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Inheritance

Single inheritance in Java: superclass and subclass, the is-a relationship, inherited members and protected, the super(…) call and constructor order, the root class Object.

  • 25 cards
  • 4 topics
  • Free
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  1. Concepts
  2. Questions
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  4. Scenarios

A few cards from the deck

  • Inheritance: what is it?

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    The mechanism by which a class takes over the attributes and methods of another, which it can extend or change.

    It avoids copying shared code: Dog extends Animal gives Dog everything Animal does, and Dog only writes what is specific to it.

  • What does running new Puppy(); print?

    ```java
    class Animal {
    Animal() { System.out.println("Animal"); }
    }

    class Dog extends Animal {
    Dog() { System.out.println("Dog"); }
    }

    class Puppy extends Dog {
    Puppy() { System.out.println("Puppy"); }
    }
    `

    • Animal, Dog, Puppy
    • Puppy, Dog, Animal
    • Puppy only
    • Animal, Puppy
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    Animal, Dog, Puppy

    Each constructor starts by calling its parent's, implicitly through super(). Execution climbs to the root, then the bodies run from the most general to the most specialised.

  • In Java, a class declares its parent class with the keyword … and can have only ….

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    In Java, a class declares its parent class with the keyword extends and can have only one.

    That is single inheritance. To take over several contracts, a class can however implement several interfaces.

  • Superclass: what is it?

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    A class from which another derives, and whose accessible members the latter takes over.

    Also called parent class. In class Dog extends Animal, Animal is the superclass: it knows nothing about Dog.

  • To reuse its storage, a developer wants the Stack class to inherit from the Array class. What should you answer?

    • Java forbids inheriting from a class used as internal storage
    • A stack is not an array: it should contain one, not inherit from it
    • Inheritance is only possible from an abstract class
    • A subclass cannot add methods to those of its parent
    Show the answer

    A stack is not an array: it should contain one, not inherit from it

    To inherit is to say "is a": the Stack would inherit index access, which breaks its last-in, first-out principle. Code reuse is done by composition: an attribute of type Array.

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