Java game loop stutters

Question

As practice I tried to make a simple program where all you do is control the square around the screen. I`ve tried to make a game loop with interpolation however I am having issues with it, It often stutters and I dont know why. Ive tried removing Thread.sleep(1); to see if this is the issue. if I remove this it actually will decrease the stutter and the fps will jump from around 300 fps to around 8000 fps. But of course if I made this game more complex, I wouldnt be able to get 8000 fps so I wanted to see if the stuttering is still here if I only have around 300 fps so what I did is add a bunch of squares to the screen. This dropped the fps to around 300 and what I noticed is that the stuttering was back. Ive looked at Game loop and Fix your timestep but I couldnt find a solution. What am I doing wrong?

Game loop:

    int targetUps = 60;
    int timePerUpdate = 1_000_000_000 / targetUps;
    long oldTime = System.nanoTime();
    long newTime;
    int accumulator = 0;
    int upsTimer = 0;
    int upsCounter = 0;
    int fpsCounter = 0;

    while(true) {
        newTime = System.nanoTime();
        accumulator += newTime - oldTime;
        upsTimer += newTime - oldTime;
        oldTime = newTime;

        while(accumulator >= timePerUpdate) {
            update();
            accumulator -= timePerUpdate;
            upsCounter++;
        }

        render(Math.min(1.0f, (double) accumulator / timePerUpdate));
        fpsCounter++;

        if(upsTimer >= 1_000_000_000) {
            System.out.println("UPS: " + upsCounter + " | " + "FPS: " + fpsCounter);
            fpsCounter = 0;
            upsCounter = 0;
            upsTimer = 0;
        }
        try {
            Thread.sleep(1);
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

render method:

private void render(double delta) {
    BufferStrategy bs = window.getBufferStrategy();
    Graphics g = bs.getDrawGraphics();
    g.setColor(Color.DARK_GRAY);
    g.fillRect(0,0,Window.WIDTH, Window.HEIGHT);
    player.render(g, delta);
    bs.show();
    g.dispose();
}

Player`s render method:

 public void render(Graphics g, double delta) {
    g.setColor(Color.GREEN);
    double renderX = oldX + (x - oldX) * delta;
    double renderY = oldY + (y - oldY) * delta;
    g.fillRect((int) renderX, (int) renderY, 32, 32);
}

Answer 1

You need to use delta like:

state = currentState * delta + previousState * ( 1.0 - delta );

When accumulator equals timePerUpdate (60 FPS) delta will be 0.

Since your code never uses (1.0 - delta),

 double renderX = oldX + (x - oldX) * delta;
 double renderY = oldY + (y - oldY) * delta;

means you're ignoring the update, and thus stutter.