Java Type System

javatype.md

Java Type System

Concepts

• Statically typed language: variables have definite types, it is a compile-time error to assign a value of an incompatible type to a variable, e.g., Java
• Dynamically typed language: only check type compatiblity at runtime, e.g., JavaScript
• Mutliple Inheritance of State, Implementation, and Type

Types in Java

• Top-level types:
  • Primitive types
  • Reference types
    • Array
    • Class
      • enums are a special kind of of classes, implicitly extend java.lang.Enum
    • Interface
      • annotations are a special kind of interfaces, implicitly extend java.lang.annotation.Annotation
• Nested types:
  • Static member types
  • Nonstatic member classes
  • Local classes
  • Anoymous classes

Inheritance in Java

1. Three types of inheritance are supported for classes: single, multilevel and hierarchical. Let A, B, C being classes

   • single A <-- B
   • multilevel A <-- B <-- C
   • hierarchical (tree) A <-- B, A <-- C

2. Multiple inheritance is NOT supported for classes

   • multiple A <-- C, B <-- C
   • why Java doesn’t support multiple inheritance?
     • the diamond problem: A <-- B, A <-- C, B <-- D, C<-- D
       • if B and C both have a method with same signature foo(), call D.foo will be a problem wether to call B's foo or C's foo method
     • simplicity:
       • multiple inheritance creates problem in casting, constructor chaining and so on.
       • very few scenarios need multiple inheritance

3. Multiple inheritance is supported for Interfaces

   • default methods (introduced in Java 8, before interfaces only have abstract methods)

Generics

• Type parameter <T> always stand in for reference types. It is not possible to use a primitive type as value for a type parameter.
  • bounded type parameter: <T extends Number>
  • mutiple bounds: <T extends A & B & C>
  • subtyping
• Type Inference
• Type Erasure
• Wildcards <?>
  • bounded wildcards: <? extends Number>
  • subtyping

Lambda Expressions