静态成员变量修改问题

先看一段代码:

//Student类	
public class Student {
	 String name;
	 int age;
	 static String country ;//静态修饰的成员变量
	
	public Student(){}
	
	public Student(String name,int age,String country){
		this.name = name;
		this.age = age;
		this.country = country;
	}
	//成员方法
	public void show(){
		System.out.println(name+","+age+","+country);
	}
}
//测试类
public class Test2 {
	public static void main(String[] args){
		Student.country ="中国";
		//创建学生对象
		Student s1 = new Student();
		
		s1.name="小哀";
		s1.age=20;
		s1.show();
		
		Student s2 = new Student();
		s2.name="小兰";
		s2.age=19;
		s2.country="Japan";//给小兰的country赋值
		s2.show();
		
		Student s3 = new Student();
		s3.name = "新一";
		s3.age = 19;
		s3.show();
		
		Student s4 = new Student();
		s4.name = "小朱";
		s4.age = 23;
		s4.country = "Chinese";//给小朱的country赋值
		s4.show();
		
		Student s5 = new Student();
		s5.name = "小丽";
		s5.age = 21;
		s5.show();
	}
}

输出结果:
小哀,20,中国
小兰,19,Japan
新一,19,Japan
小朱,23,Chinese
小丽,21,Chinese

虽然static 可以使contry让所有对象共享,但如果单独给一个对象的country赋值,下面新创建的对象的country不赋值的话,他们的country值和这一对象的country值相同.
如果想让所有对象共享这一成员变量,并不能改变,可以再给这一成员变量加上final,测试类中也不能再给country赋值:

//Student类	
public class Student {
	 String name;
	 int age;
	 static final String country ;//静态修饰的成员变量,加上final,值无法改变
	
	public Student(){}
	
	public Student(String name,int age){//构造方法中也不用给country初始化
		this.name = name;
		this.age = age;
	}
	//成员方法
	public void show(){
		System.out.println(name+","+age+","+country);
	}
}