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();
    }
}

这是一个样本考试的问题，输出被询问，正确答案是Kind：falcon，Id：2;种类：鹰，Id：2;

这有什么意义？对于猎鹰不应该是1，那么鹰，因为它的静态id将是2？

Answer 1

private static int id = 0;

static表示所有实例都将共享值

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

假设您这样做了

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

区别在于打印报表的位置。

编辑：获取所需的输出

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，您将获得

种类：猎鹰id：1
善良：老鹰身份：2

Answer 2

 public class Bird
  {
 private static int id = 0;   //initial value of id =0;
 private String kind;

 public Bird(String requiredKind)
 {
 id = id + 1;               //for first bird 0+1 so id=1 ,second bird 1+1 so id=2 
 kind = requiredKind;       
 }

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

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

关于java类的示例考试

2 个答案: