Interfaces

An Interface is like a contract, it guarantees the preconditions required to produce the desired postconditions.

• Like an extreme case of an abstract class.
• Java does not support multiple inheritance– interfaces can be used as a means to implement their behaviour.
• Specifies a set of methods that any class that implements the interface must have.
  • Contains method headings and constant definitions only.
  • Contains no instance variables nor any complete method definitions.
• Must be declared as public.
• Interfaces are also a type.
  • A method may be written with a parameter of an interface type.
  • That parameter will accept as an argument any class that implements the interface.
• Use the interface keyword instead of class within its definition.
• Methods implementing it must use the implements modifier.
  • A method (including abstract methods) can implement multiple interfaces.
  • If multiple interfaces are implemented they can’t have conflicting method definitions.
• Interfaces may be also be derived by including the extends keyword in its definition.

Comparable Interface

• Allows collections that implement the interface to be automatically sorted by Collections.sort or Arrays.sort.
• Objects that implement this interface can be used as keys in a map or set without the need to specify a comparator.
• Only has one method heading that must be implemented: public int compareTo (Object other)
• Same kind of behavior as C’s strcmp; compareTo must return…
  • A negative if the calling object comes before.
  • Zero if the calling object is equal.
  • A positive number if the calling object comes after.
• If the parameter other is not of the same type as the class being defined then a ClassCastException should be thrown.

Iterator Interface

• public interface Iterator<E>
• next throws NoSuchElementException if the iteration has no more elements.
  • You probably don’t want to call next more than once per loop.
• Implemented by all Collections.

Return	Method	Description
boolean	hasNext()	Returns true if iterations has more elements.
E	next()	Returns the next element in the iteration.
void	remove()	Removes last element returned by iterator.