Question

每当我调用pennyCount方法或removePenny方法时，我都会得到一个空指针异常错误，我不明白，因为我的HashSet应该在构造函数中填充。为什么我得到这个，我该如何解决？

import java.util.HashSet;

public class Pocket
{

private HashSet<Penny> penniesSet;



public Pocket(int numOfPennies){

    HashSet<Penny> penniesSet = new HashSet<Penny>();

    for(int n = 0; n < numOfPennies; n++){
        penniesSet.add(new Penny());}



}

public int pennyCount(){
    return penniesSet.size();

}

public Penny removePenny(){

    if(penniesSet.size() == 0){
        return null;
    }
    else{
        Penny toRemove = penniesSet.iterator().next();
        penniesSet.remove(toRemove);
        return toRemove;

    }
}

}

Answer 1

在构造函数中创建两个具有相同名称的HashSet，一个字段和一个local。将会发生的是局部变量将被实例化。到达pennyCount（）时，该字段仍为空。

private HashSet<Penny> penniesSet; //first here

public Pocket(int numOfPennies){
    HashSet<Penny> penniesSet = new HashSet<Penny>(); //then here

要纠正这样做。

private HashSet<Penny> penniesSet;

public Pocket(int numOfPennies){
    penniesSet = new HashSet<Penny>();

Answer 2

在构造函数中，您有以下

HashSet<Penny> penniesSet = new HashSet<Penny>();

应该是

penniesSet = new HashSet<Penny>();

Answer 3

在构造函数中，您声明了一个新的HashSet penniesSet，它应该是：

public Pocket(int numOfPennies){

        penniesSet = new HashSet<Penny>();

        for(int n = 0; n < numOfPennies; n++){
            penniesSet.add(new Penny());}
}

否则你的pennieSet永远不会被初始化。

Answer 4

尝试

public Pocket(int numOfPennies){

    penniesSet = new HashSet<Penny>(); // was HashSet<Penny> penniesSet = new HashSet<Penny>();

    for(int n = 0; n < numOfPennies; n++){
        penniesSet.add(new Penny());}



}

Answer 5

这样改变。

public Pocket(int numOfPennies){

    penniesSet = new HashSet<Penny>();

    for(int n = 0; n < numOfPennies; n++){
        penniesSet.add(new Penny());}

}

Answer 6

变化

public Pocket(int numOfPennies){

    HashSet<Penny> penniesSet = new HashSet<Penny>();

    for(int n = 0; n < numOfPennies; n++){
        penniesSet.add(new Penny());}



}

到

public Pocket(int numOfPennies){

    penniesSet = new HashSet<Penny>(); //set the instance variable instead of creating a local variable

    for(int n = 0; n < numOfPennies; n++){
        penniesSet.add(new Penny());}



}

Answer 7

您正在构造函数中创建新的Set对象，而不是填充类的字段。尝试：

penniesSet = new HashSet<Penny>();

为什么我收到java.lang.NullPointerException错误

7 个答案: