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Java三大特性之多态

Java三大特性之多态

作者: Java编程日记 | 来源:发表于2022-04-06 14:41 被阅读0次

1.多态

上篇文章我们给大家介绍了Java中三大特性中的继承，本文给大家介绍另一个特性——多态。首先给大家介绍的就是多态的概念。

1.1多态的概述

  • **什么是多态： **

同一个对象，在不同时刻表现出来的不同形态

  • 多态的前提：

  • 要有继承或实现关系

  • 要有方法的重写

  • 要有父类引用指向子类对象

1.2多态中的成员访问特点

  • 成员访问特点

  • 成员变量：

编译看父类，运行看父类

  • 成员方法：

编译看父类，运行看子类

为了让大家更好地理解该知识点，我们简单的给大家举一个案例，具体如下：

  • 动物类

<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Animal {
public int age = 40;

public void eat() {
    System.out.println("动物吃东西");
}

}</pre>

  • 猫类

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Cat extends Animal {
public int age = 20;
public int weight = 10;
@Override
public void eat() {
System.out.println("猫吃鱼");
}

public void playGame() {
    System.out.println("猫捉迷藏");
}

}</pre>

  • 测试类

<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class AnimalDemo {
public static void main(String[] args) {
//有父类引用指向子类对象
Animal a = new Cat();

    System.out.println(a.age);

// System.out.println(a.weight);

    a.eat();

// a.playGame();
}
}</pre>

该案例执行的结果如下：

[图片上传失败...(image-7bfa7f-1649227279677)]

1.3多态的优点和缺点

  • 优点

提高程序的扩展性。定义方法时候，使用父类型作为参数，在使用的时候，使用具体的子类型参与操作

  • 缺点

不能使用子类的特有成员

1.4多态中的转型

  • 向上转型

父类引用指向子类对象就是向上转型

  • 向下转型

格式： 子类型 对象名 = (子类型)父类引用;

为了让大家更好的理解这个知识点，我们稍微改一下上述的代码；

  • 动物类

<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Animal {
public void eat() {
System.out.println("动物吃东西");
}
}</pre>

  • 猫类

<pre class="prettyprint hljs scala" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Cat extends Animal {
@Override
public void eat() {
System.out.println("猫吃鱼");
}

public void playGame() {
    System.out.println("猫捉迷藏");
}

}</pre>

  • 测试类

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class AnimalDemo {
public static void main(String[] args) {
//多态
//向上转型
Animal a = new Cat();
a.eat();
// a.playGame();

    //向下转型
    Cat c = (Cat)a;
    c.eat();
    c.playGame();
}

}</pre>

以上的案例实现的代码执行结果具体如下：

[图片上传失败...(image-2c7dc0-1649227279677)]

1.5多态的案例

  • 案例需求

请采用多态的思想实现猫和狗的案例，并在测试类中进行测试

  • 代码实现

  • 动物类

<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Animal {
private String name;
private int age;

public Animal() {
}

public Animal(String name, int age) {
    this.name = name;
    this.age = age;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public int getAge() {
    return age;
}

public void setAge(int age) {
    this.age = age;
}

public void eat() {
    System.out.println("动物吃东西");
}

}</pre>

  • 猫类

<pre class="prettyprint hljs scala" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Cat extends Animal {

public Cat() {
}

public Cat(String name, int age) {
    super(name, age);
}

@Override
public void eat() {
    System.out.println("猫吃鱼");
}

}</pre>

  • 狗类

<pre class="prettyprint hljs scala" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Dog extends Animal {

public Dog() {
}

public Dog(String name, int age) {
    super(name, age);
}

@Override
public void eat() {
    System.out.println("狗吃骨头");
}

}</pre>

  • ** 测试类 **

<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class AnimalDemo {
public static void main(String[] args) {
//创建猫类对象进行测试
Animal a = new Cat();
a.setName("加菲");
a.setAge(5);
System.out.println(a.getName() + "," + a.getAge());
a.eat();

    a = new Cat("加菲", 5);
    System.out.println(a.getName() + "," + a.getAge());
    a.eat();
}

}</pre>

以上案例代码的实现思路如下：

[图片上传失败...(image-89e4c5-1649227279676)]

2.抽象类

2.1抽象类的概述

当我们在做子类共性功能抽取时，有些方法在父类中并没有具体的体现，这个时候就需要抽象类了！ 在Java中，一个没有方法体的方法应该定义为抽象方法，而类中如果有抽象方法，该类必须定义为抽象类！

2.2抽象类的特点

  • 抽象类和抽象方法必须使用 abstract 关键字修饰

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">//抽象类的定义
public abstract class 类名 {}

//抽象方法的定义
public abstract void eat();</pre>

  • 抽象类中不一定有抽象方法，有抽象方法的类一定是抽象类
  • 抽象类不能实例化

抽象类如何实例化呢？参照多态的方式，通过子类对象实例化，这叫抽象类多态

  • 抽象类的子类

要么重写抽象类中的所有抽象方法 要么是抽象类

2.3抽象类的成员特点

  • 成员的特点

  • 成员变量

  • 既可以是变量

  • 也可以是常量

  • 构造方法

  • 空参构造

  • 有参构造

  • 成员方法

  • 抽象方法

  • 普通方法

接下来我们还是对以上的案例稍作改动来说明该特点；具体代码如下：

  • 动物类

<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public abstract class Animal {

private int age = 20;
private final String city = "北京";

public Animal() {}

public Animal(int age) {
    this.age = age;
}

public void show() {
    age = 40;
    System.out.println(age);

// city = "上海";
System.out.println(city);
}

public abstract void eat();

}</pre>

  • **猫类 **

<pre class="prettyprint hljs scala" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Cat extends Animal {
@Override
public void eat() {
System.out.println("猫吃鱼");
}
}</pre>

  • 测试类

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class AnimalDemo {
public static void main(String[] args) {
Animal a = new Cat();
a.eat();
a.show();
}
}</pre>

以上案例代码的实现思路如下：

[图片上传失败...(image-d545fe-1649227279676)]

2.4抽象类的案例

  • 案例需求

请采用抽象类的思想实现猫和狗的案例，并在测试类中进行测试

  • 具体用代码实现如下：

  • 动物类

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public abstract class Animal {
private String name;
private int age;

public Animal() {
}

public Animal(String name, int age) {
    this.name = name;
    this.age = age;
}

public String getName() {
    return name;
}

public void setName(String name) {
    this.name = name;
}

public int getAge() {
    return age;
}

public void setAge(int age) {
    this.age = age;
}

public abstract void eat();

}</pre>

  • **猫类 **

<pre class="prettyprint hljs scala" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Cat extends Animal {

public Cat() {
}

public Cat(String name, int age) {
    super(name, age);
}

@Override
public void eat() {
    System.out.println("猫吃鱼");
}

}</pre>

  • **狗类 **

<pre class="prettyprint hljs scala" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class Dog extends Animal {

public Dog() {
}

public Dog(String name, int age) {
    super(name, age);
}

@Override
public void eat() {
    System.out.println("狗吃骨头");
}

}</pre>

  • **测试类 **

<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public class AnimalDemo {
public static void main(String[] args) {
//创建对象，按照多态的方式
Animal a = new Cat();
a.setName("加菲");
a.setAge(5);
System.out.println(a.getName()+","+a.getAge());
a.eat();
System.out.println("--------");

    a = new Cat("加菲",5);
    System.out.println(a.getName()+","+a.getAge());
    a.eat();
}

}</pre>

具体执行的结果如下：

[图片上传失败...(image-9f95b3-1649227279675)]

3.修饰符

3.1 package

  • 1、 包的概念

  • 包就是文件夹，用来管理类文件的

  • 2、 包的定义格式

<pre class="hljs lua" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 0.75em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">package 包名; (多级包用.分开)
package com.heima.demo;
</pre>

  • 3、 **带包编译&带包运行 **

  • 带包编译： javac –d . 类名.java

<pre class="prettyprint hljs css" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">javac -d . com.heima.demo.HelloWorld.java
java 包名+类名
java com.heima.demo.HelloWorld
</pre>

3.2 import

  • **导包的意义 **

使用不同包下的类时，使用的时候要写类的全路径，写起来太麻烦了

为了简化带包的操作，Java就提供了导包的功能

  • **导包的格式 **

格式： import 包名;

范例： import java.util.Scanner;

  • 示例代码（没有使用导包，创建的Scanner对象）

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">package com.heima;

public class Demo {
public static void main(String[] args) {
// 1. 没有导包，创建Scnaner对象
java.util.Scanner sc = new java.util.Scanner(System.in);
}
}</pre>

  • 示例代码（使用导包后，创建的Scanner对象）

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">package com.heima;

import java.util.Scanner;

public class Demo {
public static void main(String[] args) {
// 1. 没有导包，创建Scnaner对象
Scanner sc = new Scanner(System.in);
}
}</pre>

3.3 权限修饰符

[图片上传失败...(image-428b7a-1649227279675)]

3.4 final

  • fianl关键字的作用

  • final代表最终的意思，可以修饰成员方法，成员变量，类

  • final修饰类、方法、变量的效果

  • fianl修饰类：该类不能被继承（不能有子类，但是可以有父类）

  • final修饰方法：该方法不能被重写

  • final修饰变量：表明该变量是一个常量，不能再次赋值

3.5 final修饰局部变量

  • fianl修饰基本数据类型变量
  • final 修饰指的是基本类型的数据值不能发生改变
  • final修饰引用数据类型变量
  • final 修饰指的是引用类型的地址值不能发生改变，但是地址里面的内容是可以发生改变的

为了方便大家理解相应的知识点，我们给出一个简单的案例：

<pre class="prettyprint hljs java" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">public static void main(String[] args){
final Student s = new Student(23);
s = new Student(24); // 错误
s.setAge(24); // 正确
}</pre>

3.6 static

  • **static的概念 **

  • static关键字是静态的意思，可以修饰【成员方法】，【成员变量】

  • **static修饰的特点 **

  1. 被类的所有对象共享，这也是我们判断是否使用静态关键字的条件
  2. 可以通过类名调用当然，也可以通过对象名调用 【推荐使用类名调用】

为了方便大家的理解该知识点，我们给出一个简单的案例，具体实现如下：

<pre class="prettyprint hljs cs" style="padding: 0.5em; font-family: Menlo, Monaco, Consolas, "Courier New", monospace; color: rgb(68, 68, 68); border-radius: 4px; display: block; margin: 0px 0px 1.5em; font-size: 14px; line-height: 1.5em; word-break: break-all; overflow-wrap: break-word; white-space: pre; background-color: rgb(246, 246, 246); border: none; overflow-x: auto; font-style: normal; font-variant-ligatures: normal; font-variant-caps: normal; font-weight: 400; letter-spacing: normal; orphans: 2; text-align: start; text-indent: 0px; text-transform: none; widows: 2; word-spacing: 0px; -webkit-text-stroke-width: 0px; text-decoration-thickness: initial; text-decoration-style: initial; text-decoration-color: initial;">class Student {

public String name; //姓名
public int age; //年龄
public static String university; //学校   共享数据！所以设计为静态！

public void show() {
    System.out.println(name + "," + age + "," + university);
}

}

public class StaticDemo {
public static void main(String[] args) {
// 为对象的共享数据赋值
Student.university = "51CTO博客";

    Student s1 = new Student();
    s1.name = "一计之长";
    s1.age = 30;
    s1.show();

    Student s2 = new Student();
    s2.name = "风清扬";
    s2.age = 33;
    s2.show();
}

}</pre>

该案例具体执行的结果如下：

[图片上传失败...(image-36c9de-1649227279675)]

3.7 static访问特点

  • static的访问特点

  • 非静态的成员方法

  • 能访问静态的成员变量

  • 能访问非静态的成员变量

  • 能访问静态的成员方法

  • 能访问非静态的成员方法

  • 静态的成员方法

  • 能访问静态的成员变量

  • 能访问静态的成员方法

  • 总结成一句话就是 ：

  • 静态成员方法只能访问静态成员

总结

本文给大家介绍了Java中的三大特性之一——多态，主要介绍了多态的概念、成员访问特点、优点以及缺点；另外还介绍了抽象类以及Java中的修饰符。并且最后就是通过一些案例说明该知识点是如何使用的，帮助大家更好的理解。

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