SE450: Java: Types, Interfaces, Concrete and Abstract Classes [9/41]

Java allows for single inheritance in class extension, but multiple inheritance in interface implementation

        class Foo extends Bar implements Baz, Boo {
            // implementation
        }
        

interfaces can be thought of as "pure virtual" abstract classes

Diamond inheritance is supported in Java with interfaces only

abstract classes are classes with abstract methods. They can't be instantiated, and the abstract methods must be implemented by subclasses, unless the subclass is itself abstract

The Type relationships in Java are:

• if class C₁ extends class C₂, then C₁ is a subtype of C₂
• if interface I₁ extends interface I₂, then I₁ is a subtype of I₂
• if class C implements interface I, then C is a subtype of I
• For every interface I, I is a subtype of Object
• For every type T, reference or primitive type, T[] is a subtype of Object
• If type T₁ is a subtype of T₂, then T₁[] is a subtype of type T₂[]

For every class C (that is not Object), C is a subtype of Object. This has implications for every class that you write.

A value of a subtype can appear wherever a value of its supertype is expected. In other words, a value of a subtype can always substitute for a value of its supertype.