java-继承和多态

注意
本文档属于个人笔记,个人水平有限,请酌情采纳,有任何错误可在评论区指出

继承满足“is-a”规则,即Manager is a Employee

/images/all/image-20221113161049769.png

如果子类的构造方法没有显式地调用超类的构造方法,则将自动地调用超类的无参构造,如果没有超类没有定义无参构造方法,编译报错。

this关键字的用途:

  • 引用隐式参数
  • 调用该类其他的构造方法

super关键字的用途:

  • 调用超类的方法
  • 调用超类的构造方法

调用其他构造方法的语句只能出现在构造方法中的第一行

import java.util.Date;
import java.util.GregorianCalendar;

public class ManagerTest {
    public static void main(String[] args) {
        Manager boss = new Manager("Carl Cracker", 80000, 1987, 12, 15);
        boss.setBonus(5000);

        Employee[] staff = new Employee[3];

		// 这里实际上使用了多态
        staff[0] = boss;
        staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
        staff[2] = new Employee("Tommy Tester", 40000, 1990, 3, 15);

 
        for (Employee e : staff) {
            System.out.println("name=" + e.getName() + ",salary=" + e.getSalary());
        }
    }
}

class Employee {

    private String name;
    private double salary;
    private Date hireDay;

    /**
     * @param n     the employee's name
     * @param s     the salary
     * @param year  the hire year
     * @param month the hire month
     * @param day   the hire day
     */
    public Employee(String n, double s, int year, int month, int day) {
        name = n;
        salary = s;
        GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
        hireDay = calendar.getTime();
    }

    public String getName() {
        return name;
    }

    public double getSalary() {
        return salary;
    }

    public Date getHireDay() {
        return hireDay;
    }

    public void raiseSalary(double byPercent) {
        double raise = salary * byPercent / 100;
        salary += raise;
    }
}

class Manager extends Employee {
    private double bonus;

    /**
     * @param n     the manager's name
     * @param s     the salary
     * @param year  the hire year
     * @param month the hire month
     * @param day   the hire day
     */
    public Manager(String n, double s, int year, int month, int day) {
        // 调用父类的构造方法必须出现在子类子类构造方法的第一行
        super(n, s, year, month, day);
        bonus = 0;
    }

    @Override
    public double getSalary() {
        double baseSalary = super.getSalary();
        return baseSalary + bonus;
    }

    public void setBonus(double b) {
        bonus = b;
    }

}

“is-a”规则的另一种表述法是置换法则:程序中出现超类对象的任何地方都可以用子类对象置换

Java中的对象变量都是多态的,Employee变量既可以引用一个Employee对象,也可以引用Employee的任何一个子类(比如Manager)的对象

当把子类的对象赋给父类的变量时,就发生了向上造型

// Employee是声明类型
Employee[] staff = new Employee[3];
// 等号右边是动态类型(可能是Employee对象,也可能是它的子类对象)
staff[0] = boss;
staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
staff[2] = new Employee("Tommy Tester", 40000, 1990, 3, 15);

注意: 不能将一个超类的引用赋给子类变量(不是所有雇员都是经理)

  • 当通过对象变量调用函数的时候,调用哪个函数这件事情叫做绑定

    • 静态绑定:根据变量的声明类型来决定

    • 动态绑定:根据变量的动态类型来决定

java中默认绑定都是动态绑定

如果想让一个类无法被继承,可以在class关键字前加上final关键字,这个类的所有方法也将自动加上final关键字

如果想让某个类的方法不能被重写,可以在方法名前加上final关键字

将方法或类声明为final主要目的是确保它们不会在子类中改变。

有时候希望将超类转换为子类,这样就能调用子类的方法。但这一般是超类的设计问题。应该避免这种转换。

将超类转换为子类之前,应该使用instanceof进行检查, 避免出现类型转换异常(ClassCastException)

Employee[] staff = new Employee[3];
// staff[0]引用的是Manager对象, staff[1]和staff[2]引用的是Employee对象
staff[0] = boss;
staff[1] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
staff[2] = new Employee("Tommy Tester", 40000, 1990, 3, 15);

// 假设要将Employee对象转换为Manager对象
Manager ma = (Manager) staff[0];
// Error, staff[1]引用的是Employee对象,会报ClassCastException异常
Manager mb = (Manager) staff[1];
// 正确做法: 转换前需要检查该对象是否属于转化后的对象类型
if (staff[1] instanceof Manager) {
    Manager mb = (Manager) staff[1];
}

// 注意null不会抛出异常
System.out.println(null instanceof  Manager); //输出false

抽象类天然支持多态性,因为抽象类不能实例化,只能引用非抽象子类的对象

  • 抽象类不一定包含抽象方法,有抽象方法一定要定义为抽象类
  • 抽象类可以包含具体数据(比如name)和具体方法(比如getName)
  • 抽象类也有构造方法
  • 继承抽象类的子类必须实现所有的抽象方法
  • 抽象类永远不能实例化,所有只能通过子类对象调用对应实现的抽象方法。
import java.util.Date;
import java.util.GregorianCalendar;

public class PersonTest
{
   public static void main(String[] args)
   {
      Person[] people = new Person[2];

      // fill the people array with Student and Employee objects
      people[0] = new Employee("Harry Hacker", 50000, 1989, 10, 1);
      people[1] = new Student("Maria Morris", "computer science");

      // print out names and descriptions of all Person objects
      for (Person p : people)
         System.out.println(p.getName() + ", " + p.getDescription());
   }
}

abstract class Person
{
   private String name;

   public Person(String n)
   {
      name = n;
   }

   public abstract String getDescription();

   public String getName()
   {
      return name;
   }

}

class Employee extends Person
{
   public Employee(String n, double s, int year, int month, int day)
   {
      // 调用抽象类的构造方法
      super(n);
      salary = s;
      GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
      hireDay = calendar.getTime();
   }

   public double getSalary()
   {
      return salary;
   }

   public Date getHireDay()
   {
      return hireDay;
   }

   public String getDescription()
   {
      return String.format("an employee with a salary of $%.2f", salary);
   }

   public void raiseSalary(double byPercent)
   {
      double raise = salary * byPercent / 100;
      salary += raise;
   }

   private double salary;
   private Date hireDay;
}

class Student extends Person
{
   /**
    * @param n the student's name
    * @param m the student's major
    */
   public Student(String n, String m)
   {
      // pass n to superclass constructor
      super(n);
      major = m;
   }

   public String getDescription()
   {
      return "a student majoring in " + major;
   }

   private String major;
}

所有class默认继承自Object

除基本类型之外,所有的对象数组和基本类型数组都继承了Object

System.out.println(new Object() instanceof Object); // true
System.out.println(new int[2] instanceof Object); // true
System.out.println(new Person[2] instanceof Object); // true

重写object.equals

package EqualsTest;

import java.util.Date;
import java.util.GregorianCalendar;

/**
 * This program demonstrates the equals method.
 * @version 1.11 2004-02-21
 * @author Cay Horstmann
 */
public class EqualsTest
{
   public static void main(String[] args)
   {
      Employee alice1 = new Employee("Alice Adams", 75000, 1987, 12, 15);
      Employee alice2 = alice1;
      Employee alice3 = new Employee("Alice Adams", 75000, 1987, 12, 15);
      Employee bob = new Employee("Bob Brandson", 50000, 1989, 10, 1);

      System.out.println("alice1 == alice2: " + (alice1 == alice2));

      System.out.println("alice1 == alice3: " + (alice1 == alice3));

      System.out.println("alice1.equals(alice3): " + alice1.equals(alice3));

      System.out.println("alice1.equals(bob): " + alice1.equals(bob));

      System.out.println("bob.toString(): " + bob);

      Manager carl = new Manager("Carl Cracker", 80000, 1987, 12, 15);
      Manager boss = new Manager("Carl Cracker", 80000, 1987, 12, 15);
      boss.setBonus(5000);
      // System.out.println(对象),会自动调用该对象所属类的toString()方法
      System.out.println("boss.toString(): " + boss);
      System.out.println("carl.equals(boss): " + carl.equals(boss));
      System.out.println("alice1.hashCode(): " + alice1.hashCode());
      System.out.println("alice3.hashCode(): " + alice3.hashCode());
      System.out.println("bob.hashCode(): " + bob.hashCode());
      System.out.println("carl.hashCode(): " + carl.hashCode());
   }
}

class Employee
{
   public Employee(String n, double s, int year, int month, int day)
   {
      name = n;
      salary = s;
      GregorianCalendar calendar = new GregorianCalendar(year, month - 1, day);
      hireDay = calendar.getTime();
   }

   public String getName()
   {
      return name;
   }

   public double getSalary()
   {
      return salary;
   }

   public Date getHireDay()
   {
      return hireDay;
   }

   public void raiseSalary(double byPercent)
   {
      double raise = salary * byPercent / 100;
      salary += raise;
   }

   @Override
   public boolean equals(Object otherObject)
   {
      // 1.检查是否引用自同一个对象(实际上就是检查两个对象的地址是否相同)
      // 注意我们的目的是判断两个对象的对应数据域是否相同,而不是只对比对象地址
      // 每个对象对应不同的hashCode,hashCode值即该对象的地址
      // 只比较相同对象的话意义不大
      if (this == otherObject) return true;

      // 2.比较的对象为null,返回false
      if (otherObject == null) return false;

      // 3.判断是否是同一个类的对象
      if (getClass() != otherObject.getClass()) return false;

      // 4.这里我们已经知道比较的同一个class的非空对象
      Employee other = (Employee) otherObject;

      // 5.检查它们的数据域是否相等
      // 注: 如果要检查两个数组是否相等,可以使用Arrays.equals()
      return name.equals(other.name) && salary == other.salary && hireDay.equals(other.hireDay);
   }

   // 如果重写了equals方法,必须重写hashCode方法
   // 这样就能保证equals返回true时,两个对象的hashCode也一样
   public int hashCode()
   {
      return 7 * name.hashCode() + 11 * new Double(salary).hashCode() + 13 * hireDay.hashCode();
      // 组合多个散列值时,可以使用下面这种方式
      //       return Objects.hash(name,salary,hireDay);
   }

   public String toString()
   {
      return getClass().getName() + "[name=" + name + ",salary=" + salary + ",hireDay=" + hireDay
            + "]"; 
   }

   private String name;
   private double salary;
   private Date hireDay;
}

class Manager extends Employee
{
   public Manager(String n, double s, int year, int month, int day)
   {
      super(n, s, year, month, day);
      bonus = 0;
   }

   public double getSalary()
   {
      double baseSalary = super.getSalary();
      return baseSalary + bonus;
   }

   public void setBonus(double b)
   {
      bonus = b;
   }

   public boolean equals(Object otherObject)
   {
      if (!super.equals(otherObject)) return false;
      Manager other = (Manager) otherObject;
      // super.equals checked that this and other belong to the same class
      return bonus == other.bonus;
   }

   public int hashCode()
   {
      return super.hashCode() + 17 * new Double(bonus).hashCode();
   }

   public String toString()
   {
      return super.toString() + "[bonus=" + bonus + "]";
   }

   private double bonus;
}

包装器类是不可变的

如果在一个条件表达式中混合使用IntegerDouble类型,Integer值就会拆箱,提升为double, 再装箱为Double

由于包装类引用可以为null,所以可能会抛出NullPointerException异常

Integer i1 = 1;
Double d1 = 2.0;
System.out.println(true ? i1 : d1); // 输出1.0

Integer i2 = null;
// error, 会抛出NullPointerException异常
System.out.println(i2 * 3);

找出数组中的最大值

public class Main {
    public static void main(String[] args) {
        System.out.println(max(1,2,8.8,9.9)); // 9.9
    }

    public static double max(double... values) {
        // 负无穷
        double largest = Double.NEGATIVE_INFINITY;
        for (int i = 0; i < values.length; i++) {
            if (values[i] > largest) largest = values[i];
        }
        return largest;
    }

}
import java.util.Scanner;

public class EnumTest {
    public static void main(String[] args) {
      Scanner in = new Scanner(System.in);
      System.out.print("Enter a size: (SMALL, MEDIUM, LARGE, EXTRA_LARGE) ");
      String input = in.next().toUpperCase();
      // 创建指定名字和类的枚举常量
      Size size = Enum.valueOf(Size.class, input);
      // toString()方法返回枚举常量名
      System.out.println("size=" + size);
      System.out.println("abbreviation=" + size.getAbbreviation());
      // 判断枚举常量时直接使用"=="
      if (size == Size.EXTRA_LARGE)
         System.out.println("Good job--you paid attention to the _.");
    }
}

enum Size {
    SMALL("S"), MEDIUM("M"), LARGE("L"), EXTRA_LARGE("XL");

    private Size(String abbreviation) {
        this.abbreviation = abbreviation;
    }

    public String getAbbreviation() {
        return abbreviation;
    }

    private String abbreviation;
}

获取class对象的三种方式:

  • 使用Object类的getClass()
  • 使用Class类的静态方法forName
  • 直接使用.class

jvm为每一种类型管理一个Class对象

// 第一种方式
Employee e = new Manager("Carl Cracker", 80000, 1987, 12, 1);
System.out.println(e.getClass());

// 第二种方式(注意要捕获异常)
// 动态修改要加载的类名
String className = "ManagerTest.Employee";
try {
    System.out.println(Class.forName(className));
} catch (ClassNotFoundException ex) {
    throw new RuntimeException(ex);
}

// 第三种方式
// Class类实际上是一种泛型类
Class<Employee> employeeClass = Employee.class;
// 一个Class 对象实际上表示的是一个类型, 而这个类型未必一定是一种类
System.out.println(int.class); // 输出int

利用反射创建一个有参构造的对象

注:new newInstance()使用的是无参构造方法

try {
    Class<?> aClass = Class.forName("ManagerTest.Employee");
    // 如果要使用有参构造方法,需要先获取一个构造方法
    Constructor<?> c = aClass.getConstructor(String.class, double.class, int.class, int.class, int.class);
    // 利用上面的构造方法创建对象
    Employee e = (Employee)c.newInstance("Carl Cracker", 80000, 1987, 12, 15);
    System.out.println(e.getName());
} catch (ClassNotFoundException | InvocationTargetException | NoSuchMethodException ex) {
    throw new RuntimeException(ex);
}

通过反射机制获取类的成员变量,方法,构造方法等

package cc.bnblogs;

import java.lang.reflect.Field;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Arrays;

public class Main {
    public static void main(String[] args) throws
            ClassNotFoundException, InstantiationException,
            IllegalAccessException, NoSuchFieldException {
        Class<?> aClass = Class.forName("cc.bnblogs.Point");
        Point p = (Point)aClass.newInstance();

        // 获取类的成员变量
        // 只能获取和修改public型的值
        Field x = aClass.getDeclaredField("x");
        Field y = aClass.getDeclaredField("y");
        Field str = aClass.getDeclaredField("color");
                
        // 要获取private变量的值,需要使用setAccessible(true)
        x.setAccessible(true);
        Object o = x.get(p);
      	System.out.println(o);
        

        // 获取Point类的所有成员变量
        Field[] fields = aClass.getDeclaredFields();
        System.out.println(Arrays.toString(fields));

        // error,获取对象的私有成员变量
        // System.out.println(x.get(p));
        System.out.println(str.get(p));


        try {
            // 获取Point类的setColor方法
            Method mSetColor = aClass.getMethod("setColor", String.class);
            // 修改point的颜色
            mSetColor.invoke(p,"black");

            System.out.println(p.getColor());

            // 获取所有声明的方法
            Method[] methods = aClass.getDeclaredMethods();
            System.out.println(Arrays.toString(methods));
            
            // 使用Modifier类获取权限修饰符的名称
            System.out.println(Modifier.toString(aClass.getModifiers()));

        } catch (NoSuchMethodException | InvocationTargetException e) {
            throw new RuntimeException(e);
        }
    }
}


class Point {
    private int x;
    private int y;

    public String color;

    Point() {

    }

    Point(int x,int y) {
        this.x = x;
        this.y = y;
    }


    public String getColor() {
        return color;
    }

    public void setColor(String color) {
        this.color = color;
    }

    public int getX() {
        return x;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getY() {
        return y;
    }

    public void setY(int y) {
        this.y = y;
    }

}

反射机制实现访问父类的成员变量

Person e = new Employee("Harry Hacker", 50000, 1989, 10, 1);
// 获取Employee的父类
Class<?> aClass = Class.forName("PersonTest.Employee").getSuperclass();
// 不能这样获取
// e.getClass(); 父类引用子类对象,e实际上还是Employee类
System.out.println(aClass);
// 获取父类的成员变量name
Field name = aClass.getDeclaredField("name");
name.setAccessible(true);
Object o = name.get(e);
System.out.println(o);

利用反射创建泛型数组

java.lang.reflect包中的Array类允许动态地创建数组。例如, 将这个特性应用到Array 类中的copyOf方法实现中,实现数组的动态拓展

import java.lang.reflect.Array;
import java.util.Arrays;

public class CopyOfTest {
    public static void main(String[] args) {
        int[] a = {1, 2, 3};
        a = (int[]) goodCopyOf(a,10);
        System.out.println(Arrays.toString(a));
        
        String[] b = {"Tom", "Dick", "Harry"};
        b = (String[]) goodCopyOf(b,10);
        System.out.println(Arrays.toString(b));

        System.out.println("The following call will generate an exception.");
        // Object不能强制转换为String
        // b = (String[]) badCopyOf(b,10);
    }

    public static Object[] badCopyOf(Object[] a, int newLength) {
        Object[] newArray = new Object[newLength];
        System.arraycopy(a,0,newArray,0,Math.min(a.length,newLength));
        return newArray;
    }

    public static Object goodCopyOf(Object a, int newLength) {
        Class cl = a.getClass();
        if (!cl.isArray()) return null;
        Class componentType = cl.getComponentType();
        int length = Array.getLength(a);
        Object newArray = Array.newInstance(componentType,newLength);
        System.arraycopy(a,0,newArray,0,Math.min(length,newLength));
        return newArray;
    }
}

利用反射调用任意方法

  • 调用静态方法,不需要获取类对象
  • 调用非静态方法,需要获取类对象
  • 反射调用私有静态方法(getDeclaredMethod,setAccessible)
  • 反射调用私有非静态方法(getDeclaredMethod,setAccessible)
package CopyofTest;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;

public class CopyOfTest {
    public static void main(String[] args) throws NoSuchMethodException, ClassNotFoundException, InvocationTargetException, InstantiationException, IllegalAccessException, NoSuchFieldException {
        Class<?> aClass = Class.forName("CopyofTest.Point");
        // 有参构造器
        Constructor<Point> constructor = Point.class.getConstructor(int.class, int.class);
        Point p = constructor.newInstance(1, 2);

        // 调用get和set方法
        Method getX = aClass.getMethod("getX");
        System.out.println(getX.invoke(p));

        Method setX = aClass.getMethod("setX",int.class);
        setX.invoke(p,10);
        System.out.println(getX.invoke(p));

        // 调用private非静态方法
        Method privateMethod = aClass.getDeclaredMethod("printStr");
        privateMethod.setAccessible(true);
        String str = (String) privateMethod.invoke(p);
        System.out.println(str);

        // 调用private静态方法
        Method privateStaticMethod = aClass.getDeclaredMethod("print");
        privateStaticMethod.setAccessible(true);

        // 调用静态函数, 第一个对象参数设置为null
        privateStaticMethod.invoke(null);
    }
}


class Point {
    private int x;
    private int y;

    public Point(int x,int y) {
        this.x = x;
        this.y = y;
    }

    public void setX(int x) {
        this.x = x;
    }

    public int getX() {
        return x;
    }

    public void setY(int y) {
        this.y = y;
    }

    public int getY() {
        return y;
    }

    private String printStr() {
        return "point";
    }

    private static void print() {
        System.out.println("print");
    }
}