uni/year2/semester1/logseq-stuff/pages/Introduction to Inheritance.md

• #CT2106 - Object-Oriented Programming
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• Object Equality #card

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  • When you use == with reference variables, you are checking if the variables point to the same object.
    • So, using == on strings will only return true if the Strings are references to the same object. It will return to false even if the strings contain the same data.
    • The value of a string variable is the memory location where its String object is stored.
  • When checking for equality between objects, you must use the equals method.
    • The equals method is an instance method that ^^all objects of built-in classes have.^^
      • However, for any class that you define, you will have to write your own equals method.
        • All equals methods must have the following method signature:

          • public boolean equals(Object object)
            

    • Its specific purpose is to define equality between objects.
    • It returns a boolean value.
    • It is commutative.
      • str1.equals(str4) returns the same value as str4.equals(str1).
    • Example:

      • String str1 = "Java";
        String str2 = "Ja";
        String str3 = "va";
        String str4 = str2 + str3;
        
        str1.equals(str4) ? System.out.println("true") : System.out.println("false");
        
        

• instanceof #card

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  • instanceof is an operator that is used to determine if a variable is pointing to an object with a particular type.

    • System.out.println(bike2 instanceof Bicycle ? "true" : "false");
      

• Object

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  • What is the type of Object obj? #card card-last-interval:: 4 card-repeats:: 2 card-ease-factor:: 2.7 card-next-schedule:: 2022-11-18T16:42:30.590Z card-last-reviewed:: 2022-11-14T16:42:30.590Z card-last-score:: 5
    • obj is a variable whose type is java.lang.Object.
  • What is java.lang.Object? #card card-last-interval:: 4 card-repeats:: 2 card-ease-factor:: 2.7 card-next-schedule:: 2022-11-18T16:42:35.503Z card-last-reviewed:: 2022-11-14T16:42:35.504Z card-last-score:: 5
    • java.lang.Object is a class that provides the ^^most generic definition^^ of an object in Java.
    • It is the parent class of every class in Java.
    • For example. A Bicycle object is a Bicycle object and a java.lang.Object object.

• Casting

  • Bicycle bike1 = (Bicycle) myObject;
    String str1 = (String) obj;
    

    • Here, we can cast (convert) a variable from a higher type (Object), to a lower type (Bicycle).
      • This is allowed, as anObject point to a Bicycle object - we can check this using instanceof.
      • obj points to a String object - we can check this using instanceof.
  • Note that the variable type being converted is ^^not the object.^^

• Class Hierarchy

  • Is-a Relationships

    • Java organises all its classes in a class hierarchy.
      • For example, a car is a type of vehicle, which is a type of object.
    • These relationships can be described as "is-a" relationships.
      • A car is-a vehicle; a vehicle is-a(n) object.
    • We refer the higher-up types as parents and the lower types as children.
      • Car is-a child of Vehicle.
      • Vehicle is-a parent of Car.
      • Object is the parent of Vehicle & Car.

  • Key Ideas in Class Hierarchy

    • The top of the hierarchy represents the ^^most generic attributes & behaviours.^^
    • The bottom (sometimes referred to as "leaves") represent the ^^most specific attributes & behaviours.^^
    • Each level inherits and customises the attributes & behaviours from the level above it.
    • java.lang.Object is the superclass, the parent of all classes in Java.
    • Every class in Java has the java.lang.Object as its superclass (parent).
    • 
      • All the classes shown above inherit (receive) methods from the superclass java.lang.Object.
        • What is OOP Inheritance? #card card-last-interval:: 5.52 card-repeats:: 3 card-ease-factor:: 2.46 card-next-schedule:: 2022-11-20T04:38:03.406Z card-last-reviewed:: 2022-11-14T16:38:03.406Z card-last-score:: 5
          • Inheritance is the means by which objects automatically receive features (fields) & behaviours (methods) from their superclass.
      • The methods of this superclass are available to all objects of this Class, even though these methods may not be shown in the Class code.
        • For example: .equals().

        • Generic Methods

          • All the methods provided by the java.lang.Object are generic.
            • They only relate to java.lang.Object classes, not the subclasses.
            • When a subclass inherits these methods, it needs to customise them.
              • This is why we had to override .equals() with our own version for the example Bicycle class.

  • Overriding

    • What is overriding? #card card-last-interval:: 29.99 card-repeats:: 4 card-ease-factor:: 2.56 card-next-schedule:: 2022-12-14T19:04:47.436Z card-last-reviewed:: 2022-11-14T20:04:47.437Z card-last-score:: 5
      • Overriding is when you write your own version of a method that you have inherited from a superclass.
        • It is creating a specific version of a method inherited from a parent (superclass) class.
      • When overriding a method, you must keep every part of the method signature the same - You can only change the code in the method body.
        • Its name, its parameter types & order, its access level (e.g., public, protected), and its return type.

    • Annotation

      • It is good practice to annotate your overridden methods using @Override.
        • You code will compile & run without it, but it is considered good practice to annotate the methods that are overridden inherited from the superclass.

      • @Override
        public boolean equals(Object obj)
        {
          obj == null ? return false;
        
          if (obj instanceof Bicycle)
          {
            Bicycle bike = (Bicycle) obj;
            if (this.speed == bike.getSpeed() && this.gear == bike.getGear())
            {
              return true;
            }
          }
          return false;
        }