Check for multiple key presses in Java Processing.

key.pde

class bindKey {
  IntList keylist;
  public bindKey() {
    keylist = new IntList();
  }
  public void pressKey(int key) {
    if (!keylist.hasValue(key)) keylist.append(key);
  }
  public void releaseKey(int key) {
    if (keylist.hasValue(key)) {
      for (int i = 0; i < keylist.size(); i++) {
        if (keylist.get(i) == key) {
          keylist.remove(i);
          return;
        }
      }
    }
  }
  public Boolean isPressed(int check){
    return keylist.hasValue(check);
  }
}

main.pde

bindKey keys;
void setup(){
  size(800,600);
  keys = new bindKey();
}
void draw(){
  if(keys.isPressed(UP) && keys.isPressed(DOWN)) background(0);
  else background(255);
  
}
void keyPressed(KeyEvent evt){
  keys.pressKey(evt.getKeyCode());
}
void keyReleased(KeyEvent evt){
  keys.releaseKey(evt.getKeyCode());
}