java - 创建对象后，创建新对象会更改第一个对象的字段

Question

我正在制作多人俄罗斯方 block 。 TetrisGame 类只是使用循环运行游戏，而 Tetris 类构造函数创建一个新棋子(不用担心 board2 类，它只是绘制棋子)。所以这是两个类:

import java.awt.BorderLayout;
import java.awt.event.KeyAdapter;
import java.awt.event.KeyEvent;
import java.io.*;
import java.net.MalformedURLException;

import javax.sound.sampled.AudioInputStream;
import javax.sound.sampled.AudioSystem;
import javax.sound.sampled.Clip;
import javax.sound.sampled.LineUnavailableException;
import javax.sound.sampled.UnsupportedAudioFileException;
import javax.swing.JFrame;

@SuppressWarnings("serial")
public class TetrisGame extends JFrame
{
private board2 myBoard;
private int score;
static Tetris_RafArian tetromino, tet2;
private int speed = 500;
private boolean pause;
private boolean start;

public TetrisGame() throws InterruptedException
{
    super("Tetris");

    setLayout(new BorderLayout());
    setDefaultCloseOperation(EXIT_ON_CLOSE);
    setVisible(true);

    keyListener();
    pause=true;
    start=true;
    score=0;
    while(true)
    {
        if(start)
        {
            myBoard = new board2(10, 20);
            add(myBoard, BorderLayout.CENTER);
            tet2 = new Tetris_RafArian(myBoard, 20, 0, 12, 12);
            tetromino = new Tetris_RafArian(myBoard, 4, 0, 12, 6);
            pack();
            runGame();
        }
    score=0;    
    Thread.sleep(1);
    }

}

/**
 * Listens for key action
 */
public void keyListener()
{
    addKeyListener(new KeyAdapter() {
        public void keyPressed(KeyEvent e) {
            switch (e.getKeyCode()) {
            case KeyEvent.VK_UP:
                if (pause && start)
                    tet2.rotateTetro();
                break;
            case KeyEvent.VK_W:
                if (pause && start)
                    tetromino.rotateTetro();
                break;
            case KeyEvent.VK_LEFT:
                if (tet2.checkWall(0) && pause && start)
                    tet2.move(0);
                break;
            case KeyEvent.VK_A:
                if (tetromino.checkWall(0) && pause && start)
                    tetromino.move(0);
                break;
            case KeyEvent.VK_RIGHT:
                if (tet2.checkWall(1) && pause && start)
                    tet2.move(1);
                break;
            case KeyEvent.VK_D:
                if (tetromino.checkWall(1) && pause && start)
                    tetromino.move(1);
                break;

            case KeyEvent.VK_DOWN:
                if (tet2.checkCollision(tet2.tetro()) && pause && start)
                    tet2.fall();
                break;
            case KeyEvent.VK_S:
                if (tetromino.checkCollision(tetromino.tetro()) && pause && start)
                    tetromino.fall();
                break;

            case KeyEvent.VK_SPACE:
                if (pause && start)
                    speed = 10;
                break;
            case KeyEvent.VK_P:
                if (pause && start)
                    pause = false;
                else
                    pause = true;
                if (!start)
                    start = true;
            }
            myBoard.repaint();
        }
    });
}

/**
 * Runs a single game
 * @throws InterruptedException
 */
public void runGame() throws InterruptedException
{

    //System.out.println(tetromino.checkCollision(tetromino.tetro));
    while(tetromino.checkCollision(tetromino.tetro())||tetromino.tetro()[1][1]!=0)
    {
        while(tetromino.checkCollision(tetromino.tetro()))
        {
            if(pause)
            {
                //tetromino.fall();
                tet2.fall();
                myBoard.repaint();
                Thread.sleep(speed);
            }

        }

        speed = 500;
        int x=0;
        for(int i=0 ; i<20 ; i++)
            if(myBoard.checkRow(i))
            {
                myBoard.removeRow(i);
                score++;
                x++;

            }
        score+=x;
        myBoard.setScore(score);
        tetromino.newTetris();
    }
    start=false;
    myBoard.setEnd(true);
    myBoard.repaint();
}

public static void main(String[] args) throws InterruptedException, MalformedURLException, LineUnavailableException, UnsupportedAudioFileException, IOException
{
    new TetrisGame();
}
}

<小时/>

public class Tetris_RafArian
{
private static board2 myBoard;
private static int color, temColor;
private static int alt=0;
private static int xShift, yShift;
private static int xPos, yPos;
private static int[][] tetro, temTetro, shadTetro; // this is public because Game class needs access

public Tetris_RafArian(board2 bd, int x, int y, int tx, int ty)
{
    myBoard=bd;
    xShift=x;
    yShift=y;
    xPos=tx;
    yPos=ty;

    color = (int)(Math.random()*7)+1;
    temColor = (int)(Math.random()*7)+1;

    tetro = chooseRandom(color);
    temTetro = chooseRandom(temColor);

    shadTetro = new int[4][2];
    for(int i=0; i<4 ; i++)
        for(int j=0; j<2; j++)
            shadTetro[i][j]=tetro[i][j];
    setShadTetro();

    myBoard.setTetris(temTetro, temColor, xPos, yPos);
    myBoard.setTetris(tetro, color, xShift, yShift);
}

public int[][] tetro()
{
    return tetro;
}

/**
 * makes the shadow of the tetris
 */
private void setShadTetro()
{
    while(checkCollision(shadTetro))    
        for(int i=0 ; i<4 ; i++)
            shadTetro[i][1]++;

    myBoard.setTetris(shadTetro, color+7, xShift, yShift);
}

/**
 * A replica of the tetris constructor
 * Helps to simply redefine all the values
 * since creating multiple tetris objects is unneccessary
 */
public void newTetris()
{
    color = temColor;
    tetro = temTetro;

    for(int i=0; i<4 ; i++)
        for(int j=0; j<2; j++)
            shadTetro[i][j]=tetro[i][j];
    setShadTetro();

    myBoard.setTetris(temTetro, 0, xPos, yPos);

    temColor = (int)(Math.random()*7)+1;
    temTetro = chooseRandom(temColor);

    myBoard.setTetris(temTetro, temColor, xPos, yPos);
    myBoard.setTetris(tetro, color, xShift, yShift);
}

/**
 * chooses a tetromino based on the random number inserted
 * @param c
 * @return the desired tetromino
 */
private static int[][] chooseRandom(int c)
{
    switch(c)
    {
    case 1:
        return new int[][] {{-1,0},{0,0},{1,0},{1,1}};//J
    case 2:
        return new int[][] {{-1,1},{0,0},{-1,0},{1,0}};//L
    case 3:
        return new int[][] {{-1,0},{0,0},{1,0},{2,0}};//I
    case 4:
        return new int[][] {{-1,0},{0,0},{0,1},{1,1}};//Z
    case 5:
        return new int[][] {{-1,1},{0,0},{0,1},{1,0}};//S
    case 6:
        return new int[][] {{1,0},{0,0},{1,1},{0,1}};//O
    case 7:
        return new int[][] {{-1,0},{0,0},{1,0},{0,1}};//T
    }

    return new int[1][1];
}

/**
 * Checks for floor collision
 * @param tet
 * @return true, if collided
 * @return false, if not
 */
public boolean checkCollision(int[][] tet)
{
    for(int i=0; i<4 ; i++)
    {
        if(tet[i][1]+1>19)
        {
            //System.out.println("1");
            return false;
        }
        if(tet.equals(tetro))
        {
            int val = myBoard.getValue(tet[i][0]+xShift , tet[i][1]+1);
            if(val>0 && val<8 && !contains(tet, tet[i][0], tet[i][1]+1))
            {
                //System.out.println("2");
                return false;
            }
        }

        else if(myBoard.getValue(tet[i][0]+xShift, tet[i][1]+1)!=0 && !contains(tet, tet[i][0], tet[i][1]+1))
        {
            //System.out.println("3");
            return false;
        }
    }

    return true;
}

/**
 * checks to see if the inputed coordinates are part of the tetromino or not
 * @param tet
 * @param r
 * @param c
 * @return true, if tetromino contains those coordinates
 * @return false, if not
 */
private static boolean contains(int[][] tet, int r, int c)
{
    for(int i=0 ; i<4 ; i++)
        if(r==tet[i][0] && c==tet[i][1])
            return true;

    return false;
}

/**
 * checks for wall collision
 * @param direction
 * @return true, if collided
 */
public boolean checkWall(int d)
{
    if(d>0)
        for(int i=0; i<4 ; i++)
        {
            if(tetro[i][0]>4)
                return false;
            int val = myBoard.getValue(tetro[i][0]+xShift+1, tetro[i][1]);
            if(val>0 && val<8 && !contains(tetro, tetro[i][0]+1, tetro[i][1]))
                return false;
        }

    else
        for(int i=0; i<4 ; i++)
        {
            if(tetro[i][0]<-3)
                return false;
            int val = myBoard.getValue(tetro[i][0]+xShift-1, tetro[i][1]);
            if(val>0 && val <8 && !contains(tetro, tetro[i][0]-1, tetro[i][1]))
                return false;
        }

    return true;
}

/**
 * makes the tetromino fall one step
 */
public void fall()
{
    myBoard.setTetris(tetro, 0, xShift, yShift);

    for(int i=0 ; i<4 ; i++)
        tetro[i][1]++;

    myBoard.setTetris(tetro, color, xShift, yShift);
}

/**
 * moves the tetromino left or right
 * @param dir
 */
public void move(int dir)
{
    myBoard.setTetris(tetro, 0, xShift, yShift);

    if(dir>0)
    {
        for(int i=0 ; i<4 ; i++)
            //if(tetro[i][0]<5)
                tetro[i][0]++;
    }

    else
    {
        for(int i=0 ; i<4 ; i++)
            // if(tetro[i][0]>-3)
                tetro[i][0]--;
    }

    myBoard.setTetris(shadTetro, 0, xShift, yShift);

    for(int i=0 ; i<4 ; i++)
        for(int j=0 ; j<2 ; j++)
            shadTetro[i][j] = tetro[i][j];
    setShadTetro();

    myBoard.setTetris(tetro, color, xShift, yShift);
    myBoard.repaint();
}

/**
 * rotates the tetromino
 */
public void rotateTetro()
{
    if(color==6) return;
    alt++;  // this "alt" field let's pieces 's', 'z', and 'i' alternate between clockwise and counter-clockwise,
            // since these pieces would shift left/right otherwise.

    myBoard.setTetris(tetro, 0, xShift, yShift);
    myBoard.setTetris(shadTetro, 0,xShift, yShift);

    switch(checkRotation())
    {
    case 0:
        for(int i=0 ; i<4 ; i++)
            tetro[i][0]--;

        if(color==3)
            for(int i=0 ; i<4 ; i++)
                tetro[i][0]--;
        break;

    case 1:
        for(int i=0 ; i<4 ; i++)
            tetro[i][0]++;
        break;

    case 2:
        for(int i=0 ; i<4 ; i++)
            tetro[i][1]--;
        break;

    case 3:
        for(int i=0 ; i<4 ; i++)
            tetro[i][1]++;

        if(color==3)
            for(int i=0 ; i<4 ; i++)
                tetro[i][1]++;

        break;

    case 4:
        myBoard.setTetris(tetro, color, xShift, yShift);
        return;
    }

    for(int i=0; i<4 ; i++)
        tetro[i] = rotPointOffset(tetro[i][0],tetro[i][1],tetro[1][0],tetro[1][1]);

    for(int i=0 ; i<4 ; i++)
        for(int j=0 ; j<2 ; j++)
            shadTetro[i][j] = tetro[i][j];
    setShadTetro();

    myBoard.setTetris(tetro, color,xShift, yShift);
    myBoard.repaint();
}

/**
 * checks to see if rotating causes it to go through the walls or into other pieces
 * @return type of collision
 */
public static int checkRotation()
{
    int[]tet;

    for(int i=0; i<4 ; i++)
    {
        tet= rotPointOffset(tetro[i][0],tetro[i][1],tetro[1][0],tetro[1][1]);
        if(tet[0]>5)
            return 0;
        if(tet[0]<-xShift)
            return 1;
        if(tet[1]>19)
            return 2;
        if(tet[1]<0)
            return 3;
        if(myBoard.getValue(tet[0]+xShift, tet[1])!=0 && !contains(tetro, tet[0], tet[1]))
            return 4;
    }

    return 5;
}

/**
 * rotates coordinate (x,y) about (a,b)
 * @param x
 * @param y
 * @param a
 * @param b
 * @return
 */
public static int[] rotPointOffset(int x, int y, int a, int b)
{
    if(color==7 || color==1 || color==2)
        return new int[] {y-b+a,a-x+b};

    else
    {
        if(alt%2==0)
            return new int[] {y-b+a,a-x+b};
        else
            return new int[] {b-y+a,x-a+b};
    }
}
}

<小时/>

现在不要被所有的方法和其他方法冲昏了头脑。我的问题与 TetrisGame 构造函数有关，我在其中创建了两个俄罗斯方 block 对象:tetromino 和 tet2。当我第一次创建 tet2 时，它有自己的俄罗斯方 block (只是一个数组)。但是，在下一行中创建 tetromino 后，tet2 对象的俄罗斯方 block (以及所有其他字段)会发生更改。

现在，我评估了形势。基本上，我创建的这两个对象只是俄罗斯方 block 类的访问器，并且由于某种原因，它们没有像我期望的那样拥有自己独特的一组字段值。我还使用 toString() 方法确认了它们的内存位置是独立的。我以前曾使用相同的技术制作过多人游戏，但由于某种原因这不起作用。如有任何建议，我将不胜感激。谢谢你。

此外，对于所有想要在计算机上尝试此操作的爱好者，这里是 board2 类:

import java.awt.Color;
import java.awt.Dimension;
import java.awt.Font;
import java.awt.Graphics;

import javax.swing.JPanel;

@SuppressWarnings("serial")
public class board2 extends JPanel
{
private int[][] mtx;
private int ver , hor;
private int sz;

//private int[][] nextTet;
//private int nextCol;
private int score;

private Color color;//, nexColor;
private boolean end;

public board2(int r, int c)
{
    hor = r; ver = c;
    sz=30;
    mtx=new int[c][r+16];

    setPreferredSize(new Dimension((hor+16)*sz,ver*sz+15));
    setBackground(Color.WHITE);
    repaint();
}

public boolean checkRow(int r)
{
    for(int i=0 ; i<10 ; i++)
        if(mtx[r][i]==0)
            return false;

    return true;
}

public void removeRow(int r) throws InterruptedException
{
    for(int i=r ; i>=0 ; i--)
        for(int j=0 ; j<10 ; j++)
            if(i>0)
                mtx[i][j]=mtx  [i-1][j];
            else
                mtx[i][j]=0;

    //mtx[0]=new int[26];
}

public void paintComponent(Graphics g)
{
    super.paintComponent(g);

    g.setColor(Color.BLACK);
    g.drawLine(300, 0, 300, 600);
    g.drawLine(306, 0, 306, 580);
    g.drawLine(26, 580, 306, 580);
    for(int i=0 ; i<ver ; i++)
        g.drawLine(306, i*28+20, 300, i*sz);
    g.drawLine(0, 600, 300, 600);
    for(int i=0 ; i<hor+1 ; i++)
        g.drawLine(i*sz, 600, i*28+26, 580);

    for(int i=ver-1 ; i>9 ; i--)
        for(int j=hor+15 ; j>12 ; j--)
        {
            setColor(mtx[i][j]);
            if(mtx[i][j]>0)
            {
                //for(int k=0 ; k<26 ; k++)
                //{
                    g.setColor(color.darker().darker());
                    //g.drawLine(j*sz+k, i*sz+k, (j+1)*sz+k, i*sz+k);
                    g.fillPolygon(new int[] {j*sz, j*sz, j*28+26, j*28+26}, new int[] {i*sz, i*sz+sz, i*28+28+20, i*28+20}, 4);
                    g.setColor(color.darker());
                    g.fillPolygon(new int[] {j*sz, j*sz+sz, j*28+26+28, j*28+26}, new int[] {i*sz, i*sz, i*28+20, i*28+20}, 4);

                    //g.drawLine(j*sz+k, i*sz+k, j*sz+k, (i+1)*sz+k);
                //}
                g.setColor(color);
                g.fillRect(j*sz, i*sz, sz, sz);
                if(mtx[i][j]<8)
                g.setColor(Color.BLACK);
                else
                    g.setColor(Color.GRAY);

                g.drawRect(j*sz, i*sz, sz, sz);
                g.drawPolygon(new int[] {j*sz, j*sz, j*28+26, j*28+26}, new int[] {i*sz, i*sz+sz, i*28+28+20, i*28+20}, 4);
                g.drawPolygon(new int[] {j*sz, j*sz+sz, j*28+26+28, j*28+26}, new int[] {i*sz, i*sz, i*28+20, i*28+20}, 4);
                //g.fillPolygon(new int[] {j*sz+15, j*sz+45, j*sz+15, j*sz+45}, new int[] {i*sz+15, i*sz+15, i*sz+45, i*sz+45}, 4);

            }
            //g.drawRect(j*sz, i*sz, sz, sz);

        }

    for(int i=ver-1 ; i>9 ; i--)
        for(int j=0 ; j<13 ; j++)
        {
            setColor(mtx[i][j]);
            if(mtx[i][j]>0)
            {
                //for(int k=0 ; k<26 ; k++)
                //{
                g.setColor(color.darker().darker());
                // g.drawLine(j*sz+k, i*sz+k, (j+1)*sz+k, i*sz+k);
                g.fillPolygon(new int[] { j * sz + sz, j * sz + sz, j * 28 + 26 + 28, j * 28 + 26 + 28 },
                        new int[] { i * sz, i * sz + sz, i * 28 + 28 + 20, i * 28 + 20 }, 4);
                g.setColor(color.darker());
                g.fillPolygon(new int[] { j * sz, j * sz + sz, j * 28 + 26 + 28, j * 28 + 26 },
                        new int[] { i * sz, i * sz, i * 28 + 20, i * 28 + 20 }, 4);

                    //g.drawLine(j*sz+k, i*sz+k, j*sz+k, (i+1)*sz+k);
                //}
                g.setColor(color);
                g.fillRect(j*sz, i*sz, sz, sz);
                if(mtx[i][j]<8)
                    g.setColor(Color.BLACK);
                else
                    g.setColor(Color.GRAY);

                g.drawRect(j*sz, i*sz, sz, sz);
                g.drawPolygon(new int[] {j*sz+sz, j*sz+sz, j*28+26+28, j*28+26+28}, new int[] {i*sz, i*sz+sz, i*28+28+20, i*28+20}, 4);
                g.drawPolygon(new int[] {j*sz, j*sz+sz, j*28+26+28, j*28+26}, new int[] {i*sz, i*sz, i*28+20, i*28+20}, 4);

            }
            //g.drawRect(j*sz, i*sz, sz, sz);

        }
    for(int i=0 ; i<10 ; i++)
        for(int j=hor+15 ; j>12 ; j--)
        {
            setColor(mtx[i][j]);
            if(mtx[i][j]>0)
            {
                //for(int k=0 ; k<26 ; k++)
                // {
                g.setColor(color.darker());
                g.fillPolygon(new int[] { j * sz, j * sz + sz, j * 28 + 26 + 28, j * 28 + 26 },
                        new int[] { i * sz+sz, i * sz+sz, i * 28 + 28+20, i * 28 + 28 +20 }, 4);
                g.setColor(color.darker().darker());
                // g.drawLine(j*sz+k, i*sz+k, (j+1)*sz+k, i*sz+k);
                g.fillPolygon(new int[] { j * sz, j * sz, j * 28 + 26, j * 28 + 26 },
                        new int[] { i * sz, i * sz + sz, i * 28 + 28 + 20, i * 28 + 20 }, 4);

                // g.drawLine(j*sz+k, i*sz+k, j*sz+k, (i+1)*sz+k);
                // }
                g.setColor(color);
                g.fillRect(j*sz, i*sz, sz, sz);
                if(mtx[i][j]<8)
                    g.setColor(Color.BLACK);
                else
                    g.setColor(Color.GRAY);

                g.drawRect(j*sz, i*sz, sz, sz);
                g.drawPolygon(new int[] {j*sz, j*sz, j*28+26, j*28+26}, new int[] {i*sz, i*sz+sz, i*28+28+20, i*28+20}, 4);
                g.drawPolygon(new int[] {j*sz, j*sz+sz, j*28+26+28, j*28+26}, new int[] {i*sz+sz, i*sz+sz, i*28+28+20, i*28+28+20}, 4);
            }
            //g.drawRect(j*sz, i*sz, sz, sz);

        }
    for(int i=0 ; i<10 ; i++)
        for(int j=0 ; j<13 ; j++)
        {
            setColor(mtx[i][j]);
            if(mtx[i][j]>0)
            {
                //for(int k=0 ; k<26 ; k++)
                //{
                    g.setColor(color.darker().darker());
                    //g.drawLine(j*sz+k, i*sz+k, (j+1)*sz+k, i*sz+k);
                    g.fillPolygon(new int[] {j*sz+sz, j*sz+sz, j*28+26+28, j*28+26+28}, new int[] {i*sz, i*sz+sz, i*28+28+20, i*28+20}, 4);
                    g.setColor(color.darker());
                    g.fillPolygon(new int[] {j*sz, j*sz+sz, j*28+26+28, j*28+26}, new int[] {i*sz+sz, i*sz+sz, i*28+20+28, i*28+20+28}, 4);

                    //g.drawLine(j*sz+k, i*sz+k, j*sz+k, (i+1)*sz+k);
                //}
                g.setColor(color);
                g.fillRect(j*sz, i*sz, sz, sz);
                if(mtx[i][j]<8)
                g.setColor(Color.BLACK);
                else
                    g.setColor(Color.GRAY);

                g.drawRect(j*sz, i*sz, sz, sz);
                g.drawPolygon(new int[] {j*sz+sz, j*sz+sz, j*28+26+28, j*28+26+28}, new int[] {i*sz, i*sz+sz, i*28+28+20, i*28+20}, 4);
                g.drawPolygon(new int[] {j*sz, j*sz+sz, j*28+26+28, j*28+26}, new int[] {i*sz+sz, i*sz+sz, i*28+20+28, i*28+20+28}, 4);
                //g.fillPolygon(new int[] {j*sz+15, j*sz+45, j*sz+15, j*sz+45}, new int[] {i*sz+15, i*sz+15, i*sz+45, i*sz+45}, 4);

            }
            //g.drawRect(j*sz, i*sz, sz, sz);

        }


    Font font = new Font("Typo Round Regular Demo", Font.PLAIN, 20);
    g.setFont(font);
    g.setColor(Color.BLACK);
    g.drawString("NEXT:", (hor+1)*sz, 260);
    g.setFont(font.deriveFont(30f));

    g.drawString("SCORE:"+score, 345, 30);

    if(end)
    {
        g.setColor(Color.WHITE);
        g.fillRect(20, 225, 440, 160);
        g.setColor(Color.BLACK);
        g.drawRect(20, 225, 440, 160);
        g.setFont(font.deriveFont(60f));
        g.drawString("GAME OVER", 63, 330);

    }

}

public int getValue(int r, int c)
{
    return mtx[c][r];
}

public void setTetris(int[][] tet, int c, int xShift, int yShift)
{

    for(int i=0 ; i<4 ; i++)
    {
        //System.out.println((tet[i][0]+xShift)+","+(tet[i][1]+yShift));
        mtx[tet[i][1]+yShift][tet[i][0]+xShift]=c;}
    //System.out.println("\n\n");
}

public void sendNext(int[][] tetro, int color)
{
    for(int i=8; i<11 ; i++)
        for(int j=11; j<15 ; j++)
            mtx[i][j]=0;
    for(int i=0 ; i<4; i++)
        mtx[tetro[i][1]+9][tetro[i][0]+12]=color;
}

private void setColor(int i)
{
    switch(i)
    {
    case 1:
        color = Color.BLUE;
        break;
    case 2:
        color = new Color(255,128,0);
        break;
    case 3:
        color = Color.CYAN;
        break;
    case 4:
        color = Color.RED;
        break;
    case 5:
        color = new Color(128,255,0);
        break;
    case 6:
        color = Color.YELLOW;
        break;
    case 7:
        color = Color.MAGENTA;
        break;
    case 8:
        color = new Color(190,190,255);
        break;
    case 9:
        color = new Color(255,222,190);
        break;
    case 10:
        color = new Color(190,255,255);
        break;
    case 11:
        color = new Color(255,190,190);
        break;
    case 12:
        color = new Color(222,255,190);
        break;
    case 13:
        color = new Color(255,255,190);
        break;
    case 14:
        color = new Color(255,190,255);
    }
}

public void setEnd(boolean s)
{
    end =s;
}

public void setScore(int s)
{
    score = s;
}

}

<小时/>

对于所有已被注释掉的代码行，我深表歉意。我应该把它们修剪掉!

Answer 1

您需要删除您想要该对象的多个实例的类中变量的静态修饰符。静态意味着类中的变量只有一个值，例如，它可用于跟踪从球类创建了多少个球。

参见this了解更多详情。