园区现有游客

interface Addable {
void add();
}

interface Reducible {
void reduce();
}

interface Changeable extends Addable, Reducible {
int UNITS = 30;
}

class Number implements Changeable {
static int number;

@Override
public void add() {
    number += Changeable.UNITS;
}

@Override
public void reduce() {
    number -= Changeable.UNITS;
}

public void output() {
    System.out.println("园区现有游客：" + number + "人。");
}

}

public class Test {
public static void main(String[] args) {
Number number = new Number();
for (int i = 1; i <= 7; i++) {
number.add();
}
for (int i = 1; i <= 4; i++) {
number.reduce();
}
number.output();
}
}