接口和继承如何影响Java中类与对象之间的关系？

Question

As I continue to expand my knowledge of OOP programming, it's becoming a bit more difficult to tie all the concepts back together. Separately, I know what an object, class, interface, and inheritance is. However, I fail to see "the big picture."

An object embodies a specific concept (class) and stores specific data to that class. It's an instance of a class, to be more specific.

A class defines the common set of attributes and behaviors among all objects of its class ("the template")

Interfaces are essentially a to-do list of method headers. It expects its children to implement these abstract methods. It does this in an attempt to create a standard way of doing something in unrelated classes (ex: compareTo() in Comparable)

Inheritance allows subclasses to extend the design space for objects by adding more functionality and behaviors while still retaining that of the superclass.

All these concepts affect the way objects interact with classes, but I fail to see how inheritance and interfaces have anything to do with objects. This is a rather conceptual question, but what exactly is this "big picture?" How do all these concepts affect the relationship between classes and objects?

Answer 1

Inheritance is the OO concept which has many types like Single, Hybrid etc. Java supports Multiple inheritance in the way of Interface. Interface allows you to define the behaviour or functionality in abstract level. It leaves the children to implement those behaviour or functionality. For eg(given in official docs): You can define the behaviour of a Bicycle like gearUp, gearDown, pedal, break, etc., and let the brands to implement their style and functionality. Below is the java doc link. https://docs.oracle.com/javase/tutorial/java/concepts/interface.html

Answer 2

Inheritance in simple terms is to acquire the properties / behaviours of an other class. In order to achieve Inheritance we have to use a keyword extends in JAVA.

Class B extends Class A means Class B will act just like class A and all the methods and Variables in class A can be referred using class B also. ie. Whatever class A possesses it is shared by class B too.

Whereas Interface is a Type Definition Block (User-Defined). An abstract (incomplete) class can be called Interface. An Interface allows only Incomplete methods. In order to Inherit an Interface we have a keyword implements in JAVA. If a Class implements an Interface it is bound to contract.

That is: Either the class implementing the Interface should be declared abstract OR the class should over-ride the method in Interface and Complete it.( ie. provide method body).

Example:

Interface Car

{ public void display(); //Incomplete Method, Method has no Implementation.

}

class B implements Car

{ public void display()

{ System.out.println(“HI”);

} }

Here display method is over-rided and completed in class B. Hence no need to declare class B as abstract.

NOTE: Multiple Inheritance using class is not possible in JAVA. ie. A child class can extend or have only one parent class.

Multiple Inheritance is possible in JAVA using Interfaces. ie. An interface can extend more than one interface / implement more than one interface.

Interface Objects cannot be created, because it is incomplete.

Interface methods should be made non-static, because it is incomplete and should be over-rided in order to make it complete. If interface methods are static, it cannot be over-rided, because only one copy of static members are present and that is loaded during class loading.