Inheritance

• Inheritance: when a subclass shares methods and variables with a superclass

  • The subclass can have its own specific methods and variables

  • Using subclasses prevents repetition

• Special rules: 

  • A subclass can only inherit from one superclass (called the diamond problem)

  • One superclass can have multiple subclasses

• Using this, a class hierarchy can be made

Making a subclass

• To make a subclass: “public class insert subclass name extends insert super class name”

  • Ex: public class X extends Y

    • This would make X a subclass of the superclass Y

  • Inheritance creates an “is-a” relationship between the subclass and superclass

    • Ex: rectangle (subclass) is a quadrilateral (superclass)

Constructors

• Constructors are not inherited from superclasses

• To use a constructor from a superclass, you can call the constructor implicitly or explicitly

• The super keyword (explicit)

  • super is a constructor

  • It lets you use the constructor of the superclass with the parameters of the subclass 

    • Instance variables are initialized

    • This is especially useful when you want to do the same thing the superclass does but with different input variables 

    • This assumes you’re not changing anything about the implementation of the superclass’s method

  • public class firstClass {

    int x = 1;

    }

    


    class secClass extends firstClass {

    int x = 15;

    void printNum() {

    System.out.println(“x is “ + x);

    System.out.println(“x is also “ + super.x);

    }

    }

• When you don’t call the superclass’s constructor explicitly, Java calls the constructor implicitly

  • Java calls the superclass’s constructor with no arguments

• All constructors in the class hierarchy execute beginning with the Object constructor 

  • The Object Class is the superclass of all classes in java.lang (more on this later)

Overriding methods

• When a public method from the subclass has the same method signature as a public method from the superclass

  • Method signature: the combination of the method’s name and its parameters (order of parameters does matter!!)

• Subclasses automatically inherit all public methods from the superclass (these methods stay public)

• @Override is used to indicate that the method directly below it overrides a method from a superclass (@Override is an annotation)

• public class firstClass {

  public String rand() {

  return “banana”;

  }

  


  @Override

  public class secClass extends firstClass {

  public String rand() {

  return “apple”;

  }

  }

Using inheritance hierarchies

• Think of inheritance hierarchies as a waterfall 

  • Change in the top superclass creates changes in all the subclasses beneath it

• Subclasses can have indirect superclasses

  • Ex: say X is a subclass of Y and Y is a subclass of Z

    • X is a subclass of Y and Z

• Polymorphism 

  • Polymorphism states that we can assign an object in subclass X to a reference of type Y (if Y is the superclass)

  • In fact, a reference of type Y can refer to an object of type X or Y

The object superclass 

• The Object class is the superclass of all classes 

• Part of java.lang

• We often overwrite or modify the methods from the Object class (like equals() or toString())