java - 关于 Java 类的示例考试

Question

 public class Bird
{
  private static int id = 0;
  private String kind;

   public Bird(String requiredKind)
   {
    id = id + 1;
    kind = requiredKind;
   }

    public String toString()
   {
        return "Kind: " + kind + ", Id: " + id + "; ";
   }

     public static void main(String [] args)
     {
         Bird [] birds = new Bird[2];
         birds[0] = new Bird("falcon");
         birds[1] = new Bird("eagle");
         for (int i = 0; i < 2; i++)
         System.out.print(birds[i]);
         System.out.println();
    }
} 

这是一个来自样本考试的问题，询问输出，正确答案是 Kind: falcon, Id: 2;种类:鹰，ID:2；

这有什么意义呢？ falcon 的 id 不应该是 1，那么 eagle 因为它的静态 id 应该是 2？

Answer 1

private static int id = 0;

static 表示所有实例将共享该值

因此，每次实例化 Bird 时，值都会增加

public Bird(...){
    id = id + 1;
}

Bird bird1 = new Bird("faclon"); // id = 1
Bird bird2 = new Bird("eagle"); // id = 2

System.out.println(birdl);  // falcon id 2
System.out.println(bird2);  // eagle id 2

假设你这样做了

Bird bird1 = new Bird("falcon"); // id = 1
System.out.println(bird1);  // falcon id = 1

Bird bird2 = new Bird("eagle"); // id = 2
System.out.println(bird2);  // eagle id 2

System.out.println(bird1); // falcon id = 2

Bird bird = new Bird("elephant"); // id = 3

System.out.println(bird1); // falcon id 3

区别在于打印语句的位置。

编辑:所需的输出

Bird[] birds = new Bird[2];
String[] birdNames = {"falcon", "eagle"};

for (int i = 0; i < birds.length; i++){
    birds[i] = new Bird(birdNames[i]);
    System.out.println(birds[i]);
}

在上面的代码中，你会得到

kind: falcon id: 1
kind: eagle id: 2