Functional Interfaces turn this:
glfwSetFramebufferSizeCallback(window, (framebufferSizeCallback = new GLFWFramebufferSizeCallback() {

    @Override
    public void invoke(long window, int width, int height) {
        onResize(width, height);
    }
}));

Into this:
glfwSetFramebufferSizeCallback(window, (framebufferSizeCallback = GLFWFramebufferSizeCallback.create(this::onResize)));