activity_main.xml

<RelativeLayout 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:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.androidcallback.MainActivity" >

    <TextView
        android:id="@+id/textViewPokemonName"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/editTextPokemonId"
        android:layout_alignParentTop="true" />

    <EditText
        android:id="@+id/editTextPokemonId"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentLeft="true"
        android:layout_below="@+id/textViewPokemonName"
        android:layout_marginTop="10dp"
        android:ems="10"
        android:hint="Pokemon Id" />

    <Button
        android:id="@+id/buttonSearchPokemonId"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignLeft="@+id/editTextPokemonId"
        android:layout_alignRight="@+id/editTextPokemonId"
        android:layout_below="@+id/editTextPokemonId"
        android:layout_marginTop="10dp"
        android:text="Search" />

</RelativeLayout>

HttpGetAsyncTask.java

package com.example.androidcallback;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.ClientProtocolException;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicNameValuePair;

import android.os.AsyncTask;

public class HttpGetAsyncTask extends
		AsyncTask<BasicNameValuePair, String, String> {

	public final String url;

	private OnHttpGetListener listener;

	public HttpGetAsyncTask(String url) {
		this.url = url;
	}

	@Override
	protected String doInBackground(BasicNameValuePair... params) {

		String returnValue = "";

		StringBuilder urlBuilder = new StringBuilder();
		urlBuilder.append(url);

		for (BasicNameValuePair pair : params) {
			urlBuilder.append(pair.getName());
			urlBuilder.append("=");
			urlBuilder.append(pair.getValue());
		}

		HttpClient client = new DefaultHttpClient();
		HttpGet get = new HttpGet(urlBuilder.toString());

		try {
			StringBuilder responseBuilder = new StringBuilder();
			HttpResponse response = client.execute(get);

			HttpEntity entity = response.getEntity();
			InputStream content = entity.getContent();
			BufferedReader reader = new BufferedReader(new InputStreamReader(
					content));
			String line;
			while ((line = reader.readLine()) != null) {
				responseBuilder.append(line);
			}

			returnValue = responseBuilder.toString();

		} catch (ClientProtocolException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return returnValue;
	}

	public void setOnHttpGetListener(OnHttpGetListener l) {
		listener = l;
	}

	public void onPreExecute() {
		if (listener != null)
			listener.onHttpGetPreExecute();
	}

	public void onPostExecute(String result) {
		if (listener != null)
			listener.onHttpGetSuccess(result);
	}

	public interface OnHttpGetListener {
		public void onHttpGetSuccess(String result);

		public void onHttpGetPreExecute();
	}
}

MainActivity.java

package com.example.androidcallback;

import org.json.JSONException;
import org.json.JSONObject;

import android.app.Activity;
import android.app.ProgressDialog;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import com.example.androidcallback.HttpGetAsyncTask.OnHttpGetListener;

public class MainActivity extends Activity implements OnHttpGetListener,
		OnClickListener {

	TextView textViewPokemonName;
	EditText editTextPokemonId;
	Button buttonSearchPokemonId;
	ProgressDialog progressDialog;

	@Override
	protected void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.activity_main);
		textViewPokemonName = (TextView) findViewById(R.id.textViewPokemonName);
		editTextPokemonId = (EditText) findViewById(R.id.editTextPokemonId);
		buttonSearchPokemonId = (Button) findViewById(R.id.buttonSearchPokemonId);

		buttonSearchPokemonId.setOnClickListener(this);
		searchPokemonById(99);
	}

	private void searchPokemonById(int pokemonId) {
		HttpGetAsyncTask task = new HttpGetAsyncTask(
				"http://pokeapi.co/api/v1/pokemon/" + pokemonId);
		task.setOnHttpGetListener(this);
		task.execute();
	}

	@Override
	public void onHttpGetSuccess(String result) {
		try {
			JSONObject json = new JSONObject(result);
			String pokemonName = json.getString("name");

			textViewPokemonName.setText(pokemonName);

		} catch (JSONException e) {
			e.printStackTrace();
		}
		progressDialog.dismiss();
	}

	@Override
	public void onClick(View v) {
		String id = editTextPokemonId.getText().toString();
		searchPokemonById(Integer.parseInt(id));
	}

	@Override
	public void onHttpGetPreExecute() {
		progressDialog = ProgressDialog.show(this, "Pokemon Search",
				"Searching Pokemon please wait", true);
	}

}