Java Online Compiler

About the Java online compiler

Java is the backbone of enterprise software and Android, and it is what most Indian universities teach for object-oriented programming. This online Java compiler builds and runs your class without any JDK installation, so you can practise anywhere — including from a phone.

Everything lives inside a class with a public static void main(String[] args) entry point. From there you can practise inheritance, interfaces, collections such as ArrayList and HashMap, exception handling, and the Scanner class for reading input.

• →Completing college OOP practicals without a local JDK
• →Practising collections: ArrayList, HashMap, HashSet
• →Learning inheritance and interface implementation
• →Preparing for placement coding rounds

Java sample programs you can run right now

Copy any snippet into the editor above and press Run. Each one is short on purpose — retype it from memory afterwards.

class Animal {
  String name;
  Animal(String name) { this.name = name; }
  void speak() { System.out.println(name + " makes a sound"); }
}

class Dog extends Animal {
  Dog(String name) { super(name); }
  @Override void speak() { System.out.println(name + " barks"); }
}

public class Main {
  public static void main(String[] args) {
    new Dog("Bruno").speak();
  }
}

import java.util.*;

public class Main {
  public static void main(String[] args) {
    Map<String, Integer> marks = new HashMap<>();
    marks.put("Aarav", 78);
    marks.put("Priya", 91);
    for (Map.Entry<String, Integer> e : marks.entrySet())
      System.out.println(e.getKey() + " -> " + e.getValue());
  }
}

Common Java errors and how to fix them

Java syntax cheatsheet

Java compiler FAQs

Keep going after the compiler

Running snippets builds speed; the 17-chapter course builds understanding. Work through the Java chapters, take the quiz, then claim your certificate.

Other compilers: JavaScript compiler, TypeScript compiler, Python 3 compiler, C compiler, C++ compiler, Go compiler, Rust compiler, HTML compiler, CSS compiler, SQLite compiler

Java tutorial: five steps from blank editor to working program

These steps cover the Java you need for school, college and entry-level interviews: classes and main, typed variables, control flow, arrays and collections, objects, and exception handling.

1. 1. Every program lives in a class

   Java runs the static main method of the class you execute. The class name and file name normally match, and printing uses System.out.println.

   public class Main {
       public static void main(String[] args) {
           System.out.println("Hello, Java");
       }
   }

2. 2. Declare the type of every variable

   Java is statically typed: int, double, boolean, char and String each hold a specific kind of value, and the compiler rejects mismatches.

   int marks = 87;
   double percent = marks / 100.0;
   boolean passed = marks >= 50;
   System.out.println(percent + " " + passed);

3. 3. Branch and loop

   if/else, switch, for and while behave as in C-family languages. Integer division truncates, which is why the example above divides by 100.0.

   for (int i = 1; i <= 5; i++) {
       if (i % 2 == 0) System.out.println(i + " even");
       else System.out.println(i + " odd");
   }

4. 4. Store many values with arrays and ArrayList

   An array has a fixed length; an ArrayList grows as you add. Use the ArrayList for anything whose size you do not know upfront.

   import java.util.ArrayList;
   
   public class Main {
       public static void main(String[] args) {
           ArrayList<String> names = new ArrayList<>();
           names.add("Aarav");
           names.add("Diya");
           for (String n : names) System.out.println(n);
       }
   }

5. 5. Model things as objects

   A class groups fields with methods. Keep fields private and expose behaviour through methods — that is encapsulation, the idea most Java interviews start from.

   class Student {
       private final String name;
       private final int marks;
   
       Student(String name, int marks) {
           this.name = name;
           this.marks = marks;
       }
   
       boolean passed() { return marks >= 50; }
       String getName() { return name; }
   }
   
   public class Main {
       public static void main(String[] args) {
           Student s = new Student("Diya", 72);
           System.out.println(s.getName() + " " + s.passed());
       }
   }

Practice exercises with solutions

Try each one in the editor above before opening the solution — the struggle is where the learning happens.

public class Main {
    public static void main(String[] args) {
        int sum = 0;
        for (int i = 1; i <= 100; i++) sum += i;
        System.out.println(sum);
    }
}

public class Main {
    public static void main(String[] args) {
        String s = "coding";
        String out = "";
        for (int i = s.length() - 1; i >= 0; i--) out += s.charAt(i);
        System.out.println(out);
    }
}

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        String text = "code play code learn";
        HashMap<String, Integer> freq = new HashMap<>();
        for (String w : text.split(" "))
            freq.put(w, freq.getOrDefault(w, 0) + 1);
        System.out.println(freq);
    }
}

Why learn Java?

Java is the language most Indian universities and technical interviews use to teach and test object-oriented programming and data structures.

It runs on the JVM, so the same compiled program works on Windows, macOS, Linux and Android without changes.

What Java is used for

• →Android app development
• →Enterprise backends with Spring Boot
• →Data-structures and algorithm practice for interviews
• →Large-scale systems in banking and telecom

Ready for structured practice? The 17-chapter Java course takes these ideas one at a time, with a quiz and a certificate at the end.