor, to the values passed in to the constructor.

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Activity 3.4.3.

The following class defines a Fraction with the instance variables numerator and denominator. Complete the body of its 2 constructors. The no-argument constructor should set the default instance variable values to 1 rather than 0 since a fraction with 0 in the denominator is not valid. The constructor with parameters should copy those parameters into the instance variables. What will the code print out? Try tracing through it. View it in the Java visualizer by clicking on the Show CodeLens button below.

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public class Fraction
{
    //  instance variables
    private int numerator;
    private int denominator;

// no-arg constructor: set instance variables to default values of 1
    public Fraction()
    {
        // 1. set numerator to 1

// 2. set denominator to 1
    }

// constructor w/args: set instance variables to init parameters
    public Fraction(int initNumerator, int initDenominator)
    {
        // 3. set numerator to initNumerator

// 4. set denominator to initDenominator

}

// Print fraction
    public void print()
    {
        System.out.println(numerator + "/" + denominator);
    }

// main method for testing
    public static void main(String[] args)
    {
        Fraction f1 = new Fraction();
        Fraction f2 = new Fraction(1, 2);
        // What will these print out?
        f1.print();
        f2.print();
    }
}
====
// Test Code for Lesson 5.2.0.1 - Fraction
import static org.junit.Assert.*;

import org.junit.Test;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void test() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "1/1\n1/2";

boolean passed = getResults(expect, output, "Running main");
        assertTrue(passed);
    }
    @Test
   public void testCodeContains1()
   {
       boolean check1 =
               checkCodeContains("numerator = 1 in Fraction()", "numerator = 1");
       assertTrue(check1);
   }
   @Test
   public void testCodeContains2()
   {
       boolean check1 =
               checkCodeContains("denominator = 1 in Fraction()", "denominator = 1");
       assertTrue(check1);
   }
   @Test
   public void testCodeContains3()
   {
       boolean check1 =
               checkCodeContains("numerator = initNumerator in 2nd constructor", "numerator = initNumerator");
       assertTrue(check1);
   }
   @Test
   public void testCodeContains4()
   {
       boolean check1 =
               checkCodeContains("denominator = initDenominator in 2nd constructor", "denominator = initDenominator");
       assertTrue(check1);
   }
}

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Activity 3.4.4.

The following class defines a Car with the instance variables model and year, for example a Honda 2010 car. However, some of the code is missing. First, fill in the code to create a Car constructor. Then, fill in the code to call the constructor in the main method to create 2 Car objects. The first car should be a 2023 Ford and the second car should be a 2010 Honda. Run your program and make sure it works and prints out the information for both cars.

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public class Car
{
   //  instance variables
   private String model;
   private int year;

// constructor: set instance variables to init parameters
   public Car(String initModel, int initYear)
   {
       // 1. set the instance variables to the init parameter variables

}

// Print Car info
   public void print()
   {
     System.out.println("Car model: " + model);
     System.out.println("Car year: " + year);
   }

// main method for testing
   public static void main(String[] args)
   {
       // 2. Call the constructor to create 2 new Car objects with different
       // values The first car should be a 2023 Ford and the second car
       // should be a 2010 Honda.

Car car1 =
       Car car2 =

car1.print();
       car2.print();
   }
}
====
// Test Code for Lesson 5.2.0 - Car
import static org.junit.Assert.*;

import org.junit.Test;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main");
        String expect = "Car model: Ford\nCar year: 2023\nCar model: Honda\nCar year: 2010";

boolean passed = getResults(expect, output, "Running main");
        assertTrue(passed);
    }
}

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Subsection 3.4.3 Coding Challenge : Student Class

We encourage you to work in pairs for this challenge to create a Student class with constructors.

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First, brainstorm in pairs to do the Object-Oriented Design for a Student class. What data should we store about Students? Come up with at least 4 different instance variables. What are the data types for the instance variables?
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Write a Student class below that has your 4 instance variables and write a constructor that has 4 parameters to set all of the instance variables.
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Add a print() method that uses System.out.println to print out all the instance variables.
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Add a main method that constructs at least 2 Student objects using the constructor with different values and then calls their print() methods.
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Project 3.4.5.

Create a class Student with 4 instance variables, a constructor, and a print method. Write a main method that creates 2 Student objects with the constructor and calls their print() method.

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/**
 * class Student with 4 instance variables, a constructor, a print method, and a
 * main method to test them.
 */
public class Student
{
    // Write 4 instance variables

// Write a constructor with 4 parameters to initialize all of the instance
    // variables

// Write a print method that prints all the instance variables
    public void print()
    {

}

// main method
    public static void main(String[] args)
    {
        // Construct 2 Student objects using the constructor with different values

// call their print() methods

}
}
====
// Test Code for Lesson 5.2.1 - Challenge - Student
import static org.junit.Assert.*;

import org.junit.Test;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    public RunestoneTests()
    {
        super("Student");

Object[] values = new Object[] {"Name", 0};
        setDefaultValues(values);
    }

@Test
    public void testCallConstructors()
    {
        String code = getCodeWithoutComments();
        String search = "= new Student(";

int num = countOccurences(code, search);

String expect = search + "...) x 2";
        String actual = search + "...) x " + num;

boolean passed = getResults(expect, actual, "Checking that you made 2 Student objects");
        assertTrue(passed);
    }

@Test
    public void testConstructors()
    {
        String code = getCodeWithoutComments();
        String search = "public Student(";

int num = countOccurences(code, search);

String expect = search + "...) x 1";
        String actual = search + "...) x " + num;

boolean passed = getResults(expect, actual, "Checking that you made a constructor");
        assertTrue(passed);
    }

@Test
    public void testPrint()
    {
        String code = getCodeWithoutComments();
        String search = ".print()";

int num = countOccurences(code, search);

String expect = search + " x 2";
        String actual = search + " x " + num;

boolean passed = getResults(expect, actual, "Checking that you called print 2 times");
        assertTrue(passed);
    }

@Test
    public void testPrivateVariables()
    {
        String expect = "4 Private";
        String output = testPrivateInstanceVariables();

boolean passed = getResults(expect, output, "Checking for 4 Private Instance Variable(s)");
        assertTrue(passed);
    }
}

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Subsection 3.4.4 Design a Class for your Community

In the last lesson, you came up with a class of your own choice relevant to you or your community. In this lesson, you will add a constructor to this class.

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Consult your completed worksheet or your code in lesson 3.3 Community Challenge for the class name and its 3 instance variables that you created. Copy them into the active code exercise below.
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Add a constructor with 3 parameters to set all of the instance variables to the given parameters.
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Write a print() method that uses System.out.println to print out all the instance variables.
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Write a main method that constructs at least 2 objects of your class using the constructor and then calls their print() methods.
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Project 3.4.6.

Copy your class with its 3 instance variables from lesson 3.3 Community Challenge. Add a constructor with 3 parameters to set all of the instance variables to the given parameters. Write a print() method that uses System.out.println to print out all the instance variables. Write a main method that constructs at least 2 objects of your class using the constructors and then calls their print() methods.

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public class          // Add your class name here!
{
    // 1. copy in your instance variables for class from the last lesson

// 2. Add a constructor with 3 parameters to set all of the instance variables to the given parameters.

// 3. Write a print() method that uses System.out.println to print out all the instance variables.

// 4. Write a main method that constructs at least 2 objects of your class
    // using the constructor and then calls their print() methods.
    public static void main(String[] args)
    {
       // Construct 2 objects of your class to test the constructors

// call the objects print() methods

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

import org.junit.*;

import java.io.*;

public class RunestoneTests extends CodeTestHelper
{
    @Test
    public void testPrivateVariables()
    {
        String expect = "3 Private";
        String output = testPrivateInstanceVariables();
        boolean passed = false;
        if (Integer.parseInt(expect.substring(0, 1)) <= Integer.parseInt(output.substring(0, 1)))
            passed = true;
        passed = getResults(expect, output, "Checking private instance variable(s)", passed);
        assertTrue(passed);
    }

/* No longer required
    @Test
    public void testDefaultConstructor()
    {
        String output = checkDefaultConstructor();
        String expect = "pass";

boolean passed = getResults(expect, output, "Checking default constructor");
        assertTrue(passed);
    }
    */

@Test
    public void testConstructor3()
    {
        String output = checkConstructor(3);
        String expect = "pass";

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

@Test
    public void testPrint()
    {
        String output = getMethodOutput("print");
        String expect = "More than 15 characters";
        String actual = " than 15 characters";

if (output.length() < 15)
        {
            actual = "Less" + actual;
        }
        else
        {
            actual = "More" + actual;
        }
        boolean passed = getResults(expect, actual, "Checking print method");
        assertTrue(passed);
    }

@Test
    public void testMain() throws IOException
    {
        String output = getMethodOutput("main"); // .split("\n");
        String expect = "3+ line(s) of text";
        String actual = " line(s) of text";
        int len = output.split("\n").length;

if (output.length() > 0)
        {
            actual = len + actual;
        }
        else
        {
            actual = output.length() + actual;
        }
        boolean passed = len >= 3;

getResults(expect, actual, "Checking output", passed);
        assertTrue(passed);
    }
}

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Subsection 3.4.5 Summary

(AP 3.4.A.1) An object’s state refers to its attributes and their values at a given time and is defined by instance variables belonging to the object. This defines a has-a relationship between the object and its instance variables.
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(AP 3.4.A.2) A constructor is used to set the initial state of an object, which should include initial values for all instance variables. When a constructor is called, memory is allocated for the object and the associated object reference is returned. Constructor parameters, if specified, provide data to initialize instance variables.
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A constructor must have the same name as the class! Constructors have no return type!
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(AP 3.4.A.4) When no constructor is written, Java provides a no-parameter constructor, and the instance variables are set to default values according to the data type of the attribute. This constructor is called the default constructor. (Note that AP 3.4.A.3 is covered in lesson 3.6).
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(3.4.A.5) Default values used by the default constructor:
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- The default value for an attribute of type int is 0.
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- The default value of an attribute of type double is 0.0.
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- The default value of an attribute of type boolean is false.
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- The default value of a reference type is null.
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Subsection 3.4.6 AP Practice

Activity 3.4.7.

Consider the definition of the Cat class below. The class uses the instance variable isSenior to indicate whether a cat is old enough to be considered a senior cat or not.

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public class Cat
{
    private String name;
    private int age;
    private boolean isSenior;

    public Cat(String n, int a)
    {
        name = n;
        age = a;
        if (age >= 10)
        {
            isSenior = true;
        }
        else
        {
            isSenior = false;
        }
    }
}

Which of the following statements will create a Cat object that represents a cat that is considered a senior cat?

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Cat c = new Cat (“Oliver”, 7);
The age 7 is less than 10, so this cat would not be considered a senior cat.
Cat c = new Cat (“Max”, “15”);
An integer should be passed in as the second parameter, not a string.
Cat c = new Cat (“Spots”, true);
An integer should be passed in as the second parameter, not a boolean.
Cat c = new Cat (“Whiskers”, 10);
Correct!
Cat c = new Cat (“Bella”, isSenior);
An integer should be passed in as the second parameter and isSenior would be undefined outside of the class.

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Activity 3.4.8.

Consider the following class definition. Each object of the class Cat will store the cat’s name as name, the cat’s age as age, and the number of kittens the cat has as kittens. Which of the following code segments, found in a class other than Cat, can be used to create a cat that is 5 years old with no kittens?

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public class Cat
{
    private String name;
    private int age;
    private int kittens;

    public Cat(String n, int a, int k)
    {
        name = n;
        age = a;
        kittens = k;
    }
    public Cat(String n, int a)
    {
        name = n;
        age = a;
        kittens = 0;
    }
    /* Other methods not shown */
}

I.   Cat c = new Cat("Sprinkles", 5, 0);
II.  Cat c = new Cat("Lucy", 0, 5);
III. Cat c = new Cat("Luna", 5);

I only
Option III can also create a correct Cat instance.
II only
Option II will create a cat that is 0 years old with 5 kittens.
III only
Option I can also create a correct Cat instance.
I and III only
Good job!
I, II and III
Option II will create a cat that is 0 years old with 5 kittens.

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Activity 3.4.9.

Consider the following class definition.

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public class Cat
{
    private String color;
    private boolean isHungry;
    /* missing constructor */
}

The following statement appears in a method in a class other than Cat. It is intended to create a new Cat object c with its attributes set to “black” and true. Which of the following can be used to replace missing constructor code in the class definition so that the object c below is correctly created?

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Cat c = new Cat("black", true);

public Cat(String c, boolean h)
{
    c = "black";
    h = true;
}

The constructor should be changing the instance variables, not the local variables.

public Cat(String c, boolean h)
{
    c = "black";
    h = "true";
}

The constructor should be changing the instance variables, not the local variables.

public Cat(String c, boolean h)
{
    c = color;
    h = isHungry;
}

The constructor should be changing the instance variables, not the local variables.

public Cat(String c, boolean h)
{
    color = black;
    isHungry = true;
}

The constructor should be using the local variables to set the instance variables.

public Cat(String c, boolean h)
{
    color = c;
    isHungry = h;
}

Correct!

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Subsection 3.4.7 AP Classroom Progress Check Unit 3 Part A

This lesson ends the section for the College Board Unit 3 part A. You can now do the College Board Progress Check for Unit 3 Part A in the AP Classroom.

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You have attempted of activities on this page.

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