为什么我将null作为数组的值？ [英] Why am I getting null as a value of array?

问题描述

我有一个Hra1课程，它定义了游戏规则（游戏= hra）。问题是，我得到一个空值，例如poleMinci == null，尽管数组poleMinci是在构造函数中创建的。换句话说，玩家的移动方法总是返回false。

I have a class Hra1, which defines the rules of a game (game=hra). The problem is, that I am getting a null value, e.g. poleMinci==null, despite the array poleMinci is created in the constructor. In other words, the player's move method always returns false.

构造函数：

public Hra1()
{
    Mince [] poleMinci = new Mince[20];
    poleMinci[0] = new Mince("stříbrná", "coin.png");
    poleMinci[3] = new Mince("stříbrná", "coin.png");
    poleMinci[4] = new Mince("zlatá", "coin_gold.png");
    poleMinci[8] = new Mince("stříbrná", "coin.png");
    poleMinci[10] = new Mince("stříbrná", "coin.png");
    poleMinci[12] = new Mince("stříbrná", "coin.png");
}

玩家的移动方式：

public boolean tahHrace(Tah tah){
    if (poleMinci != null){
        int odkud = tah.getZPozice();
        int kam = tah.getNaPozici();
        boolean kamPrazdne;
        if (poleMinci [kam] != null) 
            kamPrazdne = false;
            else 
            kamPrazdne = true;

        if (kam > odkud && poleMinci [odkud] != null && kamPrazdne == true){
            poleMinci [kam] = poleMinci [odkud];
            poleMinci [odkud] = null;
            System.out.println("hráč táhl z pozice "+tah.getZPozice()
            + " na pozici "+tah.getNaPozici());
            return true;
        }
        else
             return false;

    }
    else
    return false;
}

推荐答案

您正在隐藏变量：

public Hra1()
{
    // the following variable's *scope* is inside of this constructor only
    // outside of the constructor, the local variable below doesn't exist.
    Mince [] poleMinci = new Mince[20]; 

    poleMinci[0] = new Mince("stříbrná", "coin.png");
    poleMinci[3] = new Mince("stříbrná", "coin.png");
    poleMinci[4] = new Mince("zlatá", "coin_gold.png");
    poleMinci[8] = new Mince("stříbrná", "coin.png");
    poleMinci[10] = new Mince("stříbrná", "coin.png");
    poleMinci[12] = new Mince("stříbrná", "coin.png");
}

在该构造函数中，由于poleMinci是在构造函数内部声明的，因此它是可见的仅在构造函数内部。如果类中有一个同名变量，它将为null。要解决此问题，请不要在本地重新声明变量。执行：

In that constructor, since poleMinci was declared inside of the constructor, it is visible only inside of the constructor. If you have a variable of the same name in the class it will be null. To fix this, don't re-declare the variable locally. Do:

public Hra1()
{

    // Mince [] poleMinci = new Mince[20]; // **** not this ****
    poleMinci = new Mince[20]; // **** but this. note the difference? ****

    poleMinci[0] = new Mince("stříbrná", "coin.png");
    poleMinci[3] = new Mince("stříbrná", "coin.png");
    poleMinci[4] = new Mince("zlatá", "coin_gold.png");
    poleMinci[8] = new Mince("stříbrná", "coin.png");
    poleMinci[10] = new Mince("stříbrná", "coin.png");
    poleMinci[12] = new Mince("stříbrná", "coin.png");
}

有关此问题的更多信息，请查看变量阴影。大多数IDE都会警告您可能正在执行此操作，或者设置允许它们执行此操作。我使用Eclipse并设置了我的IDE来警告我。您可能也希望这样做。

For more on this problem, check out Variable Shadowing. Most IDE's either will warn you that you might be doing this, or have a setting that will allow them to do this. I use Eclipse and have set my IDE to warn me. You might wish to do this too.

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