在其声明中包含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，如果一个类至少具有一个抽象方法，则必须将该类声明为抽象。
如果类被声明为抽象，则无法实例化。
要使用抽象类，您必须从另一个类继承它，并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。
本节为您提供了抽象类的示例。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。
/* 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-
/* 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("乔治W.-", "Houston, TX", 43);
System.out.println("\n Call mailCheck using Employee reference--");
e.mailCheck();
}
}当您编译上面的类时，它给您以下错误-
Employee.java:46: Employee is abstract; cannot be instantiated
Employee e = new Employee("乔治W.-", "Houston, TX", 43);
^
1 error我们可以通过以下方式像具体类一样继承Employee类的属性-
/* 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，您不能实例化Employee类，但是可以实例化Salary类，并且使用此实例，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();
}
}这产生以下结果-
输出结果
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