Instance methods in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

Output of instance method call

What is the output of this Java program when run?

Java

public class Car {
    private String model;
    public Car(String model) {
        this.model = model;
    }
    public String getModel() {
        return model;
    }
    public static void main(String[] args) {
        Car car = new Car("Tesla");
        System.out.println(car.getModel());
    }
}

A Tesla

B car

C null

D Compilation error

💡 Hint

Look at what the getModel() method returns.

The instance method getModel() returns the model field, which was set to "Tesla" in the constructor. So the output is "Tesla".

❓ Predict Output

intermediate

Instance method modifying object state

What will be printed after running this Java code?

Java

public class Counter {
    private int count = 0;
    public void increment() {
        count++;
    }
    public int getCount() {
        return count;
    }
    public static void main(String[] args) {
        Counter c = new Counter();
        c.increment();
        c.increment();
        System.out.println(c.getCount());
    }
}

A 1

B 0

C Compilation error

D 2

💡 Hint

Each call to increment increases count by one.

The increment method increases the count field by 1 each time it is called. It is called twice, so count becomes 2.

🔧 Debug

advanced

Identify the error in instance method usage

What error will this Java code produce when compiled or run?

Java

public class Person {
    private String name;
    public Person(String name) {
        this.name = name;
    }
    public void printName() {
        System.out.println(name);
    }
    public static void main(String[] args) {
        printName();
    }
}

A No error, prints the name

B Cannot make a static reference to the non-static method printName()

C Compilation error: missing semicolon

D NullPointerException at runtime

💡 Hint

Static methods cannot call instance methods without an object.

The main method is static and tries to call printName() which is an instance method without an object. This causes a compilation error.

❓ Predict Output

advanced

Instance method with parameter and return value

What is the output of this Java program?

Java

public class Calculator {
    public int multiply(int a, int b) {
        return a * b;
    }
    public static void main(String[] args) {
        Calculator calc = new Calculator();
        int result = calc.multiply(3, 4);
        System.out.println(result);
    }
}

A 12

B 7

C 34

D Compilation error

💡 Hint

Multiply 3 by 4.

The multiply method returns the product of the two parameters. 3 * 4 = 12.

🧠 Conceptual

expert

Understanding instance method behavior with multiple objects

Consider this Java code. What will be printed when it runs?

Java

public class Light {
    private boolean isOn = false;
    public void switchOn() {
        isOn = true;
    }
    public void switchOff() {
        isOn = false;
    }
    public String getState() {
        return isOn ? "On" : "Off";
    }
    public static void main(String[] args) {
        Light kitchenLight = new Light();
        Light bedroomLight = new Light();
        kitchenLight.switchOn();
        System.out.println(kitchenLight.getState() + " " + bedroomLight.getState());
    }
}

A On On

B Off Off

C On Off

D Compilation error

💡 Hint

Each object has its own isOn state.

kitchenLight switches on, so its isOn is true. bedroomLight remains off, so its isOn is false. Output is "On Off".