Java 中继承的需求是什么？
众所周知，继承是面向对象编程语言最重要的特性之一，无论是 Java、C++还是任何其他面向对象编程语言。但是继承的必要性是什么呢？为什么它是一个如此重要的概念
继承 可以定义为一个对象获取父对象所有属性(即数据成员)和行为(即成员函数或方法)的机制。
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)。也就是说，子类继承父类的属性(方法和字段)。
认为没有继承就能完成某件事是对时间和资源的滥用，因为我们不得不一次又一次地编写相同的代码。
让我们举个例子，三个班级人，员工和学生有一些属性，一些共同的和一些不同的。请参见下面的代码。
爪哇
// 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，我们必须在学生和员工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。
爪哇
// 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，与第一个没有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更少，因此我们不得不一次又一次地定义名称、年龄及其相关方法。
也就是说，我们已经利用了可重用性的概念，因为我们已经一次又一次地重用了 Person 类中编写的代码。
此外，上面的示例显示了方法重写(因为 Person 类的 toString 函数在 Student 和 Employee 类中都被重写，这也是继承的一个主要特性。
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，并在不同的类之间创建更好的连接，并实现方法重写。
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