An API is an interface used between the user and the server. There are two types of API’s used which offers a different mechanism for connecting and receiving data. Since connecting an API with an android application requires an internet connection. So make sure that permission is provided in an android manifest file. Write the following in AndroidManifest.xml

<uses-permission android:name="android.permission.INTERNET"/>

Create a layout file

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"

    xmlns:tools="http://schemas.android.com/tools"

    android:layout_width="match_parent"

    android:layout_height="match_parent"

    android:orientation="vertical"

    android:paddingLeft="@dimen/activity_horizontal_margin"

    android:paddingRight="@dimen/activity_horizontal_margin"

    android:paddingTop="@dimen/activity_vertical_margin"

    android:paddingBottom="@dimen/activity_vertical_margin"

    tools:context="com.sample.foo.simplewebapi.MainActivity">

    <EditText

        android:id="@+id/emailText"

        android:layout_width="match_parent"

        android:layout_height="wrap_content"

        android:inputType="textEmailAddress"

        android:hint="Enter email address"/>

    <Button

        android:id="@+id/queryButton"

        android:layout_width="wrap_content"

        android:layout_height="wrap_content"

        android:layout_gravity="end"

        style="@style/Base.Widget.AppCompat.Button.Borderless"

        android:text="Search"/>

    <RelativeLayout

        android:layout_width="match_parent"

        android:layout_height="match_parent"

        android:gravity="center">

        <ProgressBar

            android:id="@+id/progressBar"

            android:layout_width="wrap_content"

            android:layout_height="wrap_content"

            android:indeterminate="true"

            android:layout_centerHorizontal="true"

            android:visibility="gone" />

        <ScrollView

            android:layout_width="match_parent"

            android:layout_height="match_parent">

            <TextView

                android:id="@+id/responseView"

                android:layout_width="wrap_content"

                android:layout_height="wrap_content" />

        </ScrollView>

    </RelativeLayout>

</LinearLayout>

You need to create the connection with the server in the background thread, so to do that either you can use AsynTask class or any network library. A sample for AsyncTask is given below.

class RetrieveFeedTask extends AsyncTask<Void, Void, String> {
        private Exception exception;
        protected void onPreExecute() {
            progressBar.setVisibility(View.VISIBLE);
            responseView.setText("");
        }
        protected String doInBackground(Void... urls) {
            String email = emailText.getText().toString();
            // Do some validation here
            try {
                URL url = new URL(API_URL + "email=" + email + "&apiKey=" + API_KEY);
                HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();
                try {
                    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(urlConnection.getInputStream()));
                    StringBuilder stringBuilder = new StringBuilder();
                    String line;
                    while ((line = bufferedReader.readLine()) != null) {
                        stringBuilder.append(line).append("\n");
                    }
                    bufferedReader.close();
                    return stringBuilder.toString();
                }
                finally{
                    urlConnection.disconnect();
                }
            }
            catch(Exception e) {
                Log.e("ERROR", e.getMessage(), e);
                return null;
            }
        }
        protected void onPostExecute(String response) {
            if(response == null) {
                response = "THERE WAS AN ERROR";
            }
            progressBar.setVisibility(View.GONE);
            Log.i("INFO", response);
            responseView.setText(response);
        }
    }

AsyncTask has three methods onPreExecute(), doInBackground() and onPostExecute().

onPreExecute() is used for some task which need to be considered before making connection such as setting visibility of progress bar etc. onPostExecute() is used for processing after the connection was completed such processing the response etc.