为什么实例变量不能输出？

package exercise;

public class firstCup {

String wall;

String bowlCover;

void drink(){

System.out.println("firstCup有喝水的功能！");

}

void save(){

System.out.println("firstCup有储存的功能！");

}

}

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package exercise;

public class InitialCup {

public static void main(String[] args) {

// TODO Auto-generated method stub

    firstCup Cup=new firstCup();

    Cup.drink();

    Cup.wall="pp";

    Cup.bowlCover="pp";

    Cup.drink();

}

}