Class definition in Java - Practice Problems & Coding Challenges

Challenge - 5 Problems

❓ Predict Output

intermediate

What is the output of this Java class instantiation?

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

Java

public class Car {
    String brand;
    int year;

    public Car(String brand, int year) {
        this.brand = brand;
        this.year = year;
    }

    public void printInfo() {
        System.out.println(brand + " " + year);
    }

    public static void main(String[] args) {
        Car myCar = new Car("Toyota", 2020);
        myCar.printInfo();
    }
}

A Toyota 2020

B Car@15db9742

C null 0

D Compilation error

💡 Hint

Look at the constructor and the printInfo method to see what values are printed.

The constructor sets the brand to "Toyota" and year to 2020. The printInfo method prints these values separated by a space.

❓ Predict Output

intermediate

What happens if you try to access a private field directly?

Given this Java class, what will happen if you try to compile and run the main method?

Java

public class Person {
    private String name;

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

    public static void main(String[] args) {
        Person p = new Person("Alice");
        System.out.println(p.name);
    }
}

A Runtime NullPointerException

B Prints: null

C Compilation error: name has private access in Person

D Prints: Alice

💡 Hint

Check the access modifier of the field and where it is accessed.

Private fields are accessible within the same class, including from static methods via an instance. The program compiles and prints 'Alice'.

❓ Predict Output

advanced

What is the output of this Java class with static and instance variables?

Analyze the following Java code and determine what it prints when run.

Java

public class Counter {
    static int count = 0;
    int id;

    public Counter() {
        count++;
        id = count;
    }

    public void printId() {
        System.out.println("Counter id: " + id);
    }

    public static void main(String[] args) {
        Counter c1 = new Counter();
        Counter c2 = new Counter();
        c1.printId();
        c2.printId();
        System.out.println("Total count: " + Counter.count);
    }
}

A

Counter id: 0
Counter id: 0
Total count: 0

B

Counter id: 2
Counter id: 2
Total count: 2

C

Counter id: 1
Counter id: 2
Total count: 2

D Compilation error

💡 Hint

Static variables are shared among all instances, instance variables are unique per object.

Each new Counter increments the static count and assigns it to the instance id. So c1 has id 1, c2 has id 2, and total count is 2.

❓ Predict Output

advanced

What error does this Java class produce?

Look at this Java class code. What error will occur when compiling?

Java

public class Animal {
    String type;

    public Animal() {
        this.type = type;
    }
}

A Runtime NullPointerException

B No error, compiles successfully

C Compilation error: variable type might not have been initialized

D Compilation error: cannot assign a variable to itself

💡 Hint

Check what happens when you assign a field to itself in the constructor.

Assigning 'this.type = type;' assigns the field to itself because the parameter 'type' is missing, so it assigns the field to its own value (null). This is allowed and compiles without error.

🧠 Conceptual

expert

How many objects are created in this Java code?

Consider the following Java code snippet. How many objects are created when main runs?

Java

public class Box {
    int size;

    public Box(int size) {
        this.size = size;
    }

    public static void main(String[] args) {
        Box b1 = new Box(5);
        Box b2 = b1;
        Box b3 = new Box(10);
    }
}

A 2

B 3

C 1

D 0

💡 Hint

Remember that assigning one object reference to another does not create a new object.

Two new Box objects are created with 'new Box(5)' and 'new Box(10)'. The assignment 'Box b2 = b1;' does not create a new object, just another reference.