Lecture 4 (06-02)

for super, if there is no override in the current context, u wouldnt need to do super.method cuz there’s no ambiguity

All classes extend Object.

Object == Object compares the pointer values, not any kind of “value” like primitive types

• .equals can be overridden and changes how an object can be compared to another one
  • For instance, Strings (which are reference types — objects — by the way).
  • Doing str1 == str2 compares the addresses. Doing str1.equals(str2) compares the values of the strings depending on how .equals is implemented

By default .equals takes another object and will have the == implementation and return reference equality. However you can make it your own by overriding .equals

You cannot call methods / fields on null objects.

//Within pointer.java
@OVerride
public boolean equals(Object obj) {
	// ensure the object is not null so we can access its properties
	if (obj == null) return false;
	// To make typecasting safe to do!
	if (!(obj instanceof Point)) return false;
	// Typecast to access "point" related fields
	Point other = (Point) obj;
	return x == other.x && y == other.y;
}

any call of .equals from a object of type Point will use the overridden implementation.

• Remember the “ladder”; if Point doesn’t implement .equals, Java will check the parent, then it’s parent, etc. etc. until something implements .equals. If nothing does, it will be a syntax error (but that’s not possible since classes all implement Object and thus always has some implementation of .equals)

Equals method contract (General Contracts)

What be the equals method contract?

• Reflexive: For any non-null x, x.equals(x) should be True
• Symmetric: x.equals(y) is the same as y.equals(x)
• Transitive: If x.equals(y) is true and y.equals(z) is true, then x.equals(z) MUST be true
• Consistent: Calling x.equals(y) several times should give a consistent result
• x.equals(null) should return false

Hashing with .hashCode()

• Used for retrieval (think hash tables)
  • Think “indices”
• By default, Object returns the memory address of the object
• You can override it in all classes that overrides equals
  • And you should override it when you override equals because you might get unintended behaviour with hashCode in future scenarios
  • Because equal objects should have equal hashcodes
• A good .hashCode() usually gives an unequal hash for unequal objects
  • Instead of sequentially searching for an object, you can index it with the hashcode.
  • However, if many different objects return the same hash code, it would not be an easy O(1) index-and-done.
    • So it’s best to avoid collisions

@Override
public int hashCode() {
	return 0;
}

The above is legal, but lousy (no better than sequential searching)

toString()

yayayayayaya

Polymorphism

• Every instance of a subclass is also an instance of its superclass (not the other way around though)
• Has to do with “an object assuming many shapes”

Polymorphism: A object of a subclass can be used wherever its superclass object is used

Dynamic Binding

• A method is dynamically binded to an object depending on what instance the object is a type of.

Encapsulation

• Details of implementation are encapsulated / hidden from outside. Information hiding.

  • I.e. by using private access modifier, a method cannot be accessed from outside of the object

• There are 4 visibilities and 3 keywords:

  • Public: Accessible anywhere
  • Private: Only accessible in the object
  • Protected: Only accessible in children
  • Package-private (a field can be accessed anywhere within the package; otherwise private. This is the default)

• Rule of thumb: Keep things as private as possible

Abstract Classes

• Cannot have private memberstodo iirc. But maybe virtual methods?

Concrete = not abstract

• Abstract methods have no implementation / they are abstract descriptions of methods.
• If there is at least one abstract method, declare the class as abstract
  • You can still have concrete methods though.

Interfaces

• All methods are abstract so you dont need the abstract keyword

public interface Edible {
	public abstract void foo();
	
	final int x = 5;
}

• You can have final fields though (basically constants)

• Usually is a high-level description of “a behaviour”

• When implementing an interface, you must define every abstract method (same w/ abstract classes)

UML diagram

• dotted line and triangle = implements
• solid line and triangle = extends
• Italics = abstract (so Animal, Edible, and Fruit are abstract)
• • = public
• • = private
• # = protected