Printing Object State

Learning Goals

• Use IntelliJ to generate a toString() method
• Update a program to print object state

Introduction

Every Java class inherits a method named toString() that returns a string representation of an object. The toString() method is inherited from a superclass named Object, which is a topic we will cover in a later lesson.

By default, the toString() method returns the memory address of the object. In this lesson, we will see how to override the toString() method to return a descriptive string containing the values of the instance variables. This allows us to print object state at various points of the program execution.

Printing An Object Reference

Let's continue with the Dog class and add two print statements at the end of the main method as shown:

public class Dog {

    String name;
    String breed;
    int age;
    boolean waggingTail;

    public static void main(String[] args) {

        // create 2 Dog instances
        Dog bigDog = new Dog();
        Dog smallDog = new Dog();

        // assign new values to the fields of the first dog
        bigDog.name = "Bruno";
        bigDog.breed = "Great Dane";
        bigDog.age = 8;

        // assign new values to the fields of the second dog
        smallDog.name = "Penny";
        smallDog.breed = "Pug";
        smallDog.age = 5;
        smallDog.waggingTail = true;

        //print the state of each Dog instance
        System.out.println(bigDog);
        System.out.println(smallDog);

    }
}

When we run the main() method, we see something like this (memory addresses may differ):

Dog@4617c264
Dog@36baf30c

• Behind the scenes, the compiler converts System.out.println(bigDog); to System.out.println(bigDog.toString());.
  • The toString() method is called to get a string representation of the object for printing.
• By default, toString() returns a string containing the name of the class, followed by "@", followed by a hexadecimal value corresponding to the object's memory location on the heap.

It is not particularly useful to see the memory location as the output. Instead, we would like to see each dog's name, breed, and so on.

Using IntelliJ To Generate A toString() Implementation

Let's update the Dog class to change the implementation of the toString() method to print the values stored in the instance variables.

We can implement toString() in any manner we wish; however, the easiest thing is to just have IntelliJ generate it for us. The toString() method can be placed anywhere in the class, but we will place it directly before the main() method.

Position the cursor after the fields and before the main() method. Right-click and select "Generate" from the menu.

Select "toString()" from the menu.

Select all fields and then press "OK".

Confirm the new method named toString() has been added to the class above the main method:

@Override
public String toString() {
    return "Dog{" +
        "name='" + name + '\'' +
        ", breed='" + breed + '\'' +
        ", age=" + age +
        ", waggingTail=" + waggingTail +
        '}';
}

Unlike the main method, the toString() method signature does not include the keyword static. We will cover instance methods, which are non-static and must be invoked with an object reference, in detail in a later lesson. We'll also cover what @Override means in a later lesson on inheritance. But for now we can use the toString() method to print the state of each dog as a descriptive string.

The Dog class should appear as shown below.

public class Dog {

    String name;
    String breed;
    int age;
    boolean waggingTail;


    @Override
    public String toString() {
        return "Dog{" +
                "name='" + name + '\'' +
                ", breed='" + breed + '\'' +
                ", age=" + age +
                ", waggingTail=" + waggingTail +
                '}';
    }

    public static void main(String[] args) {

        // create 2 Dog instances
        Dog bigDog = new Dog();
        Dog smallDog = new Dog();

        // assign new values to the fields of the first dog
        bigDog.name = "Bruno";
        bigDog.breed = "Great Dane";
        bigDog.age = 8;

        // assign new values to the fields of the second dog
        smallDog.name = "Penny";
        smallDog.breed = "Pug";
        smallDog.age = 5;
        smallDog.waggingTail = true;

        //print the state of each Dog instance
        System.out.println(bigDog);
        System.out.println(smallDog);

    }
}

Running the main method now produces the output shown below.

Dog{name='Bruno', breed='Great Dane', age=8, waggingTail=false}
Dog{name='Penny', breed='Pug', age=5, waggingTail=true}

Comprehension Check

Challenge #1

Assume the following ProgrammingLanguage class:

public class ProgrammingLanguage {

    String name;
    int yearReleased;

    @Override
    public String toString() {
        return "ProgrammingLanguage{" +
                "name='" + name + '\'' +
                ", yearReleased=" + yearReleased +
                '}';
    }

    public static void main(String[] args) {
        ProgrammingLanguage language1 = new ProgrammingLanguage();
        ProgrammingLanguage language2 = new ProgrammingLanguage();

        language1.name = "Java";
        language1.yearReleased = 1995;

        language2.name = "Go";
        language2.yearReleased = 2007;

        System.out.println(language1);
    }
}

What is printed when the the main method executes?

Answer:

ProgrammingLanguage{name='Java', yearReleased=1995}

Challenge #2

Assume the following Bike class:

public class Bike {
    String color;
    int height;
    int wheels = 2;

    public static void main(String[] args) {
        Bike mountainBike = new Bike();
        System.out.println(mountainBike);
    }
}

What is printed when the the main method executes?

Answer:

Since the Bike class does not redefine the toString() method, the default implementation will print the memory address of the object, which looks something like this:

Bike@4617c264

Conclusion

Every Java class has a method named toString() that returns a string representation of an object. The default implementation returns the memory address of the object. IntelliJ can generate a different implementation that returns a string containing the values stored in the instance variables.

Resources

• Java 17 Object class