java中使用static要注意什么

package com.jk.ref;
 
class People{
String name;
private static String country="中国";
public People(String name){
this.name=name;
}
public void tell(){
System.out.println("name:"+name+"  "+"country:"+country);
}
/**
 * @return the country
 */
public static String getCountry() {
return country;
}
/**
 * @param country the country to set
 */
public static void setCountry(String country) {
People.country = country;
}
}
public class StaticDemo01 {
 
public static void main(String[] args) {
// TODO Auto-generated method stub
People.setCountry("shanghai");
People ps1=new People("zhangsan");
//People.country="上海";
ps1.tell();
People ps2=new People("lisi");
// ps2.country="上海";
ps2.tell();
People ps3=new People("wangwu");
// ps3.country="上海";
ps3.tell();
}
 
}

public class TestMain {
public static void main(String[] args) {
Super sup = new Sub(); //封装（向上造型）
sup.m1(); //父类引用无法调子类未重写方法，输出mi in Super
sup.m2();//调用子类方法m2，继承先构建父类方法，方法名相同覆盖（重写）方法，输出m2 in Sub
Sub sub = (Sub)sup; //拆箱（向下造型）
sub.m1(); //调用子类静态方法m1，先构建父类方法，方法名相同方法名相同遮蔽方法，输出m2 in Sub
sub.m2();//调用子类方法m2，继承先构建父类方法，方法名相同覆盖（重写）方法，输出m2 in Sub
}
}
class Super{ //父类
public static void m1() { //父类静态方法
System.out.println(“m1 in Super”);
}
public void m2() { //父类方法
System.out.println(“m2 in Super”);
}
}
class Sub extends Super{ //子类
public static void m1() { //子类静态方法
System.out.println(“m1 in Sub”);
}
public void m2() { //子类方法
System.out.println(“m2 in Sub”);
}
}