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我正在创建一个证券交易所类型的程序,到目前为止,我的输入已经正常工作,因此它可以正确地接受用户的命令。然而,它对输入的作用并没有按预期工作。我感到困惑的第一件事是为什么在我运行代码时它会抛出 NullPointerException。

基本上,该程序将输入 3 个内容,然后是它们之间的空格。因此,例如,我想以每股 20 美元的价格购买 30 股,我会输入如下输入:

b 30 20

然后它将它分成 3 个部分并将其存储到一个数组中。之后它将比较数组的第一个索引以查看程序应该做什么,在本例中它将购买,因此它将调用购买方法并将份额数量和份额值存储到我的 CircularArrayQueue 中。

它获取存储到节点中的份额值和份额数量,但是当我尝试使用我的 CirularArrayQueue 调用入队方法以将节点存储到队列中时,它给了我一个 NullPointerException。

我遇到的另一个问题是程序的终止。当程序看到输入的第一个索引值为“q”时,程序应该终止。我做了一个while循环,说明当布尔退出为假时它将循环。然后在 while 循环中,我做了一个 if 语句来检查 stockParts[0] 的值是否为“q”。如果是这样,它会将 quit 的值更改为 true 以便它可以结束循环,但由于某种原因它没有终止并且它仍在循环。

几个小时以来,我一直在为这些问题摸不着头脑,但我似乎找不到问题的根源。有人可以帮助我吗?以下是我的主类和 CircularArrayQueue 类的代码:

import java.util.Scanner;
import java.lang.Integer;

public class StockTran {
String command = "";
String[] stockParts = null;
CircleArrayQueue Q;
boolean quit = false;

public StockTran(String inputCommand) {
    try {
        Scanner conReader = new Scanner(System.in);
        this.command = inputCommand.toLowerCase();
        this.stockParts = command.split("\\s"); // splits the input into three parts

        buyShares(Integer.parseInt(stockParts[1]), Integer.parseInt(stockParts[2]));        //testing purpose only

        while (quit == false) {
            if (this.stockParts[0] == "q") {        // ends transaction and terminates program
                System.out.println("Share trading successfully cancelled.");
                quit = true;    
            }

            if (this.stockParts == null || this.stockParts.length > 3) {
                throw new Exception("Bad input.");
            }

            if (stockParts[0] == "b") {     // checks to see if it is a buying of shares
                int shares = Integer.parseInt(stockParts[1]);       // stores share amount
                int value = Integer.parseInt(stockParts[2]);        // stores selling value
                buyShares(shares, value);       // calls buyShares method and adds share to queue
            }
            else if (stockParts[0] == "s") {        // checks to see if it is a selling of shares
                int shares = Integer.parseInt(stockParts[1]);       // stores share amount
                int value = Integer.parseInt(stockParts[2]);        // stores selling value
                sellShares(shares, value);      // calls sellShares method
            }
            else if (stockParts[0] == "c") {        // checks to see if it is capital gain
                capitalGain();      // calls capitalGain and calculates net gain
            }
            System.out.println("Enter your next command or press 'q' to quit: ");
            command = conReader.nextLine().toLowerCase();
            stockParts = command.split("\\s");
        }

    } catch (Exception e) {
        e.printStackTrace();
    }
}


public void buyShares(int shareAmout, int shareValue) {     // takes in share total and values for each share
    Node temp = new Node(shareAmout, shareValue);       // stores values into node
    try {
        Q.enqueue(temp);        // enqueues the node into the CircularArrayQueue
        //System.out.println(Q.toString());

    } catch (FullQueueException e) {
        e.printStackTrace();
    }

}

public void sellShares(int shareAmount, int sharePrice) {   // ToDo

}

public int capitalGain() {  // ToDo
    return 0;
}

public static void main(String[] args) {
    String inputCommand = "";
    Scanner mainReader = new Scanner(System.in);

    System.out.println("Enter 'b' to purchase share, 's' to sell share, 'c' for capital gain, or 'Q' to quit: ");
    inputCommand = mainReader.nextLine();

    StockTran tran = new StockTran(inputCommand);
}
}
public class CircleArrayQueue implements Queue {
protected Node Q[];     // initializes an empty array for any element type
private int MAX_CAP = 0;        // initializes the value for the maximum array capacity
private int f, r;

public CircleArrayQueue(int maxCap) {
    MAX_CAP = maxCap;
    Q = new Node[MAX_CAP];  // sets Q to be a specific maximum size specified
    f = 0;      // sets front value to be 0
    r = 0;      // sets rear value to be 0;
}

public int size() {
    return (MAX_CAP - f + r) % MAX_CAP;     // returns the size of the CircularArrayQueue
}

public boolean isEmpty() {      // if front and rear are of equal value, Queue is empty
    return f == r;
}

public Node front() throws EmptyQueueException {        // method to get the front value of the CircularArrayQueue
    if (isEmpty()) throw new EmptyQueueException("Queue is empty.");
        return Q[f];        // returns object at front of CircularArrayQueue
}

public Node dequeue() throws EmptyQueueException {  // method to remove from the front of the CircularArrayQueue
    if (isEmpty()) throw new EmptyQueueException("Queue is empty.");
        Node temp = Q[f];       // stores front object in local variable
        Q[f] = null;        // sets the value to be null in the array
        f = (f + 1) % MAX_CAP;      // sets the new front value to be this
        return temp;        // returns the object that was originally in the front
}

public void enqueue(Node element) throws FullQueueException {       // method to add to the end of the CircualarArrayQueue
    if (size() == MAX_CAP - 1) throw new FullQueueException("Queue has reached maximum capacity.");
        Q[r] = element;     // stores the new element at the rear of array
        r = (r + 1) % MAX_CAP;      // sets the new rear value to be the location after element insertion
}
}
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2 回答 2

2

您还没有初始化引用 Q。因为它是一个字段变量,所以它默认初始化为 null。

    CircleArrayQueue Q;

当你遇到这样的问题时,你必须调试它。一种信息来源是异常的堆栈跟踪,它将告诉您异常是在哪里引发的。您还可以要求开发环境中的调试器在引发异常时自动停止。

其次,在 Java 中比较字符串时,使用 equals() 方法而不是 == 运算符。equals() 方法比较对象值。== 运算符比较指向对象的引用的值。您可以拥有两个具有不同参考值的相等对象。

于 2012-09-23T13:15:02.797 回答
0

初始化你的 CircleArrayQueue Q. 如果你不初始化它。它采用 null 作为默认值。

CircleArrayQueue q= new CircleArrayQueue(size);
于 2012-09-23T13:17:58.670 回答