Java Uppercase 可以工作，但我无法在 if 语句中使用它

Question

这个问题已经有答案了:

How do I compare strings in Java? (23 个回答)

已关闭 6 年前。

我找不到问题所在。我正在编写一个国际象棋游戏，我想显示玩家的棋步。例如:E4 至 E5。下面的代码比较旧字符串在与新字符串一起移动之前。问题是我对黑色数字使用小写字母，对白色数字使用大写字母，现在我想告诉玩家他拿了什么数字。该人物的颜色是什么并不相关。因此，我将字母设为大写，并在 if 语句中比较它是否是“P”或“K”等。但这不起作用。 .toUpperCase 可以工作，但在 if 语句处总是失败。这不是一个大问题，我可以使用 or 来询问大小写，但我真的想知道原因是什么。我希望有人能帮助我，并对我糟糕的英语技能表示歉意。

package projekt;

public class MoveList
{
    private String startPosition;
    private String endPosition;
    private String[][] position =
    {
            { "A8", "B8", "C8", "D8", "E8", "F8", "G8", "H8" },
            { "A7", "B7", "C7", "D7", "E7", "F7", "G7", "H7" },
            { "A6", "B6", "C6", "D6", "E6", "F6", "G6", "H6" },
            { "A5", "B5", "C5", "D5", "E5", "F5", "G5", "H5" },
            { "A4", "B4", "C4", "D4", "E4", "F4", "G4", "H4" },
            { "A3", "B3", "C3", "D3", "E3", "F3", "G3", "H3" },
            { "A2", "B2", "C2", "D2", "E2", "F2", "G2", "H2" },
            { "A1", "B1", "C1", "D1", "E1", "F1", "G1", "H1" } };

    private String[][] chessBoard;
    private String[][] oldChessBoard;

    public MoveList(String[][] chessBoard, String[][] oldChessBoard)
    {
        this.chessBoard = chessBoard;
        this.oldChessBoard = oldChessBoard;

        move();
    }

    public void move()
    {
        for (int i = 0; i < oldChessBoard.length; i++)
        {
            for (int j = 0; j < oldChessBoard.length; j++)
            {
                if (oldChessBoard[i][j] != chessBoard[i][j])
                {
                    if (oldChessBoard[i][j] != " " && chessBoard[i][j] == " ")
                    {
                        startPosition = position[i][j];
                    }
                    else if (oldChessBoard[i][j] == " " && chessBoard[i][j] != " ")
                    {
                        endPosition = " to " + position[i][j];
                    }
                    else if (oldChessBoard[i][j] != " " && chessBoard[i][j] != " ")
                    {
                        endPosition = " take " + position[i][j] + " " + takenFigure(oldChessBoard[i][j].toUpperCase());
                    }
                }
            }
        }
        if (startPosition != null)
        {
            System.out.println(startPosition + endPosition);
        }

    }

    private String takenFigure(String figure)
    {
        System.out.println(figure);

        if (figure == "P")
        {
            return "Pawn";
        }
        else if (figure == "K")
        {
            return "Knight";
        }
        else if (figure == "B")
        {
            return "Bishop";
        }
        if (figure == "R")
        {
            return "Rook";
        }
        else if (figure == "Q")
        {
            return "Queen";
        }
        else if (figure == "A")
        {
            return "King";
        }
        return "Nothing";
    }
} 

Answer 1

这是Java中String比较的典型情况，使用==运算符不是正确的方法。

对于基于大小写的相等，应使用 equals() 方法，否则使用 equalsIgnoreCase()。

例如语句

if (figure == "P")
{
  return "Pawn";
}

应该是

if (figure.equals("P"))
{
  return "Pawn";
}

更多详细信息在另一个 StackOverflow question 中提供。 .

事件更好，对于 takenFigure() 方法，应使用 switch 语句而不是连续的 if 语句，例如

switch (figure) {
  case "P":
    return "Pawn";
  case "K":
    return "Knight";
  ...
}