如何使用 KeyListener 同时检测多个键？

Question

The question says it all! Here is my code and my attempt at using nested if statements, which ultimately failed. I just want to make my code so when the user is holding shift and down, it loads a different image:

package Michael;

import java.awt.Canvas;
import java.awt.Color;
import java.awt.Graphics;
import java.awt.event.KeyEvent;
import java.awt.event.KeyListener;
import java.awt.image.BufferedImage;
import java.io.File;
import java.io.IOException;
import javax.imageio.ImageIO;

public class MyCanvas extends Canvas implements KeyListener /* Creates the     MyScreen method 
                                                        which extends (inherits) from 
                                                        Canvas and implements (uses) 
                                                        KeyListener. */
{
int myX = 100; // Set the initial x coordinate.
int myY = 100; // Set the initial y coordinate.

BufferedImage down; // Loads the Image.
{
    try
    {
        down = ImageIO.read(new File("Images/down.png")); /* Loads the image from 
                                                          'Images/down.png'.*/
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file."); /* If the file location is
                                                       non-existent, print on a new line
                                                       'Cannot find Image File'.*/
    }
} 

/* The exact same process is done for the next 9 images loaded, just with different,
appropriate, variable names and locations. */

BufferedImage downrun;
{
    try
    {
        downrun = ImageIO.read(new File("Images/downrun.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

BufferedImage left;
{
    try
    {
        left = ImageIO.read(new File("Images/left.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

BufferedImage right;
{
    try
    {
        right = ImageIO.read(new File("Images/right.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

BufferedImage runleft;
{
    try
    {
        runleft = ImageIO.read(new File("Images/runleft.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

BufferedImage runright;
{
    try
    {
        runright = ImageIO.read(new File("Images/runright.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

BufferedImage swoosh;
{
    try
    {
        swoosh = ImageIO.read(new File("Images/swoosh.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

BufferedImage swordraise;
{
    try
    {
        swordraise = ImageIO.read(new File("Images/swordraise.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

BufferedImage up;
{
    try
    {
        up = ImageIO.read(new File("Images/up.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

BufferedImage uprun;
{
    try
    {
        uprun = ImageIO.read(new File("Images/uprun.png"));
    }

    catch (IOException e)
    {
        System.out.println("Cannot find image file.");
    }
}

public MyCanvas() // The MyCanvas constructor.
{
    this.setSize(600, 400); // Makes the size of the canvas 600 x 400.
    this.addKeyListener(this); // Adds KeyListener.
    this.setBackground(Color.WHITE); // Make the background white.
    this.setFocusable(true); // Make the window automatically focused.
}

String image; // Creates the image variable as a string.

public void paint(Graphics g) // The paint method using Graphics.
{
    if (image == "down")
    {
        g.drawImage(down, myX, myY, 55, 55, null);
    }

    else if (image == "up")
    {
        g.drawImage(up, myX, myY, 55, 55, null);
    }

    else if (image == "left")
    {
        g.drawImage(left, myX, myY, 55, 55, null);
    }

    else if (image == "right")
    {
        g.drawImage(right, myX, myY, 55, 55, null);
    }

    else if (image == "swoosh")
    {
        g.drawImage(swoosh, myX, myY, 55, 55, null);
    }

    else if (image == "swordraise")
    {
        g.drawImage(swordraise, myX, myY, 55, 55, null);
    }

    else if (image == "downrun")
    {
        g.drawImage(downrun, myX, myY, 55, 55, null);
    }

    else
    {
        g.drawImage(right, myX, myY, 55, 55, null);
    }

}

@Override
public void keyPressed(KeyEvent e) // When the key is pressed:
{   
    if (e.getKeyCode() == KeyEvent.VK_DOWN) // If the down arrow is pressed:
    {
        image = "down";
        myY += 10; // Increase the y coordinate by 10 (moves down).
    }

    else if (e.getKeyCode() == KeyEvent.VK_UP) // If the up arrow is pressed:
    {
        image ="up";
        myY -= 10; // Decrease the y coordinate by 10 (moves up).
    }

    else if (e.getKeyCode() == KeyEvent.VK_LEFT) // If the left arrow is pressed:
    {
        image = "left";
        myX -= 10; // Decrease the x coordinate by 10 (moves left).
    }

    else if (e.getKeyCode() == KeyEvent.VK_RIGHT) // If the right arrow is pressed:
    {
        image = "right";
        myX += 10; // Increase the x coordinate by 10 (moves right)
    }

    else if (e.getKeyCode() == KeyEvent.VK_1) // If the 1 button is pressed:
    {
        image = "swordraise";
    }

    else if (e.getKeyCode() == KeyEvent.VK_SHIFT)
    {
        if (e.getKeyCode() == KeyEvent.VK_DOWN)
        {
            image = "downrun";
            myY += 50;
        }
    }

    repaint(); // Refresh the Images.
}

@Override
public void keyTyped(KeyEvent e) // When the key is typed:
{
    // TODO Auto-generated method stub
}

@Override
public void keyReleased(KeyEvent e) // When the key is released:
{
    if (e.getKeyCode() == KeyEvent.VK_1) // If the 1 button is released:
    {
        image = "swoosh";
    }

    repaint();
}
}

Much Appreciated!

Answer 1

This answer offers a slightly different approach to Michael by storing all the key codes of pressed keys in an ArrayList data structure as opposed to a boolean array.

ArrayList<int> keys=new ArrayList();
//This ArrayList will hold all the key codes of currently pressed keys.

@Override
public void keyPressed(KeyEvent k){
    if(!keys.contains(k.getKeyCode())){
        keys.add(k.getKeyCode());
    }
}

@Override
public void keyReleased(KeyEvent k){
    if(keys.contains(k.getKeyCode())){
        keys.remove(keys.indexOf(k.getKeyCode()));
    }
}

To test if the shift key is currently pressed, run this code as the key code for shift is 16:

    keys.contains(16);

Answer 2

You can create an array of booleans, representing whether one key is one or off right now .

boolean[] keys = new boolean[500];//whatever the highest keycode is
//....
public void keyPressed(KeyEvent e){
    keys[e.getKeyCode] = true;
}

public void keyReleased(KeyEvent e){
    keys[e.getKeyCode] = false;
}

Then, in your update (or in this case, your paint ) method, you can use your array like this:

public void update(){
    if (keys[KeyEvent.VK_SHIFT] && keys[KeyEvent.VK_DOWN]){
        //user is holding down both shift and down arrow key
    }
}