1. Introduction

In this tutorial, you'll learn how to connect your Android app to external servers using Application Programming Interfaces (APIs), fetch data from the servers, and display the data in your application.

After completing this tutorial, you'll be able to:

• Understand what an API is and how it works.
• Connect to an API.
• Fetch data from an API.
• Parse JSON data from API responses.

This tutorial assumes that you have a basic understanding of Java and Android Development. Familiarity with JSON would also be helpful since most APIs return data in JSON format.

2. Step-by-Step Guide

APIs are a way for applications to communicate with each other. When your Android app needs to get data from a server, it can make a request to the server's API, which then returns the data to your app.

In this tutorial, we'll use the HttpURLConnection class provided by Java to connect to an API. We'll also use the JSONObject and JSONArray classes to parse the JSON data.

To fetch data from an API:

1. Create a new HttpURLConnection. This opens a connection to the API.

2. Set the request method. This can be GET, POST, PUT, DELETE, etc., depending on what you want to do with the data.

3. Read the response. The API will send back data, which you can read with an InputStream.

4. Parse the JSON data. Since the data is usually in JSON format, you'll need to parse it into a format that Java understands.

3. Code Examples

The following Java code shows how to connect to an API, send a GET request, and read the response:

// Import the necessary libraries
import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        // Create a new thread to handle network operations
        Thread thread = new Thread(new Runnable() {
            @Override
            public void run() {
                try {
                    // Create a URL object with the API's URL
                    URL url = new URL("http://api.example.com/data");

                    // Open a connection to the URL
                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();

                    // Set the request method to GET
                    conn.setRequestMethod("GET");

                    // Get the input stream
                    InputStream in = new BufferedInputStream(conn.getInputStream());

                    // Read the input stream
                    BufferedReader reader = new BufferedReader(new InputStreamReader(in));
                    String line;
                    while ((line = reader.readLine()) != null) {
                        // Do something with the line
                    }

                    // Close the connection
                    conn.disconnect();
                } catch (Exception e) {
                    e.printStackTrace();
                }
            }
        });

        // Start the thread
        thread.start();
    }
}

In this code, we create a new Thread to handle the network operations, since Android does not allow network operations on the main thread.

We create a URL object with the API's URL, open a connection to the URL, and set the request method to GET. We then get the input stream, read it line by line, and do something with each line.

Finally, we close the connection by calling disconnect() on the HttpURLConnection object.

4. Summary

In this tutorial, you've learned how to connect to an API from an Android app, send a GET request, and read the response. You've also learned how to parse JSON data from the response.

To further your learning, consider exploring other types of API requests like POST, PUT, and DELETE, and how to send data to an API. You can also learn about AsyncTask, a class in Android that simplifies handling network operations on a separate thread.

You can find more about APIs and networking in Android in the Android Developers Guide.

5. Practice Exercises

1. Exercise 1: Connect to another API and fetch data from it. Try to find an API that returns data in a different format and parse that data.

2. Exercise 2: Modify the code to use an AsyncTask instead of a Thread.

3. Exercise 3: Send a POST request to an API. You'll need to find an API that allows POST requests and understands what data to send.

Here are the solutions to the exercises:

1. Solution 1: The solution depends on the API you chose. The process should be similar to the tutorial.

2. Solution 2: You can use an AsyncTask like this:

private class FetchDataTask extends AsyncTask<Void, Void, String> {
    protected String doInBackground(Void... params) {
        // Your networking code goes here
    }

    protected void onPostExecute(String result) {
        // The result of your networking operation is passed to this method
    }
}

You would then call this AsyncTask with new FetchDataTask().execute();.

1. Solution 3: Sending a POST request is similar to sending a GET request. The main difference is that you need to write to the OutputStream of the HttpURLConnection:

conn.setRequestMethod("POST");
OutputStream out = new BufferedOutputStream(conn.getOutputStream());
out.write(data.getBytes());
out.close();

Where 'data' is the data you want to send.