Master Kotlin Quickly: Side-by-Side Java vs Kotlin Syntax Guide

We’ve compiled a GitHub project “from-java-to-kotlin” that demonstrates side‑by‑side syntax differences between Java and Kotlin, helping Java‑proficient developers transition smoothly.

Logging

// Java
System.out.print("Amit Shekhar");
System.out.println("Amit Shekhar");
// Kotlin
print("Amit Shekhar")
println("Amit Shekhar")

Constants and Variables

// Java
String name = "Amit Shekhar";
final String name = "Amit Shekhar";
// Kotlin
var name = "Amit Shekhar"
val name = "Amit Shekhar"

Null Declaration

// Java
String otherName;
otherName = null;
// Kotlin
var otherName: String?
otherName = null

For Loops

// Java
for (int i = 1; i <= 10; i++) { }
for (int i = 1; i < 10; i++) { }
for (int i = 10; i >= 0; i--) { }
for (int i = 1; i <= 10; i += 2) { }
for (int i = 10; i >= 0; i -= 2) { }
for (String item : collection) { }
for (Map.Entry<String, String> entry : map.entrySet()) { }
// Kotlin
for (i in 1..10) { }
for (i in 1 until 10) { }
for (i in 10 downTo 0) { }
for (i in 1..10 step 2) { }
for (i in 10 downTo 0 step 2) { }
for (item in collection) { }
for ((key, value) in map) { }

Method Definitions

// Java
void doSomething() {
    // logic here
}
void doSomething(int... numbers) {
    // logic here
}
// Kotlin
fun doSomething() {
    // logic here
}
fun doSomething(vararg numbers: Int) {
    // logic here
}

The project focuses on practical code examples rather than theory, making the differences clear and easy to grasp.