Instance variables in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java code using instance variables?

Consider the following Java class and main method. What will be printed when the program runs?

Java

public class Car {
    String color = "red";
    int year = 2020;

    public static void main(String[] args) {
        Car myCar = new Car();
        System.out.println(myCar.color + " " + myCar.year);
    }
}

A red 2020

B null 0

C red 0

D null 2020

💡 Hint

Instance variables get default values if not initialized, but here they are initialized.

The instance variables 'color' and 'year' are initialized with 'red' and 2020 respectively. So the output prints these values.

❓ Predict Output

intermediate

What is the value of the instance variable after object creation?

Look at this Java class. What will be the value of 'count' for the object 'obj' after creation?

Java

public class Counter {
    int count;

    public Counter() {
        count = 5;
    }

    public static void main(String[] args) {
        Counter obj = new Counter();
        System.out.println(obj.count);
    }
}

A Compilation error

B 0

C null

D 5

💡 Hint

Instance variables get default values but constructor can change them.

The constructor sets the instance variable 'count' to 5 when the object is created. So printing obj.count outputs 5.

🔧 Debug

advanced

Why does this code print 0 instead of 10?

This Java code intends to set the instance variable 'score' to 10, but it prints 0. What is the cause?

Java

public class Game {
    int score;

    public void setScore(int score) {
        score = score;
    }

    public static void main(String[] args) {
        Game g = new Game();
        g.setScore(10);
        System.out.println(g.score);
    }
}

A The instance variable 'score' is final and cannot be changed.

B The method parameter 'score' shadows the instance variable; assignment affects only the parameter.

C The code has a syntax error and does not compile.

D Instance variables cannot be assigned inside methods.

💡 Hint

Look at variable names inside the method and how to refer to instance variables.

Inside setScore, the parameter 'score' hides the instance variable 'score'. The assignment 'score = score;' assigns the parameter to itself, leaving the instance variable unchanged at 0.

📝 Syntax

advanced

Which option correctly declares and initializes an instance variable?

Choose the correct way to declare and initialize an instance variable 'name' of type String in a Java class.

A String name = "Alice";

B static String name = "Alice";

C String name();

D String name := "Alice";

💡 Hint

Instance variables are declared inside the class but outside methods, without static keyword for instance scope.

Option A correctly declares an instance variable 'name' initialized to "Alice". Option A makes it static (class variable). Option A is a method declaration. Option A uses invalid syntax.

🚀 Application

expert

How many instance variables does this Java object have after creation?

Given the class below, how many instance variables does an object of this class have after creation?

Java

public class Person {
    String firstName;
    String lastName;
    int age;

    public Person(String firstName, String lastName) {
        this.firstName = firstName;
        this.lastName = lastName;
    }

    public static void main(String[] args) {
        Person p = new Person("John", "Doe");
    }
}

A 0

B 2

C 3

D 1

💡 Hint

Count all instance variables declared in the class, regardless of initialization.

The class declares three instance variables: firstName, lastName, and age. All exist in every object, even if age is not set explicitly.