浅谈Static关键字

1.使用static关键字声明的属性为全局属性

　　未使用static关键字指定city之前，如果需要将Tom,Jack,Mary三人的城市均改成Beijing,需要再次声明三次对象的city为Beijing

 package packageone;
 class People{
     String name;
     String city  = "Shanghai";
     public People(String name) {
         this.name = name;
     }
     public void showInfo() {
         System.out.println("姓名:"+name+","+"城市:"+city);
     }
 }
 public class StaticDemo {

     public static void main(String[] args) {
         People p1 = new People("Tom");
         p1.city = "Beijing";
         p1.showInfo();
         People p2 = new People("Jack");
         p2.city = "Beijing";
         p2.showInfo();
         People p3 = new People("Mary");
         //未声明p3对象的city属性为Beijing,则其city仍为Shanghai。
         p3.showInfo();
     }
 }

使用static关键字指定city后，只需设置city = “Beijing”一次,即可实现三个人城市的更改

 package packageone;
 class People{
     String name;
     static String city  = "Shanghai";
     public People(String name) {
         this.name = name;
     }
     public void showInfo() {
         System.out.println("姓名:"+name+","+"城市:"+city);
     }
 }
 public class StaticDemo {

     public static void main(String[] args) {
         People p1 = new People("Tom");
         p1.city = "Beijing";
         p1.showInfo();
         People p2 = new People("Jack");
         p2.showInfo();
         People p3 = new People("Mary");
         //未声明p3对象的city属性为Beijing,但其city变为Beijing。
         p3.showInfo();
     }
 }

2.使用static关键字声明的属性和方法可直接通过类名来调用（代码作为对该static应用的解释有点复杂了，同时是接口的简单使用）

 package packageone;

 interface USB {
     void start();
     void stop();
 }

 class C {
     public static void work(USB u) {
         u.start();
         System.out.println("工作中");
         u.stop();
     }
 }

 class USBDisk implements USB {
     @Override
     public void start() {
         System.out.println("U盘开始工作");
     }

     @Override
     public void stop() {
         System.out.println("U盘停止工作");
     }
 }

 class Printer implements USB {
     @Override
     public void start() {
         System.out.println("打印机开始工作");
     }

     @Override
     public void stop() {
         System.out.println("打印机停止工作");
     }
 }

 public class interfacetest {
     public static void main(String[] args) {
         //直接通过类名来调用work方法
         C.work(new USBDisk());
         C.work(new Printer());
     }

 }

3.注意：】使用static方法的时候，只能访问static声明的属性和方法，而非static声明的属性和方法是不能访问的。而非static声明的方法是可以去调用static声明的属性或方法

 package packageone;
 //由于博主我水平有限参考了别人的代码案例,但由于他的代码有较大错误，经调试成功后援引，算是自己的代码了吧~嘿嘿
 class People {
     private String name;
     private int 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;
     }

     // 使用static定义country属性
     private static String country = "China";

     // 定义static方法,修改static属性
     public static void setCountry(String c) {
         country = c;
     }

     // 取得static属性
     public static String getCountry() {
         return country;
     }

     // 通过构造方法为属性赋值(初始化操作)
     public People(String name, int age) {
         this.name = name;
         this.age = age;
     }

     public void info() {
         System.out.println("姓名:"+ getName()+"年龄:"+getAge()+"城市:"+country);
     }
 }

 public class StaticDemo {
     public static void main(String args[]) {
         People per1 = new People("张三", 20);
         People per2 = new People("李四", 21);
         People per3 = new People("王五", 23);
         System.out.println("--------- 修改前-----------");
         per1.info();
         per2.info();
         per3.info();
         System.out.println("--------- 修改后-----------");
         // 直接使用类名称调用方法来修改static属性的内容,正是因为country为static全局变量,才不需要每个人都去修改国籍
         People.setCountry("USA");
         per1.info();
         per2.info();
         per3.info();
     }
 }