为什么我将null作为数组的值？

Question

我有一个Hra1类,它定义了一个游戏规则(游戏= hra).问题是,我得到一个空值,例如poleMinci == null,尽管在构造函数中创建了数组poleMinci.换句话说,玩家的移动方法总是返回false.

构造函数:

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

玩家的移动方法:

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("hrá? táhl z pozice "+tah.getZPozice()
            + " na pozici "+tah.getNaPozici());
            return true;
        }
        else
             return false;

    }
    else
    return false;
}

Answer 1

你正在隐藏一个变量:

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

在该构造函数中,由于poleMinci是在构造函数内部声明的,因此它只在构造函数内部可见.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.做:

public Hra1()
{

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

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

有关此问题的更多信息,请查看Variable Shadowing.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并设置我的IDE来警告我.您可能也希望这样做.