In this tutorial we will discuss about the inheritance in Java. The most fundamental element of Java is the class. A class represents an entity and also, defines and implements its functionality. In Java, classes can be derived from other classes, in order to create more complex relationships.
A class that is derived from another class is called subclass and inherits all fields and methods of its superclass. In Java, only single inheritance is allowed and thus, every class can have at most one direct superclass. A class can be derived from another class that is derived from another class and so on. Finally, we must mention that each class in Java is implicitly a subclass of the Object
class.
Suppose we have declared and implemented a class A. In order to declare a class B that is derived from A, Java offers the extendkeyword that is used as shown below:
2 |
//Members and methods declarations. |
6 |
//Members and methods from A are inherited. |
7 |
//Members and methods declarations of B. |
Java supports only public inheritance and thus, all fields and methods of the superclass are inherited and can be used by the subclass. The only exception are the private members of the superclass that cannot be accessed directly from the subclass. Also, constructors are not members, thus they are not inherited by subclasses, but the constructor of the superclass can be invoked from the subclass. In order to call the constructor of the superclass Java provides the keyword super, as shown below:
03 |
System.out.println( "New A" ); |
10 |
System.out.println( "New B" ); |
A sample example to present the inheritance in Java is shown below:
Animal.java:
03 |
System.out.println( "A new animal has been created!" ); |
07 |
System.out.println( "An animal sleeps..." ); |
11 |
System.out.println( "An animal eats..." ); |
Bird.java:
01 |
public class Bird extends Animal { |
04 |
System.out.println( "A new bird has been created!" ); |
09 |
System.out.println( "A bird sleeps..." ); |
14 |
System.out.println( "A bird eats..." ); |
Dog.java:
01 |
public class Dog extends Animal { |
04 |
System.out.println( "A new dog has been created!" ); |
09 |
System.out.println( "A dog sleeps..." ); |
14 |
System.out.println( "A dog eats..." ); |
MainClass.java:
01 |
public class MainClass { |
02 |
public static void main(String[] args) { |
03 |
Animal animal = new Animal(); |
04 |
Bird bird = new Bird(); |
In this example we created three distinct classes, Animal
, Dog
and Bird
. Both Dog
and Bird
classes extend the Animal
class and thus, they inherit its members and methods. Moreover, as we can see below, each class overrides the methods of Animal
and thus, both the Dog and Bird classes redefine the functionality of Animal’s
methods.
A sample execution is shown below:
A new animal has been created!
A new animal has been created!
A new bird has been created!
A new animal has been created!
A new dog has been created!
An animal sleeps...
An animal eats...
A bird sleeps...
A bird eats...
A dog sleeps...
A dog eats...
A nested class has access to all the private members of its enclosing class, both fields and methods. Therefore, a public or protected nested class inherited by a subclass has indirect access to all of the private members of the superclass.
As already mentioned, a subclass inherits all of the public and protected members of its superclass. If the subclass is in the same package as its superclass, it also inherits the package-private members of the superclass. The inheritance in Java provides the following features:
- You can declare a field in the subclass with the same name as the one in the superclass thus, hiding it. This is called shadowing.
- You can declare new fields in the subclass that are not in the superclass.
- You can write a new instance method in the subclass that has the same signature as the one in the superclass, thus overriding it.
- You can declare new methods in the subclass that are not in the superclass.
Final abstract classes can exist in a hierarchy of types. For more information about abstract classes and how are used in Java, please refer to the Java abstract tutorial here.
Inheritance and Casting
When a class B extends a class A, then an instance of the B class is of type B, but also of type A. Thus, such an instance can be used in all cases where a class B or class A object is required. However, the reverse is not true! An instance of the class A is of course of type A, but it is not of type B.
Thus, we can use casting between the instances of classes. The cast inserts a runtime check, in order for the compiler to safely assume that the cast is used properly and is correct. If not, a runtime exception will be thrown.
A simple example that demonstrates the usage of casting is shown below:
2 |
Animal a2 = new Bird(); |
7 |
// The following statements are incorrect. |
8 |
// Dog d = new Animal(); |
9 |
// Bird b = new Animal(); |
A sample execution is shown below:
A dog eats...
A bird sleeps...
The instanceof operator
The instanceof operator can be used, in order to determine if an object is a valid instance of a specific type. It can be used to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface. A simple example is shown below:
2 |
if (d instanceof Animal) { |
Interfaces
An interface in Java is an abstract type that is used to declare and specify a set of public methods and members. An interface can be implemented by a class. In this case, the class must provide an implementation for every method defined in the interface. A significant advantage of using interfaces is the fact that in Java, multiple interfaces can be implemented by a single class.
A sample example that uses both classes and multiple interfaces is shown below:
BasketballTeam.java:
1 |
public interface BasketballTeam { |
2 |
public void printBasketballName(); |
FootballTeam.java:
1 |
public interface FootballTeam { |
2 |
public void printFootballName(); |
Team.java:
01 |
public class Team implements BasketballTeam, FootballTeam { |
03 |
private String name = null ; |
05 |
public Team(String name) { |
10 |
public void printFootballName() { |
11 |
System.out.println( "Football Team: \"" + name + " F.C.\"" ); |
15 |
public void printBasketballName() { |
16 |
System.out.println( "Basketball Team: \"" + name + " B.C.\"" ); |
19 |
public static void main(String[] args) { |
20 |
Team t = new Team( "Team A" ); |
21 |
t.printBasketballName(); |
22 |
t.printFootballName(); |
A sample execution is shown below:
Basketball Team: "Team A B.C."
Football Team: "Team A F.C."
- Summary: Java中函数参数的传递
函数调用参数传递类型(java)的用法介绍. java方法中传值和传引用的问题是个基本问题,但是也有很多人一时弄不清. (一)基本数据类型:传值,方法不会改变实参的值. public class Te ...
- SUMMARY | JAVA中的数据结构
String String类是不可修改的,创建需要修改的字符串需要使用StringBuffer(线程同步,安全性更高)或者StringBuilder(线程非同步,速度更快). 可以用“+”连接Stri ...
- Java longTime 和C#日期转换(结构+运算符重载)
前几天,因为工作原因,连到了公司的一个java系统.查看数据的时候,突然整个人都不好了,数据库中日期字段时间为毛都是整型?之前从来没有接触过java,所心就趁机了解了一下.原来,在数据库中,保存的是j ...
- JavaScript- The Good Parts Chapter 5 Inheritance
Divides one thing entire to many objects;Like perspectives, which rightly gazed uponShow nothing but ...
- Java Interview Reference Guide--reference
Part 1 http://techmytalk.com/2014/01/24/java-interview-reference-guide-part-1/ Posted on January 24, ...
- 【DateStructure】 Charnming usages of Map collection in Java
When learning the usage of map collection in java, I found serveral beneficial methods that was enco ...
- java url demo
// File Name : URLDemo.java import java.net.*; import java.io.*; public class URLDemo { public stati ...
- Java 8 Date-Time API 详解
从Java版本1.0开始就支持日期和时间,主要通过java.util.Date类. 但是,Date类设计不佳. 例如,Date中的月份从1开始,但从日期却从0开始.在JDK 1.1中使用它的许多方法已 ...
- 关于Java中基类构造器的调用问题
在<Java编程思想>第7章复用类中有这样一段话,值得深思.当子类继承了父类时,就涉及到了基类和导出类(子类)这两个类.从外部来看,导出类就像是一个与基类具有相同接口的新类,或许还会有一些 ...
随机推荐
- linux下 php 安装mysql的扩展模块
1.安装mysql-devel包 [root@DBproxy ~]# yum install mysql-devel 注:该包必须在编译php之前安装好,否则在安装php的mysql扩展模块是会碰到各 ...
- Windows平台编译SQLite 3
由于需要sqlite的x64版本只能自己编译,下载sqlite源代码.sqlite.def.Visual Studio 2013新建一个Visual C++ Empty Project,Configu ...
- .sh 的运行
cat *.sh 看一下你的那个sh文件 看第一行是#!/bin/bash 还果 #!/bin/sh 如果是/bin/bash就执行 bash your.sh 如果是/bin/sh 就执行 sh yo ...
- Office2010安装需要MSXML版本6.10.1129.0的方法
今天给朋友装Office2010,由于朋友之前使用的是绿化版的0ffice2007,所以卸载后安装Office遇到了若要安装Office2010,需要在计算机上安装MSXML版本6.10.1129.0 ...
- npm使用报错解决办法
在使用脚手架工具进行项目搭建的时候,很多时候会用到npm ,最近用npm的时候遇到一个错误: 'CALL "I:\Program Files\nodejs\\node.exe" & ...
- python中filter(),map()和reduce()的用法及区别
先看filter()方法 print(list(filter(lambda n : n % 2 == 1, range(20))))# 结果 [1, 3, 5, 7, 9, 11, 13, 15, 1 ...
- windows远程访问ubuntu下的jupyter notebook必要配置
0.生成配置文件(一般采用默认) jupyter notebook --generate-config 1.打开ipython, 创建一个密文密码 In [1]: from notebook.auth ...
- POJ 3067 - Japan - [归并排序/树状数组(BIT)求逆序对]
Time Limit: 1000MS Memory Limit: 65536K Description Japan plans to welcome the ACM ICPC World Finals ...
- Oracle 分析函数的使用(主要是rollup用法)
分析函数是oracle 8.1.6中就引入的一个全新的概念,为我们分析数据提供了一种简单高效的处理方式.在分析函数出现以前,我们必须使用自联查询,子查询或者内联视图,甚至复杂的存储过程实现的语句,现在 ...
- MongoDB的固态list
在MongoDB中当前数据库所指定的list可以是一个固态的list,所谓固态就是大小确定,如创建一个固态的集合:db.createCollection("固态集合的名字",{ca ...