What should you pay attention to when using static in java?

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”); } }