private Border blueBorder[] = new Border[2];
private Border redBorder[] = new Border[2];
private Border greenBorder[] = new Border[2];

for (int i = 0; i < 1; i++) {
    for (int j = 0; j < 2; j++) {  
           redBorder[j] = new Border("border_red.png", j * (BOARD_WIDTH - 110), i * 300);
           blueBorder[j] = new Border("border_blue.png", j * (BOARD_WIDTH - 110), BOARD_HEIGTH - 15);
           greenBorder[j] = new Border("border_green.png", j * 345, 15);
    }
}

Rectangle rGreenBorder[i] = (Rectangle) greenBorder[i].getBounds();

if (rPlayer.intersects(greenBorder[0].getBounds()))

Rectangle rGreenBorder[] = new Rectangle[greenBorder.length]; // length 2
for (int i = 0; i < greenBorder.length; i++) {
    Border border = (Border) greenBorder[i];
    rGreenBorder[i] = border.getBounds();
    
    // Exception in thread "Thread-1" java.lang.NullPointerException 
    // at java.awt.Rectangle.intersects(Rectangle.java:786)
    if (rPlayer.intersects(rGreenBorder[0])) {
          player.setX(player.getWidth() + 15);
    }
    if (rPlayer.intersects(rGreenBorder[1])) {
          player.setX(BOARD_WIDTH - player.getWidth() - 15);
    }
    
    // passing i var into square brackets works.. but that's not the point i want
    if (rPlayer.intersects(rGreenBorder[i])) {
          player.setX(BOARD_WIDTH - player.getWidth() - 15);
    }
}