Java中继承的需要是什么？

继承，正如我们都听说过的，是面向对象编程语言最重要的特性之一，无论是Java、C++ 还是任何其他 OOP 语言。但是继承的必要性是什么？为什么它是一个如此重要的概念

继承可以定义为一种机制，通过该机制一个对象可以获得父对象的所有属性（即数据成员）和行为（即成员函数或方法）。

继承的基本思想是从现有类（称为父类或基类或超类）创建新类（称为子类或派生类或子类）。即子类继承父类的属性（方法和字段）。

想在没有继承的情况下实现某事将是对时间和资源的滥用，因为我们必须一次又一次地编写相同的代码。

让我们举个例子，三个类 Person、Employee 和 Student 有一些属性，一些共同的，一些不同的。请参阅下面的代码。

Java

// Java program without inheritance
import java.io.*;
import java.math.BigDecimal;
  
// class person
class Person {
  
    private String name;
    private int age;
  
    public String getName() { return name; }
  
    public void setName(String name) { this.name = name; }
  
    public int getAge() { return age; }
  
    public void setAge(int age) { this.age = age; }
  
    public String toString()
    {
        return String.format(
            "Person :- name : %s , age : %d", name, age);
    }
}
  
//class student
class Student {
  
    private String name;
    private int age;
    private int rollno;
    private String school;
  
    public String getName() { return name; }
  
    public void setName(String name) { this.name = name; }
  
    public int getAge() { return age; }
  
    public void setAge(int age) { this.age = age; }
  
    public int getRollno() { return rollno; }
  
    public void setRollno(int rollno)
    {
        this.rollno = rollno;
    }
  
    public String getSchool() { return school; }
  
    public void setSchool(String school)
    {
        this.school = school;
    }
  
    public String toString()
    {
        return String.format(
            "Student :- name : %s , age : %d , roll : %d , school : %s",
            name, age, rollno, school);
    }
}
  
//class Employee
class Employee {
  
    private String name;
    private int age;
    private double salary;
    private String organisation;
  
    public String getName() { return name; }
  
    public void setName(String name) { this.name = name; }
  
    public int getAge() { return age; }
  
    public void setAge(int age) { this.age = age; }
  
    public double getSalary() { return salary; }
  
    public void setSalary(double salary)
    {
        this.salary = salary;
    }
  
    public String getOrganisation() { return organisation; }
  
    public void setOrganisation(String organisation)
    {
        this.organisation = organisation;
    }
  
    public String toString()
    {
        return String.format(
            "Employee :- name : %s , age : %d , organisation : %s , salary : %f",
            name, age, organisation, salary);
    }
}
  
// class PersonRunner
public class PersonRunner {
  
    public static void main(String[] args)
    {
        Person person = new Person();
        Student student = new Student();
        Employee employee = new Employee();
  
        person.setName("Saurabh");
        person.setAge(20);
  
        student.setName("Prateek");
        student.setAge(21);
        student.setRollno(101);
        student.setSchool("New Era HS");
  
        employee.setName("Sushant");
        employee.setAge(25);
        employee.setOrganisation("GeeksforGeeks");
        employee.setSalary(50000.00);
  
        System.out.println(person);
        System.out.println(student);
        System.out.println(employee);
    }
}

输出

Person :- name : Saurabh , age : 20
Student :- name : Prateek , age : 21 , roll : 101 , school : New Era HS
Employee :- name : Sushant , age : 25 , organisation : GeeksforGeeks , salary : 50000.000000

请注意，我们必须在 Student 和 Employee 类中再次编写已经在 Person 类中编写的相同代码。这需要时间和记忆。

现在仔细阅读下面我们使用继承的代码。

Java

// Java program with inheritance
import java.io.*;
  
class Person {
  
    private String name;
    private int age;
  
    public String getName() { return name; }
  
    public void setName(String name) { this.name = name; }
  
    public int getAge() { return age; }
  
    public void setAge(int age) { this.age = age; }
  
    public String toString()
    {
        return String.format(
            "Person :- name : %s , age : %d", getName(),
            getAge());
    }
}
  
// Student class is inheriting the properties of Person
// class( through extend keyword) Therefore, we don't need to
// declare name and age (and their related methods which are
// covered in Person) again.
class Student extends Person {
  
    private int rollno;
    private String school;
  
    public int getRollno() { return rollno; }
  
    public void setRollno(int rollno)
    {
        this.rollno = rollno;
    }
  
    public String getSchool() { return school; }
  
    public void setSchool(String school)
    {
        this.school = school;
    }
  
    public String toString()
    {
        return String.format(
            "Student :- name : %s , age : %d , roll : %d , school : %s",
            getName(), getAge(), getRollno(), getSchool());
    }
}
  
// Employee class is inheriting the properties of Person
// class( through extend keyword) Therefore, we don't need to
// declare name and age (and their related methods which are
// covered in Person) again.
class Employee extends Person {
  
    private double salary;
    private String organisation;
  
    public double getSalary() { return salary; }
  
    public void setSalary(double salary)
    {
        this.salary = salary;
    }
  
    public String getOrganisation() { return organisation; }
  
    public void setOrganisation(String organisation)
    {
        this.organisation = organisation;
    }
  
    public String toString()
    {
        return String.format(
            "Employee :- name : %s , age : %d , organisation : %s , salary : %f",
            getName(), getAge(), getOrganisation(),
            getSalary());
    }
}
  
public class PersonRunner {
  
    public static void main(String[] args)
    {
        Person person = new Person();
        Student student = new Student();
        Employee employee = new Employee();
  
        person.setName("Saurabh");
        person.setAge(20);
  
        student.setName("Prateek");
        student.setAge(21);
        student.setRollno(101);
        student.setSchool("New Era HS");
  
        employee.setName("Sushant");
        employee.setAge(25);
        employee.setOrganisation("GeeksforGeeks");
        employee.setSalary(50000.00);
  
        System.out.println(person);
        System.out.println(student);
        System.out.println(employee);
    }
}

输出

Person :- name : Saurabh , age : 20
Student :- name : Prateek , age : 21 , roll : 101 , school : New Era HS
Employee :- name : Sushant , age : 25 , organisation : GeeksforGeeks , salary : 50000.000000

请注意，与第一个未使用继承的示例相比，我们在上面的示例中使用继承时编写的代码较少，因此我们必须一次又一次地定义名称、年龄及其相关方法。

也就是说，我们利用了可重用性的概念，因为我们一遍又一遍地重用了在 Person 类中编写的代码。

另外，上面的例子展示了Method Overriding（因为Person类的toString函数在Student和Employee类中都被覆盖了，这也是继承的一大特点。

因此，它得出结论，继承的主要目的是实现可重用性的概念，节省我们的时间和资源，并在不同的类之间建立更好的连接，并实现方法覆盖。