Inheritance Note

Hypernym and Hyponym

We use the word hyponym for the opposite type of relationship.

• “dog”: Hypernym of “poodle”, “malamute”, “dachshund”, etc.
• “poodle”: Hyponym of “dog”

Hypernyms and hyponyms comprise a hierarchy.

• A dog “is-a” canine.
• A canine “is-a” carnivore.
• A carnivore “is-an” animal.

Interface

• Interface is a specification of what a class is able to do, not how to do it.

Overriding vs. Overloading

• If a “subclass” has a method with the exact same signature as in the “superclass”, we say the subclass overrides the method.
• Methods with the same name but different signatures are overloaded.
• the @Override annotation: the only effect of this tag is that the code won’t compile if it is not actually an overriding method.

Interface Inheritance

Specifying the capabilities of a subclass using the implements keyword is known as interface inheritance.

• Interface: The list of all method signatures.
• Inheritance: The subclass “inherits” the interface from a superclass.
• Specifies what the subclass can do, but not how.
• Subclasses must override all of these methods!

Implementation Inheritance: Default Methods

Use the default keyword to specify a method that subclasses should inherit from an interface.

• If you don’t like a default method, you can override it.

Static and Dynamic Type, Dynamic Method Selection

Every variable in Java has a “compile-time type”, a.k.a. “static type”.

• This is the type specified at declaration.

Variables also have a “run-time type”, a.k.a. “dynamic type”.

• This is the type specified at instantiation (e.g. when using new).
• Equal to the type of the object being pointed at.

Suppose we call a method of an object using a variable with:

• compile-time type X
• run-time type Y

Then if Y overrides the method, Y’s method is used instead.

• If we have a variable has static type X and dynamic type Y, then if Y overrides the method, Y's method is used instead.
• At compile time, the compiler will verify X has a method that can handle the given parameter. If multiple methods can handle, it records the most specific one.
• At runtime, if Y overrides the recorded signature, use the overridden method.

Implementation Inheritance: Extends

When a class is a hyponym of an interface, we used implements. If you want one class to be a hyponym of another class, you use extends.

• All instance and static variables.
• All methods.
• All nested classes.
  Constructor are not inherited.

Type Checking and Casting

• Compiler allows method calls based on compile-time type of variable.
• Compiler also allows assignments based on compile-time types.

Expressions have compile-time types:

• An expression using the new keyword has the specified compile-time type.
• Method calls have compile-time type equal to their declared type.

Java has a special syntax for specifying the compile-time type of any expression.

• Put desired type in parenthesis before the expression.
  • Compile-time type Dog: maxDog(frank, frankJr);
  • Compile-time type Poodle: (Poodle) maxDog(frank, frankJr);

Subtype Polymorphism

Polymorphism: “providing a single interface to entities of different types”

Interfaces and Abstract Classes

implements and extends can be used to enable interface inheritance and implementation inheritance.

• Interface inheritance: What (the class can do).
• Implementation inheritance: How (the class does it).

Interfaces may combine a mix of abstract and default methods.

• Abstract methods are what. And must be overridden by subclass.
• Default methods are how.
• Unless you use the keyword default, a method will be abstract.
• Unless you specify an access modifier, a method will be public.
• Can provide variables, but they are public static final.
• A class can implement multiple interfaces.
• Cannot be instantiated.
• Java 9 added private methods to interfaces.

Abstract classes are an intermediate level between interfaces and classes.

• Cannot be instantiated.
• Can provide either abstract or concrete methods.
• Use abstract keyword for abstract methods.
• Use no keyword for concrete methods.
• Can provide variables (any kind).
• Can provide protected and package private methods.

Abstract classes are often used as partial implementations as interfaces.

Interfaces:

• Primarily for interface inheritance. Limited implementation inheritance.
• Classes can implement multiple interfaces.

Abstract classes:

• Can do anything an interface can do, and more.
• Subclasses only extend one abstract class.

Generalized Comparison

Comparables

The industrial strength approach: Use the built-in Comparable interface.

Already defined and used by tons of libraries. Uses generics.

public interface Comparable {
     public int compareTo(T obj);
} 

• Lots of built in classes implement Comparable (e.g. String).
• Lots of libraries use the Comparable interface (e.g. Arrays.sort)
• Avoids need for casts.

public class Dog implements Comparable {
    public int compareTo(Dog uddaDog) {
        return this.size - uddaDog.size;
    }

Comparators

In some languages, we’d write two comparison functions and simply pass the one we want :

• sizeCompare()
• nameCompare()

The standard Java approach: Create sizeComparator and nameComparator classes that implement the Comparator interface.

• Requires methods that also take Comparator arguments

public interface Comparator {
    int compare(T o1, T o2);
}

public class Dog implements Comparable {
  private String name;
  private int size;
 
  public static class NameComparator implements Comparator {
    public int compare(Dog d1, Dog d2) {
        return d1.name.compareTo(d2.name);
    }
  }
  ...
}