Java Interfaces and Abstract Classes

These are some notes from a java/c class I took 

Interfaces

• There are like abstract classes
• Their default types for methods are public abstract
• None of the methods are implemented
• Fields are automatically assumed to be public, static, and final
• A class that implements an interface can be used as a type to initialize the interface type
• For instance, Playable p = new TicTacToe();
• This forces classes implementing an interface to follow it and classes using these implementers can assume that methods from the interface can be used
• Interfaces are useful when disparate classes need to play common roles
• You cannot extend more than one class, but you can implement multiple interfaces
• Interfaces can be created for each major function, so a child class can pick and choose which interfaces it needs; this is better than using a single interface for multiple functions, since you would have to create dummy functions for ones you don't need

public interface Playable{ //the playable interface is stored in Playable.java like classes
  void play();
}

public class TicTacToe implements Playable{
  public void play(){//code here
  }
}


Abstract Classes

• Are used to define general methods and fields for subclasses
• They cannot be instantiated, but you can have abstract class variables that are assigned to subclassses (of an abstract class)
• Every abstract method has to be overriden
• Not every method has to be abstract
• Private methods cannot be abstract
• The constructor can be implemented (and other methods as well)
• You can have an abstract class with no abstract methods

public abstract class Ball{
public abstract int hit(int batSpeed);
}
public class BaseBall extends Ball{
public int hit(int batSpeed){//code here}
}