Java - 抽象

根据词典，**抽象**是指处理思想而不是事件的特性。例如，当您考虑电子邮件的情况时，发送电子邮件后发生的事情等复杂细节，您的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，提及接收者的地址，然后单击发送。

Java 抽象

抽象是向用户隐藏实现细节的过程，只向用户提供功能。换句话说，用户将了解对象的功能而不是其工作方式。在 Java 编程中，抽象是使用**抽象类**和**接口**实现的。

Java 抽象类

在声明中包含**abstract**关键字的Java 类称为抽象类。

• Java 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**声明为抽象类。

• 如果一个类被声明为抽象类，则不能对其进行实例化。

• 要使用抽象类，您必须从另一个类继承它，并在其中为抽象方法提供实现。

• 如果您继承了一个抽象类，则必须为其中的所有抽象方法提供实现。

示例：Java 抽象类

本节为您提供 Java 抽象类的示例。要在 Java 中创建抽象类，只需在类声明中，在 class 关键字之前使用**abstract**关键字。

/* 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，除了抽象方法外，Employee 类与 Java 中的普通类相同。该类现在是抽象的，但它仍然有三个字段、七个方法和一个构造函数。

现在您可以尝试以下方式实例化 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;
   }
}

编译上述类时，会收到以下错误：

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

继承 Java 抽象类

我们可以像在以下方式中继承具体类一样继承 Employee 类的属性：

示例：在 Java 中继承抽象类

/* 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;
   }
}

这里，您不能实例化 Employee 类，但您可以实例化 Salary 类，并使用此实例访问 Employee 类中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

Java 抽象方法

如果您希望一个类包含特定的方法，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。

• 在方法声明中，您必须在方法名称之前放置abstract关键字。

• 抽象方法包含方法签名，但不包含方法体。

• 抽象方法的末尾将使用分号（;），而不是大括号。

示例 1：在 Java 中实现抽象方法

以下是抽象方法的示例。

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

将方法声明为抽象有两个结果：

• 包含它的类必须声明为抽象类。

• 任何继承当前类的类都必须重写抽象方法或将其自身声明为抽象类。

注意：最终，子类必须实现抽象方法；否则，您将拥有一个无法实例化的抽象类层次结构。

假设 Salary 类继承了 Employee 类，那么它应该像下面这样实现computePay()方法：

/* 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
}

示例 2：在 Java 中实现抽象方法

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