java-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();
    }
}