Java 抽象

抽象是一个隐藏实现细节并只向用户显示功能的过程。例如，我们在发送邮件的时候不需要知道具体的邮件协议和发送过程，只需要知道收件人的地址然后单击发送即可。

Java 抽象

抽象是隐藏电子邮件的过程。来自用户的实现细节，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编程中，抽象是通过抽象类和接口来实现的。

Java 抽象类

Java中使用关键字abstract来声明抽象类。

• 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，则必须声明该类抽象。

• 如果一个类被声明为抽象，则它不能被实例化。

• 要使用抽象类，您必须要从另一个类继承它，请为其中的抽象方法提供实现。

• 如果继承抽象类，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，只需在类声明中的 class 关键字之前使用 abstract 关键字即可。

/* 文件名 : 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 类是相同的与 Java 中的普通类一样。该类现在是抽象的，但它仍然具有三个字段、七个方法和一个构造函数。

现在您可以尝试通过以下方式实例化 Employee 类 -

/* 文件名 : 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;
   }
} 

When编译上面的类，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 Java 抽象类

我们可以像具体类一样继承 Employee 类的属性，如下所示 -

示例:继承Java中的抽象类

/* 文件名 : Salary.java */
public class Salary extends Employee {
   private double salary;   //年薪
   
   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，你不能实例化Employee类，但你可以实例化Salary类，并且使用这个实例你可以访问这三个类Employee类的字段和七个方法如下所示。

/* 文件名 : 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;   //年薪
   
   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

如果你想让一个类包含特定的方法但是你希望该方法的实际实现由子类决定，你可以在父类中将该方法声明为抽象。

• abstract 关键字用于将方法声明为抽象。

• 您必须在方法声明中的方法名称之前放置abstract关键字。

• 抽象方法包含方法签名，但不包含方法体。

• 抽象方法末尾将有一个符号冒号 (;)，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();
   //类定义的剩余部分
} 

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。

• 任何类继承当前类必须覆盖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 -

/* 文件名 : Salary.java */
public class Salary extends Employee {
   private double salary;   //年薪
  
   public double computePay() {
      System.out.println("Computing salary pay for " + getName());
      return salary/52;
   }
   // 类定义的其余部分
} 

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。

/* 文件名 : 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();
      // 类定义的剩余部分

   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;   //年薪

   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;
   }
   // 类定义的其余部分
} 

输出

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