Question

所以我正在使用2个类和测试器类在Arrays上做一个项目但是我一直收到nullPointerException错误。我知道错误意味着什么，我相信问题在于我在下面提供的Tank类及其构造函数更具体。 Tank类使用我的类Fish并且应该创建一系列鱼，然后测试者创建一个新的Tank对象。

public class Fish{

  private int weight;
  private String name;

  public Fish(int w, String n){
    weight = w;
    name = n;
  }

  public void feed(){
    weight = weight + 5;
  }

  public int getWeight(){
    return weight;
  }

  public void setWeight(int w){
    weight = w;
  }

  public void setName(String n){
    name = n;
  }

  public String getName(){
    return name;
  }

  public void starve(){
    weight = weight -5;
  }

  public String toString(){
    return name +"'s weight is: " + getWeight();
  }
}

这是Tank类：

import java.util.ArrayList;
public class Tank{

  private boolean isFish = true;
  private int size;
  private int index = 0;
  private Fish[] fishArray;
  private ArrayList<Fish> fishTank = new ArrayList<Fish>();

  public Tank(int i){
   fishArray = new Fish[i];
  }

  public Tank(){
    fishArray = new Fish[10];
  }

  public void feedFish(int i){
    fishArray[i].feed();
  }

  public void addFish(Fish f){
    fishArray[index] = f;
    size++;
  }

  public void starveFish(int i){
    fishArray[i].starve();
  }

  public String getNameFor(int i){
    return (fishArray[i].getName());
  }

  public int getWeightFor(int i){
    return (fishArray[i].getWeight());
  }

  public int getTotalWeight(){
    int w = 0;
    for(int i =0; i< fishArray.length; i++){
      w = fishArray[i].getWeight() + w;
    }
    return w;
  }

  public Fish getLargest(){
    int heaviest = Integer.MIN_VALUE;
    for(int i = 0; i < fishArray.length; i++){
      if (fishArray[i].getWeight() > heaviest)
        heaviest = fishArray[i].getWeight();
      index = i;
    }
    return fishArray[index];
  }

  public Fish getSmallest(){
    int small = Integer.MAX_VALUE;
    for(int i = 0; i < fishArray.length; i++){
      if (fishArray[i].getWeight() < small)
        small = fishArray[i].getWeight();
      index=i;
    }
    return fishArray[index];
  }

  public boolean doesExist(Fish f){
    for(int i = 0; i < fishArray.length; i++){
      if(fishArray[i].getWeight() == f.getWeight() && fishArray[i].getName().equalsIgnoreCase(f.getName()))
        isFish = true;
      else isFish = false;
    }
    return isFish;
  }

  public ArrayList<Fish> transfer(){
    for(int i = 0; i < fishArray.length; i ++){
      fishTank.add(fishArray[i]);}
    return fishTank;
  }
}

正如你所看到的，我正在尝试在tank类中构建一个重载的构造函数，它创建了一个fish对象的数组，但是我在使它工作时遇到了一些麻烦。我相信这是我的测试人员课程中出现错误的地方：

public class TankTest{
  public static void main(String[] args) {

    Tank tank = new Tank();
    tank.addFish(new Fish(10, "Doris"));
    tank.addFish(new Fish(12, "Marlin"));
    tank.addFish(new Fish(7, "nemo"));
    tank.addFish(new Fish(15, "bubbles"));
    tank.addFish(new Fish(18, "gill"));
    tank.addFish(new Fish(5, "peach"));

    tank.feedFish(3);
    tank.starveFish(2);
    tank.starveFish(4);
    tank.feedFish(5);
    tank.starveFish(1);
    tank.feedFish(0);
    tank.feedFish(2);
    tank.starveFish(5);
    tank.starveFish(1);
    tank.feedFish(3);
    tank.feedFish(1);
    tank.feedFish(5);
    tank.starveFish(0);



    System.out.println("The total weight is : " + tank.getTotalWeight());
    System.out.println("The largest Fish is : " + tank.getLargest().getName() );
    System.out.println("The smallest Fish is : " + tank.getSmallest().getName() );

    tank.transfer();

  }
}

任何帮助将不胜感激

Answer 1

我注意到您的addFish方法存在问题：

public void addFish(Fish f){
    fishArray[index] = f;
    size++;
}

所以你的代码应该是这样的：

public void addFish(Fish f){
    fishArray[index] = f;
    index++;
    size++;
}

您也应该增加index变量。目前，您要将所有Fish对象添加到数组的0位置。

您还应该使用size变量而不是fishArray.length来避免到达数组的null元素。 此变量应在构造函数中初始化为0，与index变量相同。

你已经实例化了6个对象（fish），但是数组有10个元素，其中4个没有被实例化。

public Tank(){ 
  fishArray = new Fish[10];
}

这将create 10 fish

中的fishArray

fishArray[0] = new fish()
fishArray[1] = new fish()
...
fishArray[5] = new fish();

但是

fishArray[6] = null
fishArray[7] = null
fishArray[8] = null
fishArray[9] = null

尝试将getTotalWeight(), getLargest() and getSmallest()更改为：

＆＃13;

public int getTotalWeight(){
    int w = 0;
    for(int i =0; i< size; i++){
        w = fishArray[i].getWeight() + w;
    }
    return w;
  }

  public Fish getLargest(){
    int heaviest = Integer.MIN_VALUE;
    for(int i = 0; i < size; i++){
        if (fishArray[i].getWeight() > heaviest) {
           heaviest = fishArray[i].getWeight();
           index = i;
        }            
    }
    return fishArray[index];
  }

  public Fish getSmallest(){
    int small = Integer.MAX_VALUE;
    for(int i = 0; i < size; i++){
        if (fishArray[i].getWeight() < small) {
           small = fishArray[i].getWeight();
           index=i;
        }            
    }
    return fishArray[index];
  }

＆＃13;

nullPointerException错误让我发疯

1 个答案: