public class cat{
    int color;
    int age;
    int id;
   
    cat(int _color, int _age, int _id){
        color = _color;
        age = _age;
        id = _id;
        }
   
    public static void main(String[] args){
        cat c1 = new cat(1,2,8);
        System.out.println(c1);
        }
   
    }   
   

Cat类没有重写toString()方法
输出时默认调用父类(Object)的toString（）方法

public String toString()
{
    return color+"，"+age+","+id;
}

或者直接这么写也行
System.out.println(c1.color + " " + c1.age + " " + c1.id);