- Packages – Two classes in different packages communicating
File: pack1/A.java
package pack1;
public class A {
public void msg() {
System.out.println(“Hello from class A”);
}
}
File: pack2/B.java
package pack2;
import pack1.A;
public class B {
public static void main(String[] args) {
A a = new A();
a.msg();
}
}
✅ 2. Static method display() without creating object
class Test {
static void display() {
System.out.println(“Static method called”);
}
public static void main(String[] args) {
Test.display();
}
}
✅ 3. Divide-by-zero exception
class DivZero {
public static void main(String[] args) {
try {
int a = 10 / 0;
} catch (ArithmeticException e) {
System.out.println(“Error: Divide by zero”);
}
}
}
✅ 4. Read content of a text file
import java.io.*;
class FileRead {
public static void main(String[] args) throws Exception {
BufferedReader br = new BufferedReader(new FileReader(“test.txt”));
String line;
while ((line = br.readLine()) != null)
System.out.println(line);
br.close();
}
}
✅ 5. try–catch–finally
class TCF {
public static void main(String[] args) {
try {
int a = 10 / 2;
} catch (Exception e) {
System.out.println(“Error”);
} finally {
System.out.println(“Finally block executed”);
}
}
}
- Create a file and write message
import java.io.*;
class WriteFile {
public static void main(String[] args) throws Exception {
FileWriter fw = new FileWriter(“hello.txt”);
fw.write(“Welcome to Java File Handling”);
fw.close();
System.out.println(“File written”);
}
}
✅ 7. Interface implemented in two classes
interface A {
void show();
}
class B implements A {
public void show() {
System.out.println(“Class B”);
}
}
class C implements A {
public void show() {
System.out.println(“Class C”);
}
}
class Test {
public static void main(String[] args) {
A obj = new B();
obj.show();
obj = new C();
obj.show();
}
}
✅ 8. Thread using Runnable
class MyThread implements Runnable {
public void run() {
System.out.println(“Thread running”);
}
}
class Main {
public static void main(String[] args) {
Thread t = new Thread(new MyThread());
t.start();
}
}
✅ 9. Custom exception AgeException
class AgeException extends Exception {
AgeException(String msg) { super(msg); }
}
class Test {
public static void main(String[] args) {
try {
int age = 15;
if (age < 18)
throw new AgeException(“Age < 18 not allowed”);
} catch (AgeException e) {
System.out.println(e.getMessage());
}
}
}
✅ 10. Multilevel inheritance (A → B → C)
class A {
void showA() { System.out.println(“A”); }
}
class B extends A {
void showB() { System.out.println(“B”); }
}
class C extends B {
void showC() { System.out.println(“C”); }
}
class Main {
public static void main(String[] args) {
C obj = new C();
obj.showA();
obj.showB();
obj.showC();
}
}
✅ 11. Student class
class Student {
String name;
int age;
Student(String n, int a) {
name = n;
age = a;
}
void display() {
System.out.println(name + ” ” + age);
}
public static void main(String[] args) {
Student s = new Student(“PPP”, 21);
s.display();
}
}
✅ 12. User-defined package mypack
File: mypack/Hello.java
package mypack;
public class Hello {
public void msg() {
System.out.println(“Hello from mypack”);
}
}
Main file
import mypack.Hello;
class Test {
public static void main(String[] args) {
new Hello().msg();
}
}
✅ 13. Child class calling parent method
class Parent {
void show() { System.out.println(“Parent method”); }
}
class Child extends Parent {
void callParent() { show(); }
public static void main(String[] args) {
new Child().callParent();
}
}
Two threads printing numbers
class T1 extends Thread {
public void run() {
for (int i = 1; i <= 5; i++) System.out.println(“T1: ” + i); } } class T2 extends Thread { public void run() { for (int i = 1; i <= 5; i++) System.out.println(“T2: ” + i); } } class Main { public static void main(String[] args) { new T1().start(); new T2().start(); } } ✅ 15. ArrayList add/display import java.util.*; class AL { public static void main(String[] args) { ArrayList list = new ArrayList<>();
list.add(“A”);
list.add(“B”);
System.out.println(list);
}
}
✅ 16. Array index out of bounds
class AIOOB {
public static void main(String[] args) {
try {
int a[] = {1, 2, 3};
System.out.println(a[5]);
} catch (ArrayIndexOutOfBoundsException e) {
System.out.println(“Index error”);
}
}
}
✅ 17. Thread sleep
class SleepDemo {
public static void main(String[] args) throws Exception {
System.out.println(“Start”);
Thread.sleep(1000);
System.out.println(“End”);
}
}
✅ 18. Thread by extending Thread
class MyThread extends Thread {
public void run() {
System.out.println(“Thread Running”);
}
public static void main(String[] args) {
new MyThread().start();
}
}
✅ 19. Single inheritance
class Parent {
void show() { System.out.println(“Parent”); }
}
class Child extends Parent {
public static void main(String[] args) {
new Child().show();
}
}
✅ 20. Car constructor
class Car {
Car(String model) {
System.out.println(“Car model: ” + model);
}
public static void main(String[] args) {
new Car(“BMW”);
}
} - Interface Animal implemented by Dog
interface Animal {
void sound();
}
class Dog implements Animal {
public void sound() {
System.out.println(“Bark”);
}
public static void main(String[] args) {
new Dog().sound();
}
}
✅ 22. Static vs non-static
class Demo {
static int a = 10; // static
int b = 20; // non-static
public static void main(String[] args) {
Demo d = new Demo();
System.out.println(a);
System.out.println(d.b);
}
}
✅ 23. Count characters in text file
import java.io.*;
class CountChar {
public static void main(String[] args) throws Exception {
FileReader fr = new FileReader(“test.txt”);
int count = 0;
while (fr.read() != -1) count++;
fr.close();
System.out.println(“Characters: ” + count);
}
}
✅ 24. Abstract class Shape & area()
abstract class Shape {
abstract void area();
}
class Square extends Shape {
int s = 5;
void area() { System.out.println(“Area = ” + (s * s)); }
public static void main(String[] args) {
new Square().area();
}
}
✅ 25. Copy one file to another
import java.io.; class Copy { public static void main(String[] args) throws Exception { FileInputStream in = new FileInputStream(“a.txt”); FileOutputStream out = new FileOutputStream(“b.txt”); int c; while ((c = in.read()) != -1) out.write(c); in.close(); out.close(); System.out.println(“Copied”); } } ✅ 26. HashMap store & display import java.util.;
class HM {
public static void main(String[] args) {
HashMap map = new HashMap<>();
map.put(1,”PPP”);
map.put(2,”ABC”);
System.out.println(map);
}
}
✅ 27. Iterator for ArrayList
import java.util.*;
class ItDemo {
public static void main(String[] args) {
ArrayList list = new ArrayList<>();
list.add(“A”);
list.add(“B”);
Iterator i = list.iterator();
while (i.hasNext())
System.out.println(i.next());
}
}
✅ 28. Overload sum()
class Calc {
int sum(int a, int b) { return a + b; }
int sum(int a, int b, int c) { return a + b + c; }
public static void main(String[] args) {
Calc c = new Calc();
System.out.println(c.sum(2, 3));
System.out.println(c.sum(2, 3, 4));
}
}
✅ 29. Default & parameterized constructors
class Demo {
Demo() { System.out.println(“Default”); }
Demo(int x) { System.out.println(“Parameterized: ” + x); }
public static void main(String[] args) {
new Demo();
new Demo(10);
}
}
✅ 30. Box calculating volume
class Box {
int v;
Box(int l, int b, int h) {
v = l * b * h;
}
void show() { System.out.println(“Volume = ” + v); }
public static void main(String[] args) {
Box b = new Box(2,3,4);
b.show();
}
}