Java-为什么执行代码行时我的JLabel图标不能立即更新？

Question

(excuse my formatting)

I'm using a JLabel (jLabel3) as a background image for a game I am making. I have another JLabel (jLabel4) as the player. When the Player reaches a certain position, the map (jLabel3) changes to ("/newpackage/Map2TreasureHunt.png") .

My code:

if ((x == (66 + 5 * 50)) && (y == (215 + 51))) {

    jLabel3.setIcon(new ImageIcon(getClass().getResource("/newpackage/Map2TreasureHunt.png")));
        x = 316; //( 66 + 5*50) 
        y = 11; // y - 4 * 51;

    } jLabel4.setLocation(x, y);

I ran this through Debug in Netbeans, and the icon is always updated after the if ends. Then, I tried two if's:

if ((x == (66 + 5 * 50)) && (y == (215 + 51))) {

    jLabel3.setIcon(new ImageIcon(getClass().getResource("/newpackage/Map2TreasureHunt.png")));
        //x = 316; //( 66 + 5*50)
       // y = 11; // y - 4 * 51;

    } 

if (jLabel3.getIcon().toString().equals("/newpackage/Map2TreasureHunt.png")){
        x = 316; //( 66 + 5*50)
        y = 11; // y - 4 * 51;
    }

The JLabel (jLabel3) then updates after BOTH if's run.

I don't quite understand what is going on here. If someone knows a more efficient way to rewrite this code, that would also be nice.

Thanks! -littleCode

Answer 1

Everything in Swing happens in a single thread: the event dispatch thread, which basically does the following:

1. wait for an event (click, key press, etc.)
2. execute the listeners for this event
3. repaint what needs to be
4. go back to 1.

So, while you're still inside the code reacting to an event, nothing will be repainted. The corollary is that, if you loop infinitely in the event dipatch thread, the UI will completely freeze.