What is the difference between java and kotlin ?

Overview

Java is a general-purpose OOP language. Kotlin is a modern JVM language developed by JetBrains that compiles to the same bytecode as Java — making them 100% interoperable while offering major improvements.

Quick Comparison

val x = 5

data class

Conciseness Comparison

javaCopy
// Java — 15 lines for a simple model
public class Person {
    private final String name;
    private final int age;
    public Person(String name, int age) { this.name = name; this.age = age; }
    public String getName() { return name; }
    public int getAge() { return age; }
    @Override public String toString() { return "Person(" + name + ", " + age + ")"; }
}

kotlinCopy
// Kotlin — 1 line
data class Person(val name: String, val age: Int)

Type Inference

javaCopy
// Java — explicit types
ArrayList<String> list = new ArrayList<String>();

kotlinCopy
// Kotlin — type inferred
val list = mutableListOf<String>()
val name = "Alice" // Kotlin knows it's a String

Functional Programming

kotlinCopy
// Kotlin — clean lambdas and higher-order functions
val numbers = listOf(1, 2, 3, 4, 5)
val evens = numbers.filter { it % 2 == 0 }  // [2, 4]
val doubled = evens.map { it * 2 }           // [4, 8]
val sum = doubled.sum()                       // 12

Sealed Classes (Pattern Matching)

kotlinCopy
sealed class Result<out T> {
    data class Success<T>(val data: T) : Result<T>()
    data class Error(val message: String) : Result<Nothing>()
    object Loading : Result<Nothing>()
}

fun handleResult(result: Result<User>) = when (result) {
    is Result.Success -> showUser(result.data)
    is Result.Error -> showError(result.message)
    Result.Loading -> showSpinner()
}

Java-Kotlin Interoperability

kotlinCopy
// Kotlin can call Java code directly
val list = java.util.ArrayList<String>()
list.add("Hello")

// Java can call Kotlin code with @JvmStatic, @JvmField
class Utils {
    companion object {
        @JvmStatic fun greet(name: String) = "Hello, $name"
    }
}
// From Java: Utils.greet("Alice")

Summary: Kotlin is a better Java — it compiles to the same JVM bytecode, works with all Java libraries, but eliminates NullPointerExceptions, requires far less code, and adds modern features like coroutines, extension functions, and sealed classes.