Comparison to Java Programming Language

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

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 - replaced with companion objects, top-level functions, extension functions, or @JvmStatic.
• Wildcard-types - replaced with declaration-site variance and type projections.
• Ternary-operator a ? b : c - replaced with if expression.

What Kotlin has that Java does not

• Lambda expressions + Inline functions = performant custom control structures
• Extension functions
• Null-safety
• Smart casts
• String templates
• Properties
• Primary constructors
• First-class delegation
• Type inference for variable and property types
• Singletons
• Declaration-site variance & Type projections
• Range expressions
• Operator overloading
• Companion objects
• Data classes
• Separate interfaces for read-only and mutable collections
• Coroutines