long currentTime = System.currentTimeMillis();
                 if (currentTime > nextSecond) {
                     nextSecond += 100;
                     frameInLastSecond = framesInCurrentSecond;
                     framesInCurrentSecond = 0;
                 }
                 framesInCurrentSecond++;
                 g.setColor(Color.GREEN);
                 g.drawString(frameInLastSecond + " fps", 20, 20);

static long nextSecond = System.currentTimeMillis() + 1000;
         static int frameInLastSecond = 0;
         static int framesInCurrentSecond = 0;
    

long lastTime = System.currentTimeMillis(); //Последнее время в ms
int fps = 0; //Счетчик FPS
while (true) { //Допустим, игровой цикл
    if (System.currentTimeMillis() - lastTime >= 1000L) {
       System.out.println(fps + " fps");
       fps = 0;
       lastTime = System.currentTimeMillis();
    }
    fps++;
}