Comparison to Java

Some Java issues addressed in Kotlin

Kotlin fixes a series of issues that Java suffers from:

• Null references are controlled by the type system.

• No raw types

• Arrays in Kotlin are invariant

• Kotlin has proper function types, as opposed to Java's SAM-conversions

• Use-site variance without wildcards

• Kotlin does not have checked exceptions

• Separate interfaces for read-only and mutable collections

What Java has that Kotlin does not

• Checked exceptions

• Primitive types that are not classes. The byte-code uses primitives where possible, but they are not explicitly available.

• Static members are replaced with companion objects, top-level functions, extension functions, or @JvmStatic.

• Wildcard-types are replaced with declaration-site variance and type projections.

• Ternary-operator a ? b : c is replaced with if expression.

• Records

• Pattern Matching

• package-private visibility modifier

What Kotlin has that Java does not

• Lambda expressions + Inline functions = performant custom control structures

• Extension functions

• Null-safety

• Smart casts (Java 16: Pattern Matching for instanceof)

• String templates (Java 21: String Templates (Preview))

• Properties

• Primary constructors

• First-class delegation

• Type inference for variable and property types (Java 10: Local-Variable Type Inference)

• Singletons

• Declaration-site variance & Type projections

• Range expressions

• Operator overloading

• Companion objects

• Data classes

• Coroutines

• Top-level functions

• Default arguments

• Named parameters

• Infix functions

• Expect and actual declarations

• Explicit API mode and better control of API surface

What's next?

Learn how to:

• Perform typical tasks with strings in Java and Kotlin.

• Perform typical tasks with collections in Java and Kotlin.

• Handle nullability in Java and Kotlin.