Question

我无法完善传输方法以从2个数组列表中传输内容。我需要使用For-Each循环来执行此操作，但此时该方法仅传输数组列表中的第一个项目，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;
    }
}

我的测试员

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方法应该是：

// 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);
    }
}

在原始示例中，当您从coins.remove(coin);转移到this.coins时，a会从b移除硬币。

从Purse a转换为Purse b的方法与您编写的方法一样，可能已在Purse类之外定义为静态，因为没有引用当前Purse this。

Answer 2

public void transfer(Purse a, Purse b)
{

    for(String coin : a.coins){
        b.coins.add(coin);
        coins.remove(coin);
}

你的错误就在这条线上：

coins.remove(coin);

这是从属于this对象的钱包中移除硬币，这与您在测试代码中转移它们的钱币相同。你添加一个硬币，然后立即从同一个钱包中删除它。你应该从钱包a中删除它们。

由于您在transfer对象上调用Purse，因此最好只有一个参数。你调用方法的Purse应该是你要么从中转移或转移到的钱包，而论证应该是另一个钱包。

钱包程序中传输方法的麻烦（ArrayList）

2 个答案: