java - 钱包程序中的传输方法出现问题(ArrayList)

Question

我在完善从 2 个数组列表传输内容的传输方法时遇到了麻烦。我需要使用 For-Each 循环来执行此操作，但目前该方法仅传输数组列表中的第一项，仅此而已。

package assignment;

import java.util.ArrayList;

/**
 * A purse holds a collection of coins
 *
 * @author type your name
 */
public class Purse {

    ArrayList<String> coins;
    ArrayList<String> rcoins;

    /**
     * Constructs an empty purse.
     */
    public Purse() {
        coins = new ArrayList<String>();
        rcoins = new ArrayList<String>();
    }

    /**
     * Add a coin to the purse
     *
     * @param coinName the coin to add
     */
    public void addCoin(String coinName) {
        coins.add(coinName);
    }

    /**
     * Return a string describing the object
     *
     * @return a string in the format "Purse[coinName1, coinName2, ...]"
     */
    public String toString() {
        if (coins.size() == 0)
            return "Purse[]";

        String output = "Purse[";

        for (String coin : coins)
            output = output + coin + ", ";

        // remove the last ", " 
        output = output.substring(0, output.length() - 2);
        return output + "]";
    }

    public String reverse() {
        if (coins.size() == 0)
            return "Purse[]";

        String output = "Reverse Purse[";

        for (String coin : coins)
            rcoins.add(0, coin);

        for (String coin : rcoins)
            output += coin + ",";

        output = output.substring(0, output.length() - 1);
        return output + "]";

    }

    public void transfer(Purse a, Purse b) {
        for (String coin : a.coins) {
            b.coins.add(coin);
            coins.remove(coin);
        }
    }

    public String sameContents(Purse a, Purse b) {
        String eq = "";
        int size;
        if (a.coins.size() > b.coins.size())
            size = b.coins.size();
        else
            size = a.coins.size();
        for (int i = 0; i < size; i++) {
            if (a.coins.get(i).equals(b.coins.get(i)))
                eq = "They are equal";
            else
                eq = "They are not equal";
        }
        return eq;
    }
}

我的测试仪

package assignment;

public class PurseTester {

    public static void main(String[] args) {
        //Create new Purses
        Purse p = new Purse();
        Purse q = new Purse();

        //Add coins
        p.addCoin("Nickel");
        p.addCoin("Quarter");
        p.addCoin("Dime");
        q.addCoin("Penny");
        q.addCoin("Quarter");

        //Print contents of Purse P and the reversed contents
        System.out.println(p.toString());
        System.out.println("\n" + p.reverse());

        //Print contents of Purse Q
        System.out.println("\nOther " + q.toString() + "\n");

        //Call the transfer method to transfer the contents of Purse Q into Purse P
        q.transfer(q, p);

        //Print contents after transfer
        System.out.println(p.toString());
        System.out.println("Other " + q.toString());

        System.out.println("\n");
        //Compare purses P and Q to see if they share contents, print reslt
        System.out.println(p.sameContents(p, q));
    }
}

Answer 1

在面向对象编程中，您的transfer方法应如下所示:

// transfer coins from an other purse to this
public void transfer(Purse from) {
    for(String coin : from.coins){
        this.coins.add(coin);
        from.coins.remove(coin);
    }
}

在您原来的示例中，当您从 a 转账时，coins.remove(coin); 将从 this.coins 中删除硬币到b。

从 Purse a 转移到 Purse b (如您编写的方法)的方法可能应该在 Purse 类之外静态定义，因为没有引用当前的Purse this。