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Java Classes & Objects Practice

Defining classes, creating objects, instance variables and methods.

Simple Class - Student

Easy

Create a class Student with fields name (String) and age (int). In main, read name and age, create a Student object, and print "Name: , Age: ".

Input Format

Two lines: name, then age.

Output Format

Name: , Age:

Sample Test Cases

Sample Input 1 Alice 18	Sample Output 1 Name: Alice, Age: 18
Sample Input 2 Bob 20	Sample Output 2 Name: Bob, Age: 20

Constraints

Explanation

Create a constructor or set fields directly, then print.

Class with Method

Easy

Create a class Rectangle with a method int area() that returns length * breadth. Read L and B, create a Rectangle object, call area(), and print the result.

Input Format

A single line with two integers L and B.

Output Format

A single integer: the area.

Sample Test Cases

Sample Input 1 4 5	Sample Output 1 20
Sample Input 2 7 3	Sample Output 2 21

Constraints

Explanation

Store fields and return the product in area().

Multiple Objects

Easy

Create a class Book with title (String) and pages (int). Read two books (title and pages each) and print both in the format " has pages", one per line.

Input Format

Four lines: title1, pages1, title2, pages2.

Output Format

Two lines: book info for each book.

Sample Test Cases

Sample Input 1

Java Basics
300
Python Basics
250

Sample Output 1

Java Basics has 300 pages
Python Basics has 250 pages

Constraints

Explanation

Create two Book objects and print their details.

Instance Variable Count

Easy

Create a class Counter with an instance variable int count (starts at 0) and a method void increment() that adds 1. Read N, call increment() N times, then print count.

Input Format

A single integer N.

Output Format

A single integer: the final count.

Sample Test Cases

Sample Input 1 5	Sample Output 1 5
Sample Input 2 0	Sample Output 2 0

Constraints

Explanation

Loop N times calling increment().

Constructor with Parameters

Medium

Create a class Circle with fields radius (double) and a constructor that takes radius. Add a method double area() that returns 3.14 * r * r. Read R, create the object via the constructor, and print the area as an integer rounded down.

Input Format

A single integer R.

Output Format

A single integer: floor(area).

Sample Test Cases

Sample Input 1 5	Sample Output 1 78
Sample Input 2 10	Sample Output 2 314

Constraints

Explanation

Use (int)Math.floor(c.area()) when printing.

Getter and Setter

Medium

Create a class Person with private field name and public getter getName() and setter setName(String). Read a name, set it with the setter, and print it with the getter.

Input Format

A single line containing a name.

Output Format

The name printed by the getter.

Sample Test Cases

Sample Input 1 Alice	Sample Output 1 Alice
Sample Input 2 Rahul	Sample Output 2 Rahul

Constraints

Explanation

Encapsulate the field behind getter/setter.

This Keyword

Medium

Create a class Employee with fields name and salary, and a constructor that uses the this keyword to assign them. Read name and salary, create the object, and print "Name: , Salary: ".

Input Format

Two lines: name, then salary.

Output Format

Name: , Salary:

Sample Test Cases

Sample Input 1

Arjun
50000

Sample Output 1

Name: Arjun, Salary: 50000

Constraints

Explanation

Use this.name = name to resolve shadowing.

Static Field and Method

Medium

Create a class MathUtils with a static method int square(int n). Read N and call MathUtils.square(N) (no object needed), printing the result.

Input Format

A single integer N.

Output Format

A single integer: N*N.

Sample Test Cases

Sample Input 1 6	Sample Output 1 36
Sample Input 2 12	Sample Output 2 144

Constraints

Explanation

Call the static method using the class name.

Method Overloading (Same Class)

Medium

Create a class Calculator with overloaded methods: int add(int a, int b) and int add(int a, int b, int c). Read three integers X, Y, Z and print add(X,Y) on one line and add(X,Y,Z) on the next.

Input Format

A single line with three integers X, Y, Z.

Output Format

Two lines: add(X,Y), then add(X,Y,Z).

Sample Test Cases

Sample Input 1 2 3 4	Sample Output 1 5 9
Sample Input 2 -1 5 2	Sample Output 2 4 6

Constraints

-10^6

Explanation

Define both overloads and call each.

Array of Objects

Hard

Create a class Student with name (String) and marks (int). Read N and then N students (name and marks each). Print the name of the student with the highest marks. If tied, print the first one.

Input Format

First line: N. Next N lines: name and marks.

Output Format

The name of the topper.

Sample Test Cases

Sample Input 1

3
Alice 85
Bob 92
Carol 78

Sample Output 1

Bob

Constraints

Explanation

Store Student[] and find the max marks.

Object as Parameter

Hard

Create a class Point with fields x and y. Write a method static double distance(Point a, Point b) that returns sqrt((a.x-b.x)^2 + (a.y-b.y)^2). Read four integers (x1 y1 x2 y2), create two Point objects, and print the distance rounded down to the nearest integer.

Input Format

A single line with four integers x1 y1 x2 y2.

Output Format

A single integer: floor(distance).

Sample Test Cases

Sample Input 1 0 0 3 4	Sample Output 1 5
Sample Input 2 1 1 4 5	Sample Output 2 5

Constraints

-1000

Explanation

Use Math.hypot or Math.sqrt.

Returning an Object

Hard

Create a class Time with hours and minutes. Write a method static Time add(Time t1, Time t2) that returns a new Time (carry minutes over 60 to hours). Read two times as "h m" each and print the sum as "h m".

Input Format

Two lines, each "h m".

Output Format

A single line: summed hours and minutes.

Sample Test Cases

Sample Input 1 1 30 2 45	Sample Output 1 4 15
Sample Input 2 5 50 6 20	Sample Output 2 12 10

Constraints

Explanation

Return a new Time object from add().

Competitive MCQs — Java Classes & Objects

Code snippets, output prediction, concepts & error spotting. Pick an answer to see instant feedback.

Score 0/ 31

Which keyword creates an object from a class?

Correct!

Wrong — correct answer is .

new ClassName() allocates and initializes an instance.

What is the output of this code?

java

class Dog {
    void bark() {
        System.out.println("Woof");
    }
}
new Dog().bark();

Correct!

Wrong — correct answer is .

new Dog().bark() invokes the method, printing "Woof".

What is the output of this code?
class Dog {
void bark() {
System.out.println("Woof");
}
}
new Dog().bark();

Correct!

Wrong — correct answer is .

new Dog().bark() invokes the method, printing "Woof".

What is a constructor?

Correct!

Wrong — correct answer is .

Constructors run when an object is created with new.

What is the name of a constructor?

Correct!

Wrong — correct answer is .

A constructor must match the class name exactly.

What is the output of this code?

java

class A {
    int x = 5;
}
System.out.println(new A().x);

Correct!

Wrong — correct answer is .

The object's field x is 5.

What is the output of this code?
class A {
int x = 5;
}
System.out.println(new A().x);

Correct!

Wrong — correct answer is .

The object's field x is 5.

What does the default constructor do?

Correct!

Wrong — correct answer is .

If no constructor is declared, Java supplies a no-arg default.

What is the output of this code?

java

class Point {
    int x;
    Point(int x) {
        this.x = x;
    }
}
System.out.println(new Point(7).x);

Correct!

Wrong — correct answer is .

The constructor stores 7 into this.x.

Q10

What is the output of this code?
class Point {
int x;
Point(int x) {
this.x = x;
}
}
System.out.println(new Point(7).x);

Correct!

Wrong — correct answer is .

The constructor stores 7 into this.x.

Q11

What is an object?

Correct!

Wrong — correct answer is .

Classes are blueprints; objects are their instances.

Q12

Which statement about instance fields is TRUE?

Correct!

Wrong — correct answer is .

Instance fields are per-object state.

Q13

What is the output of this code?

java

class A {
    static int count = 0;
    A() {
        count++;
    }
}
new A();
new A();
System.out.println(A.count);

Correct!

Wrong — correct answer is .

Two constructions increment the shared static count to 2.

Q14

What is the output of this code?
class A {
static int count = 0;
A() {
count++;
}
}
new A();
new A();
System.out.println(A.count);

Correct!

Wrong — correct answer is .

Two constructions increment the shared static count to 2.

Q15

What is the output of this code?
class Student {
String name;
Student(String n) {
name = n;
}
}
System.out.println(new Student("Aya").name);

Correct!

Wrong — correct answer is .

The constructor stores "Aya" in the name field.

Q16

What is the output of this code?
class A {
A() {
System.out.println("built");
}
}
new A();

Correct!

Wrong — correct answer is .

Creating A() runs the constructor, printing "built".

Q17

What is the output of this code?
class A {
int x;
}
System.out.println(new A().x);

Correct!

Wrong — correct answer is .

Numeric fields default to 0.

Q18

What does new do?

Correct!

Wrong — correct answer is .

new reserves heap memory and invokes the constructor.

Q19

What is the output of this code?
class A {
int val = 7;
}
A a = new A();
System.out.println(a.val);

Correct!

Wrong — correct answer is .

The instance field val holds 7.

Q20

Which statement is TRUE about instance fields?

Correct!

Wrong — correct answer is .

Instance fields belong to each individual object.

Q21

What is a field?

Correct!

Wrong — correct answer is .

Fields store the state of class instances (or the class).

Q22

What is the output of this code?
class A {
A() {
System.out.print("A");
}
}
class B extends A {
B() {
System.out.print("B");
}
}
new B();

Correct!

Wrong — correct answer is .

The parent constructor runs first automatically, printing A then B.

Q23

Which keyword refers to the current object?

Correct!

Wrong — correct answer is .

this refers to the object whose method is executing.

Q24

What is the output of this code?
class A {
int x;
A(int x) {
this.x = x;
}
}
System.out.println(new A(5).x);

Correct!

Wrong — correct answer is .

this.x distinguishes the field from the parameter.

Q25

Which access modifier makes a field visible everywhere?

Correct!

Wrong — correct answer is .

public members are accessible from any class.

Q26

What is the output of this code?
class A {
static void hi() {
System.out.println("Hi");
}
}
A.hi();

Correct!

Wrong — correct answer is .

Static methods are called through the class name.

Q27

Can a class have multiple constructors?

Correct!

Wrong — correct answer is .

Constructors overload like other methods, differing by parameters.

Q28

What is the output of this code?
class A {
int x = 1;
int x() {
return 2;
}
}
System.out.println(new A().x());

Correct!

Wrong — correct answer is .

The method x() returns 2; fields and methods can share names.

Q29

What is encapsulation at the class level?

Correct!

Wrong — correct answer is .

Private fields plus public methods is the classic encapsulated design.

Q30

What is the output of this code?
class A {
String name = "Default";
A(String name) {
this.name = name;
}
}
System.out.println(new A("Java").name);

Correct!

Wrong — correct answer is .

The parameter overrides the initial value.

Q31

Which keyword creates a new object?

Correct!

Wrong — correct answer is .

ClassName ref = new ClassName(); is the standard object creation.

Java Practice Questions

Java Classes & Objects Practice

Competitive MCQs — Java Classes & Objects