In Kotlin, variable declarations look a bit different than Java's:

val i : Int = 42

• They start with either val or var, making the declaration final ("value") or variable.

• The type is noted after the name, separated by a :

• Thanks to Kotlin's type inference the explicit type declaration can be obmitted if there is an assignment with a type the compiler is able to unambigously detect

Java	Kotlin
int i = 42;	var i = 42 (or var i : Int = 42)
final int i = 42;	val i = 42

• Kotlin does not need ; to end statements
• Kotlin is null-safe
• Kotlin is 100% Java interoperable
• Kotlin has no primitives (but optimizes their object counterparts for the JVM, if possible)
• Kotlin classes have properties, not fields
• Kotlin offers data classes with auto-generated equals/hashCode methods and field accessors
• Kotlin only has runtime Exceptions, no checked Exceptions
• Kotlin has no new keyword. Creating objects is done just by calling the constructor like any other method.
• Kotlin supports (limited) operator overloading. For example, accessing a value of a map can be written like: val a = someMap["key"]
• Kotlin can not only be compiled to byte code for the JVM, but also into Java Script, enabling you to write both backend and frontend code in Kotlin
• Kotlin is fully compatible with Java 6, which is especially interesting in regards for support of (not so) old Android devices
• Kotlin is an officially supported language for Android development
• Kotlin's collections have built-in disctinction between mutable and immutable collections.
• Kotlin supports Coroutines (experimental)

Kotlin uses == for equality (that is, calls equals internally) and === for referential identity.

Java	Kotlin
a.equals(b);	a == b
a == b;	a === b
a != b;	a !== b

See: https://kotlinlang.org/docs/reference/equality.html

In Kotlin, if, try and others are expressions (so they do return a value) rather than (void) statements.

So, for example, Kotlin does not have Java's ternary Elvis Operator, but you can write something like this:

val i = if (someBoolean) 33 else 42

Even more unfamiliar, but equally expressive, is the try expression:

val i = try {
    Integer.parseInt(someString)
}
catch (ex : Exception)
{
    42
}