Generating a random number every time in a specific time interval in java

I have a method, that draws textures at runtime.

I generate a random number, that changes the height of the texture.

The problem is, that during runtime, all possible random numbers are being created and that the texture is shown with all kind of heights.

I generate a random number with:

Random r = new Random();
int low = 0;
int high = 30;
int result1 = r.nextInt(high-low) + low;

Here's my drawTower() method:

private void drawTower() {

    batcher.draw(AssetLoader.texture1, tower1.getX(), tower1.getY() + tower1.getHeight() - result1, 
           tower1.getWidth(), midPointY - (tower1.getHeight()));    
}

You can see that there is result1, which is the randomly generated number.

Now the problem: In my method render() I use that drawTower() method, which is the reason why all possible random numbers are being created.

How can I fix this issue? I thought about generating a random number and after specific time interval, a new random number is being generated. But I don't know how it's done.

public void render(float runTime) {

    // We will move these outside of the loop for performance later.
    Wizard wizard = myWorld.getWizard();

    // Fill the entire screen with black, to prevent potential flickering.
    Gdx.gl.glClearColor(0, 0, 0, 1);
    Gdx.gl.glClear(GL20.GL_COLOR_BUFFER_BIT);

    // Begin ShapeRenderer
    shapeRenderer.begin(ShapeRenderer.ShapeType.Filled);

    // Draw Background color
    shapeRenderer.setColor(55 / 255.0f, 80 / 255.0f, 100 / 255.0f, 1);
    shapeRenderer.rect(0, 0, 136, midPointY + 66);

    // End ShapeRenderer
    shapeRenderer.end();

    // Begin SpriteBatch
    batcher.begin();      
    batcher.enableBlending(); 

    batcher.draw(AssetLoader.texture0,
            wizard.getX(), wizard.getY(), wizard.getWidth(), wizard.getHeight());

    // Draw Tower
    drawTower();

    // End SpriteBatch
    batcher.end();

}