空指针错误有帮助吗？

Question

嗨我得到一个空指针在过程中标记.测试者类位于底部.当试图打印动物园时会发生这种情况.似乎没有为"动物动物"设置名称,但我创造了动物.

它可以访问我想要的错误动物,但是我如何访问我已经制作的列表中的动物(请不要使用for参数中的":")!我知道这是错的,但像animal_list.printdetails？

 public class Zoo { 
    private Animal animal;  
    public int number_animals;//# animals allowed in zoo
    private List<Animal> animal_list;
    private String zoo_name;


public Zoo(String name){
    this.zoo_name = name;
    animal_list = new ArrayList<Animal>();
}

 public void addAnimal(Animal obj) {
        animal_list.add(obj);
 }
 public String toString(){
    String s = "In zoo " + zoo_name + " there are the following animals:\n";
    for (int i = 0; i < animal_list.size(); i++){
        s += animal.getName() + " who weighs " + animal.getWeight() + " kg.";//null pointer on this line why??? i have made an animal. How do I access this animals in the list (please no using the ":" in the for parameter)!
    }
    return s;
 }



public class Animal {

public String name;
public int weight;
private String food_type;
private int space_requirement;
private Zoo zoo;
private Animal animal;  

public Animal(String name, int weight){
    this.name = name;
    this.weight = weight;
}    
public String getName(){
    return this.name;
}
public int getWeight(){
    return this.weight;
}



public class Test {
public static void main(String[] args) {        
    Zoo diego = new Zoo("San Diego");
    Animal giffy = new Animal("Giffy", 950);
    Animal gunther = new Animal("Gunther", 950);    
    diego.addAnimal(giffy);
    diego.addAnimal(gunther);
    System.out.println(diego);

}

Answer 1

for (int i = 0; i < animal_list.size(); i++){
    s += animal.getName() + " who weighs " + animal.getWeight() + " kg.";
}

因为你没有使用你的List,所以你试图引用你的animal字段Zoo(你实际上并没有在任何地方使用它).你必须得到你Animal的animal_list

for (int i = 0; i < animal_list.size(); i++){
    s += animal_list.get(i).getName() + " who weighs " + 
         animal_list.get(i).getWeight() + " kg.";
}

另请注意,您确实应该使用StringBuilder此处而不是String使用+=和+运算符创建新对象:

public String toString() {
    StringBuilder s = new StringBuilder("In zoo "); 
    s.append(zoo_name).append(" there are the following animals:\n");
    for (int i = 0; i < animal_list.size(); i++){
        s.append(animal_list.get(i).getName());
        s.append(" who weighs ");
        s.append(animal_list.get(i).getWeight());
        s.append(" kg.\n");
    }
    return s.toString();
}

Strings在Java中是不可变的; 你无法改变它们.当您使用它们连接它们+=或者+实际上是在创建新String对象并丢弃旧对象时.编译器实际上会将它优化到最佳位置StringBuilder,但是好的做法是不要将它留给编译器.

编辑添加: 如果您不熟悉,以上是方法链接的示例

当你说的话:

String name = animal_list.get(i).getName();

它相当于:

Animal a = animal_list.get(i);
String name = a.getName();