没个类都必须有构造方法 可是不一定是一个 也可以不写默认 区别就是构造函数的参数不同 都是一类名命名的 就像你在介绍你自己时 说明的自身特征的函数 初始化

class Meal {
Meal() { System.out.println("Meal()"); }//下面的每个类都有一个构造函数
}

class Bread {
Bread() { System.out.println("Bread()"); }
}

class Cheese {
Cheese() { System.out.println("Cheese()"); }
}

class Lettuce {
Lettuce() { System.out.println("Lettuce()"); }
}

class Lunch extends Meal {
Lunch() { System.out.println("Lunch()"); }
}

class PortableLunch extends Lunch {
PortableLunch() { System.out.println("PortableLunch()");}
}

public class Sandwich extends PortableLunch {
private static Test monitor = new Test();
private Bread b = new Bread();
private Cheese c = new Cheese();
private Lettuce l = new Lettuce();
public Sandwich() {
System.out.println("Sandwich()");
}