Online Java Compiler – Run and Test Java Code in Your Browser

Write, compile, and run Java code instantly in your browser with our free online Java compiler. No installation needed — ideal for learning, testing, and quick prototyping.

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💡 Java Basics Guide for Beginners

1. Declaring Variables and Constants

Java requires you to declare the type of each variable. Use final for constants.

int age = 30;
double pi = 3.14159;
char grade = 'A';
String name = "Alice";
boolean isActive = true;

// Constants
final int MAX_USERS = 100;
final String COMPANY = "CodeUtility";

2. Conditionals (if / switch)

Use if, else if, and switch for control flow.

int x = 2;
if (x == 1) {
    System.out.println("One");
} else if (x == 2) {
    System.out.println("Two");
} else {
    System.out.println("Other");
}

switch (x) {
    case 1:
        System.out.println("One");
        break;
    case 2:
        System.out.println("Two");
        break;
    default:
        System.out.println("Other");
}

3. Loops

Use for, while, and do-while for iteration.

for (int i = 0; i < 3; i++) {
    System.out.println(i);
}

int n = 3;
while (n > 0) {
    System.out.println(n);
    n--;
}

4. Arrays

Arrays hold fixed-size sequences of the same type.

int[] numbers = {10, 20, 30};
System.out.println(numbers[1]);

5. ArrayList Manipulation

Use ArrayList for dynamic-sized lists.

import java.util.ArrayList;

ArrayList<Integer> nums = new ArrayList<>();
nums.add(1);
nums.add(2);
nums.add(3);
nums.remove(Integer.valueOf(2));

for (int num : nums) {
    System.out.print(num + " ");
}

6. Console Input/Output

Use Scanner for input and System.out for output.

import java.util.Scanner;

Scanner scanner = new Scanner(System.in);
System.out.print("Enter your name: ");
String name = scanner.nextLine();
System.out.println("Hello, " + name);

7. Functions

Define methods with return types and parameters.

public static int add(int a, int b) {
    return a + b;
}

System.out.println(add(3, 4));

8. HashMaps

Use HashMap for key-value storage.

import java.util.HashMap;

HashMap<String, Integer> ages = new HashMap<>();
ages.put("Alice", 30);
System.out.println(ages.get("Alice"));

9. Exception Handling

Use try and catch to handle runtime exceptions.

try {
    throw new Exception("Something went wrong");
} catch (Exception e) {
    System.out.println(e.getMessage());
}

10. File I/O

Use Files and Paths from java.nio.file for file handling.

import java.nio.file.*;
import java.io.IOException;

Files.writeString(Paths.get("file.txt"), "Hello File");
String content = Files.readString(Paths.get("file.txt"));
System.out.println(content);

11. String Manipulation

Java strings support many methods like length(), substring(), and contains().

String text = "Hello World";
System.out.println(text.length());
System.out.println(text.substring(0, 5));
System.out.println(text.contains("World"));

12. Classes & Objects

Java supports object-oriented programming using classes and instances.

class Person {
    String name;
    Person(String name) {
        this.name = name;
    }
    void greet() {
        System.out.println("Hi, I'm " + name);
    }
}

Person p = new Person("Alice");
p.greet();