Drawing a square on a rectangular window in opengl

I am drawing a 3d cube in LWJGL 3.It renders fine with the window size being 600 * 480.It renders a perfect cube.

But if i increase the window's width to 900 while the height remains the same. The cube have a rectangular shape.

In 640 * 480 resolution the front face of cube is :

In 900 * 480 resolution the front face of cube is :

I want to render a perfect square without it resembling a rectangle.

if required the, The render loop is as follows :

private void loop() {

GL.createCapabilities();

        // Set the clear color
        glClearColor(0f, 0.0f, 0.0f, 0.0f);

        float x = 0.5f;
        float y = 0.5f;
        float z = 0.5f;
        float[] vertices = new float[] {
                // VO
                -x, y, z,
                // V1
                -x, -y, z,
                // V2
                x, -y, z,
                // V3
                x, y, z,
                // V4
                -x, y, -z,
                // V5
                x, y, -z,
                // V6
                -x, -y, -z,
                // V7
                x, -y, -z, };

        float[] colours = new float[] {
               0.5f, 0.0f, 0.0f,
              0.0f, 0.5f, 0.0f, 
             0.0f, 0.0f, 0.5f, 
             0.0f, 0.5f, 0.5f,
              0.5f,0.0f, 0.0f, 
             0.0f, 0.5f, 0.0f,
              0.0f, 0.0f, 0.5f,
              0.0f, 0.5f, 0.5f, };

        int[] indices = new int[] {
                // Front face
                0, 1, 3, 3, 1, 2,
                // Top Face
                4, 0, 3, 5, 4, 3,
                // Right face
                3, 2, 7, 5, 3, 7,
                // Left face
                6, 1, 0, 6, 0, 4,
                // Bottom face
                2, 1, 6, 2, 6, 7,
                // Back face
                7, 6, 4, 7, 4, 5, };

        RawModel model = Loader.loadRawModel(vertices, indices, colours);
        ShaderProgram shader = null;

        Transformation transformation = new Transformation();

        try {

            shader = new ShaderProgram(Utils.loadTextFile("src/shaders/vertexShader.txt"),
                    Utils.loadTextFile("src/shaders/fragmentShader.txt"));
            shader.link();
            shader.createUniform("projectionMatrix");
            shader.createUniform("worldMatrix");
        } catch (Exception e) {
            e.printStackTrace();
        }

        float rotation = 0;


        while (!glfwWindowShouldClose(window)) {
            glEnable(GL11.GL_DEPTH_TEST);
            glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT); 

            shader.bind();

            shader.setUniform("projectionMatrix", transformation.getProjectionMatrix((float) Math.toRadians(60),
                    (float) (width / height), 0.1f, 1000f));
            shader.setUniform("worldMatrix", transformation.getWorldMatrix(new Vector3f(0, 0, -3f),
                    new Vector3f(rotation, rotation, rotation), 1));
            model.render();

            shader.unBind();

            glfwSwapBuffers(window); 
            glfwPollEvents();
        }

        shader.dispose();
        Loader.dispose();
    }

Solution

Thanks to @Derhass the problem is solved now.The problem was that i calculated the aspect ration as :

float aspectRatio = (float)(900/480);

which equals to 1.0f .

But when i added f to both width and height :

float aspectRatio = (float)(900f/480f);

It gave the right aspectRatio of 1.875.

now the cube appears as square.