对内部类的局部变量访问需要声明为final

Question

我得到了一个问题，对内部类的局部变量访问需要声明为final。从方法createGrids()->“squares[i][j]=0；”中，i是一个需要声明为final的局部变量。我不知道为什么，我在字段中添加了final，但它并不起作用。

import java.util.ArrayList;
import java.util.Random;

//略

public class Minesweeper{
    private JFrame frame;
    private int cols = 9;
    private int rows = 9;
    public static final int GRID_HEIGHT = 9;
    public static final int GRID_WIDTH = 9;
    final JButton[][] grids = new JButton[GRID_WIDTH][GRID_HEIGHT];
    final int [][] squares = new int [GRID_WIDTH][GRID_HEIGHT];
    private static int width = 500;
    private static int heigth = 400;

    private JPanel s;
    private JPanel n;
    private JPanel w;
    private int mines = 10;
    private int bomb = 1;
    private JLabel j1;
    private JPanel e;
    private JRadioButton moreGrid;
    ArrayList<Integer> list = new ArrayList<Integer>();

    public Minesweeper() {
        mines=10;
        createGrids();
        s = new JPanel();
        n = new JPanel();
        e = new JPanel();
        w = new JPanel();

        resetButton = new JButton("Rest");
        resetButton.addActionListener(new ActionListener(){
                public void actionPerformed(ActionEvent e){ createGrids();}
            });
        newGameButton = new JButton("New Game");
        frame.add(n, BorderLayout.NORTH);
        frame.add(w, BorderLayout.WEST);
        frame.add(s, BorderLayout.SOUTH);
        s.add(resetButton);
        s.add(newGameButton);
    }

    public void game()
    {
        for(int i = 0; i < GRID_WIDTH; i++) {
            for(int j = 0; j < GRID_HEIGHT; j++) {
                squares[i][j] = 0;
            }
        }
    }
    public void setRandom()
    {
        Random r = new Random();
        for(int x = 0; x < mines; x++){
            int b = r.nextInt(9);
            int c = r.nextInt(9) ;   
            squares[b][c] = bomb;   
        }
    }

    public void createGrids(){
        frame = new JFrame("Minesweeper");
        createMenuBar(frame);
        frame.setTitle("Nicholas Minesweeper");
        JPanel m = new JPanel(new GridLayout(9,9));
        for(int i = 0; i < GRID_WIDTH; i++) {
            for(int j = 0; j < GRID_HEIGHT; j++) {
                grids[i][j] = new JButton();
                grids[i][j].addActionListener(new ActionListener(){
                    public void actionPerformed(ActionEvent e){ 
                        if (squares[i][j] == 1)
                        {
                           System.out.println("BOmb");
                        }
                        else {
                            grids[i][j].setVisible(false);
                        }
                    }
                });
                m.add(grids[i][j]);
            }
        }
        frame.add(m, BorderLayout.CENTER);
        frame.setResizable(false);
        frame.setSize(width, heigth);

        frame.setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
        frame.setVisible(true);
        frame.setSize(350, 250);
        frame.setVisible(true); 
    } 
}

Answer 1

匿名内部类可以通过幕后的一个技巧访问局部变量。局部变量被实现为内部类的隐藏成员变量。它们被分配了局部变量的副本。为了防止复制值出错，Java编译器强制要求这些局部变量必须是final，这样它们就不会被更改，这样复制就会保持正确。

封闭类的字段不需要是final；使用的局部变量必须是final。必须使匿名内部类中使用的所有局部变量为final。可以通过声明要初始化为i和j值的final变量来实现这一点，并在匿名内部类中使用它们。

// Inside the for loops in the createGrids method
grids[i][j] = new JButton();
// Declare x, y final
final int x = i;
final int y = j;
grids[i][j].addActionListener(new ActionListener(){
    public void actionPerformed(ActionEvent e){ 
        // Use x, y instead of i, j inside.
        if (squares[x][y] == 1)
        {
             System.out.println("BOmb");
        }
        else {
             grids[x][y].setVisible(false);
        }
    }
 });

注意，在Java8中，这是不必要的，因为Java8编译器可以检测匿名内部类中使用的局部变量是否是“有效的final”，即不是final，但在初始化后从未更改。