继承可以解决代码复用，让编程更加靠近人的思维。当多个类存在相同的属性（变量）和方法时，可以从这些类中抽象出父类，在父类中定义这些相同的属性和方法。所有的子类不需要重新定义这些属性和方法，只需要通过extends语句来声明继承父类：
class 子类 extends 父类
这样，子类就会自动拥有父类定义的属性和方法。
*父类的public修饰符的属性和方法，protected修饰符的属性和方法，默认修饰符的属性和方法被子类继承了，父类的private修饰符的属性和方法不能被子类继承。
注意事项：
- 子类最多只能继承一个父类
- java所有的类都是object类的子类
下面是由继承Employee 类来定义Manager 类的格式， 关键字extends 表示继承。
public class Manager extends Employee{ 添加方法和域}
子类中继承父类的域和方法；
子类构造器
public Manger(String name, double salary, int year, int month, int day){ super(name, salary, year, month, day); bonus = 0;}
因为子类终的构造器不能直接访问父类的私有域，所以必须利用父类的构造器对这部分私有域进行初始化，可以使用super实现对父类构造器的调用。使用super调用构造器的语句必须是子类构造器的第一条语句。
package testbotoo;import java.time.LocalDate;public class Employee{ private String name; private double salary; private LocalDate hireDay; public Employee(String name, double salary, int year, int month, int day) { this.name = name; this.salary = salary; hireDay = LocalDate.of(year, month, day); } public String getName() { return name; } public double getSalary() { return salary; } }
package testbotoo;public class Manager extends Employee { private double bonus; /** * @param name the employee's name * @param salary the salary * @param year the hire year * @param month the dire onth * @param day the hire day */ public Manager(String name, double salary, int year, int month, int day) { super(name, salary, year, month, day); bonus = 0; } public double getSalary() { double baseSalary = super.getSalary(); return baseSalary +bonus; } public void setBonus(double b) { bonus = b; }}
package testbotoo;public class ManagerTest { public static void main(String[] args) { Manager boss = new Manager("aaa",8000,1999,12,20); boss.setBonus(5000); Employee[] staff = new Employee[3]; staff[0] = boss; staff[1] = new Employee("hary",5000,1989,3,15); staff[2] = new Employee("mayun",50000,1989,3,16); for (Employee e : staff) System.out.println("name="+e.getName()+",salary="+e.getSalary()); }}