本文介绍了JAVA中的类抽象过程，包括如何从现实抽象出类，使用类图描述，以及对象初始化。接着讲解了封装的概念，通过示例展示了如何使用封装保护代码安全，如限制对属性的直接访问。最后讨论了常量的使用，如何创建和应用常量以避免错误输入。
一、构造函数
1、从现实中抽象出类分为三步：
- 给具有共性特征的物体分类
- 找出它的属性
- 找出它的行为
2、使用类图描述类
- 用于分析和设计“类”
- 直观、容易理解
3、对象初始化
overload：一个类中有多个方法，同名不同参，此为overload（重载）。
在Dog.java中输入如下代码：
如果没有名字传入的时候，小狗的名字默认为Steve。然后我们写两个函数，一个是游泳，一个是玩碟子。
public class Dog {
public String name;
public String color;
public double age;
public int health;
/**默认构造函数
* 特点：1.构造函数的名字与类名必须一致
* 2.构造函数不能有返回值
* 3.无参构造函数，为默认方法
*/
public Dog() {
name = "Steve"; //作用：设置默认值
}
public void swim() {
System.out.println(name + " is swimming.");
}
public void playDisc() {
System.out.println(name + " is playing disc.");
}
}
然后，我们在GameUi.java文件中传入两只小狗的信息：
public class GameUi {
public static void main(String[] args) {
Dog d1 = new Dog();
Dog d2 = new Dog();
d1.name = "Annie";
d1.color = "White";
d2.name = "Blake";
d2.color = "Black";
d1.playDisc();
d2.swim();
}
}
点击运行，得到下面的结果：
Annie is playing disc.
Blake is swimming.
我们也可以使用有参函数。 在同样一块作用域中，局部变量优先于成员变量。
在Dog.java中我们写入：
public class Dog {
public String name;
public String color;
public double age;
public int health;
public Dog(String name) {
this.name = name; //this指当前对象
this.health = 70; //健康值默认70
this.color = "Black"; //默认黑色
}
public void swim() {
System.out.println(name + " is swimming. ");
}
public void playDisc() {
System.out.println(name + " is playing disc.");
}
}
在GameUi.java中我们写入：
import com.icss.bk.biz.Dog;
public class GameUi {
public static void main(String[] args) {
Dog d1 = new Dog("Annie");
Dog d2 = new Dog("Blake");
d1.name = "Annie";
d2.name = "Barron";
d2.color = "Yellow";
d1.playDisc();
System.out.println(d1.name + " is " + d1.color + ". Health status:" + " " + d1.health);
d2.swim();
System.out.println(d2.name + " is " + d2.color + ". Health status:" + " " + d2.health);
}
}
运行后我们得到结果：
Annie is playing disc.
Annie is Black. Health status: 70
Barron is swimming.
Barron is Yellow. Health status: 70
我们可以尝试写多参函数：
二、封装
面向对象（OOP）：Object Orient Program
三个重要特征：继承、封装和多态。
封装：主要作用是为了代码安全。使用关键字private，在作用域外不可被调用。
现在我们给小狗加上泳姿：
public class Dog {
public String name;
public String color;
public double age;
public int health;
private String swimType; //泳姿
public Dog(String name) {
this.name = name; //this指当前对象
this.health = 70; //健康值默认70
this.color = "Black"; //默认黑色
this.swimType = "Doggie style";
}
public void swim() {
System.out.println(name + " is swimming, using " + this.swimType + ".");
}
}
import com.icss.bk.biz.Dog;
public class GameUi {
public static void main(String[] args) {
Dog d1 = new Dog("Annie");
Dog d2 = new Dog("Blake");
d2.name = "Barron";
d2.color = "Yellow";
d2.swim();
System.out.println(d2.name + " is " + d2.color + ". Health status:" + " " + d2.health);
}
}
得到结果：
Barron is swimming, using Doggie style.
Barron is Yellow. Health status: 70
假如我们想在GameUi.java中调用swimTpye，是无法调用和改变的。
如果改变的话，需要回到Dog.java函数中去修改。假如我们根据水的深浅，来修改小狗的游泳姿势。
首先，我们新建一个包，其中，entity的意思为实体包。
实体：只有属性，没有方法。
然后，我们建一个实体类，游泳池：
package com.iscc.bk.entity;
public class Pool {
public double length; //游泳池长度
public double width; //游泳池宽度
public double depth; //游泳池高度
public Pool(double length,double width,double depth) {
this.length = length;
this.width = width;
this.depth = depth;
}
}
然后，我们给小狗定义一下，如果游泳池水深超过2米，则使用潜水的方式：
package com.icss.bk.biz;
import com.iscc.bk.entity.Pool;
public class Dog {
public String name;
public String color;
public double age;
public int health;
private String swimType; //泳姿
public Dog(String name) {
this.name = name; //this指当前对象
this.health = 70; //健康值默认70
this.color = "Black"; //默认黑色
this.swimType = "doggie style";
}
public void swim(Pool pool) {
if(pool != null) {
if(pool.depth > 2) {
this.swimType = "diving";
}
System.out.println(name + " is swimming, using " + this.swimType + ".");
}
}
public void playDisc() {
System.out.println(name + " is playing disc.");
}
}
在Game.java界面，输入代码：
package com.icss.ui;
import com.icss.bk.biz.Dog;
import com.iscc.bk.entity.Pool;
public class GameUi {
public static void main(String[] args) {
Dog d1 = new Dog("Annie");
Dog d2 = new Dog("Blake");
Pool pool = new Pool(100,25,3);
d1.name = "Annie";
d2.name = "Barron";
d2.color = "Yellow";
//d1.playDisc();
//System.out.println(d1.name + " is " + d1.color + ". Health status:" + " " + d1.health);
d2.swim(pool);
System.out.println(d2.name + " is " + d2.color + ". Health status:" + " " + d2.health);
}
}
运行后，得到结果：
Barron is swimming, using diving.
Barron is Yellow. Health status: 70
接下来，我们对健康值进行一些调整。我们可以限定，如果健康值输入为负数，或者大于100，都属于无效值。即设定范围为0道100之间。
首先，我们将health的属性由public变为private，然后进行一次封装。鼠标右键选择Source下的Generate Getters and Setters，勾选health。
我们会发现，下面多了2个方法。
在GameUi中，我们修改部分代码内容：
然后我们处理健康值不在0-100范围内的问题。
package com.icss.bk.biz;
import com.iscc.bk.entity.Pool;
public class Dog {
public String name;
public String color;
public double age;
private int health;
private String swimType; //泳姿，私有成员
public Dog(String name) {
this.name = name; //this指当前对象
this.health = 70; //健康值默认70
this.color = "Black"; //默认黑色
this.swimType = "doggie style";
}
public int getHealth() {
if(health > 100) {
this.health = 70; //将大于100的数调回70
}
else if(health < 0) {
this.health = 60; //将小于100的数调回60
}
else {
this.health = health; //正常赋值
}
return health;
}
public void setHealth(int health) {
this.health = health;
}
public void swim(Pool pool) {
if(pool != null) {
if(pool.depth > 2) {
this.swimType = "diving";
}
System.out.println(name + " is swimming, using " + this.swimType + ".");
}
}
public void playDisc() {
System.out.println(name + " is playing disc.");
}
}
然后我们把小狗的健康值调至范围以外
package com.icss.ui;
import com.icss.bk.biz.Dog;
import com.iscc.bk.entity.Pool;
public class GameUi {
public static void main(String[] args) {
Dog d1 = new Dog("Annie");
d1.setHealth(-80);
Dog d2 = new Dog("Blake");
d2.setHealth(120);
Pool pool = new Pool(100,25,3);
d1.name = "Annie";
d2.name = "Barron";
d2.color = "Yellow";
d1.playDisc();
System.out.println(d1.name + " is " + d1.color + ". Health status:" + " " + d1.getHealth());
d2.swim(pool);
System.out.println(d2.name + " is " + d2.color + ". Health status:" + " " + d2.getHealth());
}
}
运行后，我们得到结果：
Annie is playing disc.
Annie is Black. Health status: 60
Barron is swimming, using diving.
Barron is Yellow. Health status: 70
使用同样的方法，可以对name、color、age等进行封装。
三、常量
接下来，我们给小狗确定性别
然后，鼠标右键选择Source下的Generate Getters and Setters，勾选sex
接下来，为小狗赋性别
最后，我们在游泳函数中假如小狗的性别
现在，我们运行程序得到结果：
Barron: male
Barron is swimming, using diving.
Barron is Yellow. Health status: 70
为了避免多次输入male和female出现错误，我们可以这么操作
public class GameUi {
public static final String MALE = "male";
public static final String FEMALE = "female";
public static void main(String[] args) {
Dog d1 = new Dog("Annie");
d1.setHealth(-80);
d1.setSex(FEMALE);
Dog d2 = new Dog("Blake");
d2.setHealth(120);
d2.setSex(MALE);
Pool pool = new Pool(100,25,3);
d1.name = "Annie";
d2.name = "Barron";
d2.color = "Yellow";
d1.swim(pool);
System.out.println(d1.name + " is " + d1.color + ". Health status:" + " " + d1.getHealth());
d2.swim(pool);
System.out.println(d2.name + " is " + d2.color + ". Health status:" + " " + d2.getHealth());
}
}
输出得到结果：
Annie: male
Annie is swimming, using diving.
Annie is Black. Health status: 60
Barron: female
Barron is swimming, using diving.
Barron is Yellow. Health status: 70
上面的例子中，MALE和FEMALE就是常量。当然，我们还要考虑代码摆放的位置，因为放在GameUi中，在Dog中就无法调用了，因此，需要将常量封装到entity里面。我们在entity包中新建Sex类，然后将之前的两行代码从GameUi中转移至Sex中
package com.iscc.bk.entity;
public class Sex {
public static final String MALE = "male";
public static final String FEMALE = "female";
}
然后在GameUi调用的时候，我们也需要将代码进行调整
public class GameUi {
public static void main(String[] args) {
Dog d1 = new Dog("Annie");
d1.setHealth(-80);
d1.setSex(Sex.FEMALE);
Dog d2 = new Dog("Blake");
d2.setHealth(120);
d2.setSex(Sex.FEMALE);
Pool pool = new Pool(100,25,3);
d1.name = "Annie";
d2.name = "Barron";
d2.color = "Yellow";
d1.swim(pool);
System.out.println(d1.name + " is " + d1.color + ". Health status:" + " " + d1.getHealth());
d2.swim(pool);
System.out.println(d2.name + " is " + d2.color + ". Health status:" + " " + d2.getHealth());
}
}
运行后，结果和之前一致。
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