Java-抽象


根据字典,抽象是处理想法而不是事件的质量。例如,当您考虑电子邮件的情况时,复杂的细节(例如发送电子邮件后会发生什么)、电子邮件服务器使用的协议对用户是隐藏的。因此,要发送电子邮件,您只需键入内容,提及收件人地址,然后单击“发送”。

同样,在面向对象编程中,抽象是向用户隐藏实现细节的过程,仅向用户提供功能。换句话说,用户将获得有关对象做什么的信息,而不是它如何做的信息。

在Java中,抽象是通过抽象类和接口来实现的。

抽象类

在声明中包含abstract关键字的类称为抽象类。

  • 抽象类可能包含也可能不包含抽象方法,即没有主体的方法( public void get(); )

  • 但是,如果一个类至少有一个抽象方法,则该类必须声明为抽象的。

  • 如果一个类被声明为抽象类,则它不能被实例化。

  • 要使用抽象类,您必须从另一个类继承它,并提供其中抽象方法的实现。

  • 如果继承抽象类,则必须提供其中所有抽象方法的实现。

例子

本节为您提供抽象类的示例。要创建抽象类,只需在类声明中的 class 关键字之前使用abstract关键字即可。

/* File name : Employee.java */ public abstract class Employee { private String name; private String address; private int number; public Employee(String name, String address, int number) { System.out.println("Constructing an Employee"); this.name = name; this.address = address; this.number = number; } public double computePay() { System.out.println("Inside Employee computePay"); return 0.0; } public void mailCheck() { System.out.println("Mailing a check to " + this.name + " " + this.address); } public String toString() { return name + " " + address + " " + number; } public String getName() { return name; } public String getAddress() { return address; } public void setAddress(String newAddress) { address = newAddress; } public int getNumber() { return number; } }

您可以观察到,除了抽象方法之外,Employee 类与 Java 中的普通类相同。该类现在是抽象的,但它仍然具有 3 个字段、7 个方法和 1 个构造函数。

现在您可以尝试通过以下方式实例化 Employee 类 -

/* File name : AbstractDemo.java */ public class AbstractDemo { public static void main(String [] args) { /* Following is not allowed and would raise error */ Employee e = new Employee("George W.", "Houston, TX", 43); System.out.println("\n Call mailCheck using Employee reference--"); e.mailCheck(); } } abstract class Employee { private String name; private String address; private int number; public Employee(String name, String address, int number) { System.out.println("Constructing an Employee"); this.name = name; this.address = address; this.number = number; } public double computePay() { System.out.println("Inside Employee computePay"); return 0.0; } public void mailCheck() { System.out.println("Mailing a check to " + this.name + " " + this.address); } public String toString() { return name + " " + address + " " + number; } public String getName() { return name; } public String getAddress() { return address; } public void setAddress(String newAddress) { address = newAddress; } public int getNumber() { return number; } }

当你编译上面的类时,它会给出以下错误 -

Employee.java:46: Employee is abstract; cannot be instantiated
      Employee e = new Employee("George W.", "Houston, TX", 43);
                   ^
1 error

继承抽象类

我们可以像具体类一样继承 Employee 类的属性,如下所示 -

例子

/* File name : Salary.java */ public class Salary extends Employee { private double salary; // Annual salary public Salary(String name, String address, int number, double salary) { super(name, address, number); setSalary(salary); } public void mailCheck() { System.out.println("Within mailCheck of Salary class "); System.out.println("Mailing check to " + getName() + " with salary " + salary); } public double getSalary() { return salary; } public void setSalary(double newSalary) { if(newSalary >= 0.0) { salary = newSalary; } } public double computePay() { System.out.println("Computing salary pay for " + getName()); return salary/52; } }

在这里,您无法实例化 Employee 类,但您可以实例化 Salary 类,并且使用该实例您可以访问 Employee 类的所有三个字段和七个方法,如下所示。

/* File name : AbstractDemo.java */ public class AbstractDemo { public static void main(String [] args) { Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00); Employee e = new Salary("John Adams", "Boston, MA", 2, 2400.00); System.out.println("Call mailCheck using Salary reference --"); s.mailCheck(); System.out.println("\n Call mailCheck using Employee reference--"); e.mailCheck(); } } abstract class Employee { private String name; private String address; private int number; public Employee(String name, String address, int number) { System.out.println("Constructing an Employee"); this.name = name; this.address = address; this.number = number; } public double computePay() { System.out.println("Inside Employee computePay"); return 0.0; } public void mailCheck() { System.out.println("Mailing a check to " + this.name + " " + this.address); } public String toString() { return name + " " + address + " " + number; } public String getName() { return name; } public String getAddress() { return address; } public void setAddress(String newAddress) { address = newAddress; } public int getNumber() { return number; } } class Salary extends Employee { private double salary; // Annual salary public Salary(String name, String address, int number, double salary) { super(name, address, number); setSalary(salary); } public void mailCheck() { System.out.println("Within mailCheck of Salary class "); System.out.println("Mailing check to " + getName() + " with salary " + salary); } public double getSalary() { return salary; } public void setSalary(double newSalary) { if(newSalary >= 0.0) { salary = newSalary; } } public double computePay() { System.out.println("Computing salary pay for " + getName()); return salary/52; } }

输出

Constructing an Employee
Constructing an Employee
Call mailCheck using Salary reference --
Within mailCheck of Salary class 
Mailing check to Mohd Mohtashim with salary 3600.0

 Call mailCheck using Employee reference--
Within mailCheck of Salary class 
Mailing check to John Adams with salary 2400.0

抽象方法

如果您希望一个类包含特定方法,但希望该方法的实际实现由子类确定,则可以在父类中将该方法声明为抽象方法。

  • Abstract关键字用于将方法声明为抽象方法。

  • 您必须将抽象关键字放在方法声明中的方法名称之前。

  • 抽象方法包含方法签名,但不包含方法体。

  • 抽象方法末尾将有一个符号冒号 (;),而不是花括号。

以下是抽象方法的示例。

例子

public abstract class Employee { private String name; private String address; private int number; public abstract double computePay(); // Remainder of class definition }

将方法声明为抽象方法有两个后果 -

  • 包含它的类必须声明为抽象类。

  • 继承当前类的任何类都必须重写抽象方法或将自身声明为抽象方法。

注意- 最终,后代类必须实现抽象方法;否则,您将拥有无法实例化的抽象类层次结构。

假设Salary类继承Employee类,那么它应该实现computePay()方法,如下所示 -

/* File name : Salary.java */ public class Salary extends Employee { private double salary; // Annual salary public double computePay() { System.out.println("Computing salary pay for " + getName()); return salary/52; } // Remainder of class definition }

例子

以下示例展示了抽象方法的概念。

/* File name : AbstractDemo.java */ public class AbstractDemo { public static void main(String [] args) { Salary s = new Salary("Mohd Mohtashim", "Ambehta, UP", 3, 3600.00); System.out.println("salary: " + s.computePay()); } } abstract class Employee { private String name; private String address; private int number; public Employee(String name, String address, int number) { System.out.println("Constructing an Employee"); this.name = name; this.address = address; this.number = number; } public abstract double computePay(); // Remainder of class definition public String getName() { return name; } public void setName(String name) { this.name = name; } public String getAddress() { return address; } public void setAddress(String address) { this.address = address; } public int getNumber() { return number; } public void setNumber(int number) { this.number = number; } } class Salary extends Employee { private double salary; // Annual salary public Salary(String name, String address, int number, double salary) { super(name, address, number); this.salary = salary; } public double computePay() { System.out.println("Computing salary pay for " + getName()); return salary/52; } // Remainder of class definition }

输出

Constructing an Employee
Computing salary pay for Mohd Mohtashim
salary: 69.23076923076923