class web {
    private final OkHttpClient client = new OkHttpClient();
    private String html;

    public String get() {
        System.out.println("Result html " + this.html); //Здесь выводит NULL
        return this.html;
    }

    public void set(String html) {
        this.html = html;
    }

    public void httpPost(final String url, final String param) {

        FormBody.Builder formBuilder = new FormBody.Builder();
        RequestBody formBody = formBuilder.build();
        Request request = new Request.Builder()
                .url(url)
                .build();

        client.newCall(request).enqueue(new Callback() {
            @Override
            public void onFailure(Call call, IOException e) {

            }

            @Override
            public void onResponse(Call call, final Response response) throws IOException {
                if (!response.isSuccessful()) {
                    throw new IOException("Unexpected code " + response);
                } else {
                    set(response.body().string());
                }
            }

        });

    }
}