java - 如何存储一个变量使其不能被修改？

Question

抱歉，如果我弄错了这个问题的格式，这是我的第一篇文章。

基本上我的问题是我将一个对象分配为一个变量，然后更改原始对象，然后尝试使用该变量将对象恢复到其原始状态。然而，我不断发现，尽管从未对变量调用方法，但变量本身已更改为与对象现在的样子完全相同。

我尝试存储的对象是 b 并且看似更改的变量都是存储和原始的。我调用了原始方法，但没有人应该改变它，只是从它那里接收信息。我不调用存储中的任何方法。

如何，当我尝试执行 b = store;我可以确保 b 与参数中传递给它的 b 相同吗？

方法代码在这里:

public int getMove(Board b, int playerNum) throws QuitGameException{        
    original = b;
    store = b;
    int otherPlayerNum = 0;
    try{
        out.println("In house 2 on our side " + original.getSeeds(2, playerNum));
    } catch (Exception e){
        out.println("problem");
    }
    try{
        out.println("In house 2 on our side " + store.getSeeds(2, playerNum));
    } catch (Exception e){
        out.println("problem");
    }


    //Prints the current board to the user in a textual interface
    if(playerNum == 1){
        otherPlayerNum = 2;
    } else {
        otherPlayerNum = 1;
    }

    out.println("Opponent's side: ");
    for(int i=6;i>0;i--){
        try{
            out.print("House " + i + " : [" + original.getSeeds(i, otherPlayerNum)+ "] ");
        } catch (InvalidHouseException e){
            out.print("Invalid house");
        }
    }
    out.println();
    out.println("Opponent's score: " + original.getScore(otherPlayerNum));

    out.println("Computer's side: ");
    for(int i=1;i<7;i++){
        try{
            out.print("House " + i + " : [" + original.getSeeds(i, playerNum) + "] ");
        } catch (InvalidHouseException e){
            out.print("Invalid house");
        }
    }
    out.println();
    out.println("Computer's score: " + original.getScore(playerNum));

    //Each move is tried so that the score can be received, the move which produces the highest score is chosen to be returned.
    System.out.println(b.toString());
    int highestScore = b.getScore(playerNum);
    int bestHouse = 0;
    boolean moveFound = false;
    int move = 1;
    int score = 0;
    for(int i =1; i<7 ;i++){
        try{
            b.makeMove(i,playerNum);
            score = b.getScore(playerNum);
        } catch (Exception e){
            out.println("a problem");
            score = 0;
        }

        if(score>highestScore){
            bestHouse = i;
            highestScore = score;
            moveFound = true;
            move = i;
        }
        try{
            System.out.println("Seeds in side " + playerNum + " and house " +i+" = "+original.getSeeds(i, playerNum));
        } catch (Exception e){
            out.println("problem");
        }            
        b = original;
    }
    try{
        out.println("In house 2 on our side " + original.getSeeds(2, playerNum));
    } catch (Exception e){
        out.println("problem");
    }
    try{
        out.println("In house 2 on our side " + store.getSeeds(2, playerNum));
    } catch (Exception e){
        out.println("problem");
    }
    out.println("All okay? "+b.equals(store));
    out.println("All okay? "+original.equals(store));
    int side = playerNum;
    int bestScore = 0;
    int seeds = 0;

    //If no move has been found which increases the score then the first house with a seed in it is chosen to be returned
    if(!moveFound){
        for (int i =1; i<7 ;i++){
            try{
                seeds = original.getSeeds(i,playerNum);
                System.out.println("Seeds = "+ seeds);
                if (seeds>0 && !moveFound){
                    move = i;
                    moveFound = true;
                }
            } catch (Exception e){
                seeds = 0;
            }
        }
    }
    return move;

}

提前致谢。我很乐意提供更多详细信息。

Answer 1

变量不保存对象。它们持有对象的引用(或指针，如果您愿意)。将对象分配给变量不会创建该对象的任何副本。它只是让变量指向这个对象:

Board b = new Board();

这将创建一个 Board 对象，并使 b 变量指向该对象:

                b ------> board object

Board store = b;

这会将同一个棋盘对象分配给另一个变量 store。所以 b 和 store 现在都指向 board 对象。

                b ------> board object
                             ^
                store -------|

因此，如果您现在执行类似 b.setName("new name") 的操作，您会修改板对象的状态，并且由于 b 并存储两个引用相同的板对象，调用 store.getName() 将返回新名称:

                b ------> board object with new name
                             ^
                store -------|

如果你想要一个变量来保存原始的棋盘状态，你需要创建一个棋盘对象的副本:

Board store = new Board(b);

这个构造函数应该从它作为参数的板上复制所有内容:

public Board(Board other) {
    this.name = other.getName();
    ...
}