lec08.md

Inheritance, Implements

Hypernyms, Hyponyms, & Interface Inheritance

• Define reference type (interface) for hypernym (List61B.java)
• public class SLList<Blorg> implements List61B<Blorg>
  • Type parameters have to match, even if List61B<Item> interface has different name
• Mark each method from List61B w/ @Override annotation

Interface Inheritance

• Powerful tool for generalizing code

List61B<String> someList = new SLList<String>();
someList.addFirst("elk");

• When run, SLList created & address stored in someList variable
  • String "elk" inserted into SLList referred to by addFirst
  • List61B object will access/use implemented methods from corresponding subclass
  • Can be set to new SLList<>() as well, String not necessary

Default Methods & For Loops

Implementation Inheritance

• Use default keyword to specify method that subclasses should inherit from interface
• default public void print() {}
• default method of interface can't access instance variables (b/c no instance variables in interface), have to use methods already specified in interface to implement
  • Can override default methods

Dynamic Method Selection

Static Type vs Dynamic Type

• Every variable in Java has "compile-time type"/"static type"
  • Type specified at declaration, never changes
• Variables also have "run-time type"/"dynamic type"
  • Type specified at instantiation (e.g. when using new)
  • Equal to type of object being pointed at

public static void main(String[] args) {
    LivingThing lt1; // Static = LivingThing, Dynamic = null
    lt1 = new Fox(); // Static = LivingThing, Dynamic = Fox

    // Dynamic type of a1 set to same Dynamic type as lt1
    Animal a1 = lt1; // Static = Animal, Dynamic = Fox

    // Dynamic type of h1 set to same Dynamic type as a1
    Fox h1 = a1; // Static = Fox, Dynamic = Fox
    lt1 = new Squid(); // Static = LivingThing, Dynamic = Squid
}

Variable	Static Type	Dynamic Type
lt1	LivingThing	Squid
a1	Animal	Fox
h1	Fox	Fox

• When calling method of object using variable with:
  • Compile-time type X
  • Run-time type Y
• If Y overrides method, Y's method used instead
  • Known as "dynamic method selection"

Overloaded Methods

• When overloading, dynamic method selection not used → static type used instead

public static void peek(List61B<String> list) {
    System.out.println(list.getLast());
}

public static void peek(SLList<String> list) {
    System.out.println(list.getFirst());
}

SLList<String> SP = new SLList<String>();
List61B<String> LP = SP;
SP.addLast("elk");
SP.addLast("elk");
SP.addLast("cool");
peek(SP); // elk
peek(LP); // cool

Overloading vs Overriding

• Dynamic method selection only happens for overriden methods
  • When instance method of subtype overrides method in supertype, uses most specialized method available
• Dynamic method selection does not happen for overloaded methods:
  • When class has two or more methods, one for supertype, one for subtype

Interface vs Implementation Inheritance

• Interface Inheritance (what):
  • Allows generalization of code in powerful, simple way
• Implementation Inheritance (how):
  • Allows code-reuse: subclasses can rely on superclasses/interfaces
    • Gives another dimension of control to subclass designers: can decide whether or not to override default implementations
• Both cases = "is-a" relationships, not "has-a"

Dangers of Implementation Inheritance

• Harder to keep track of where something was actually implemented
• Rules for resolving conflicts can be arcane
  • E.g. two interfaces both give conflicting default methods
• Encourages overly complex code
• Breaks encapsulation