In android it is very useful to make network requests to mostly interact with an api. There are many ways to achieve that, but the simplest way - that is also recommended by Google - is the library volley.

Get Started

At first you have to add the volley library to your build.gradle

dependencies {
    ...
    compile 'com.android.volley:volley:1.0.0'
}

Next you have to add the Internet permission to your AndroidManifest.xml, if you haven’t already.

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

StringRequest

For an normal GET request were starting with an standard StringRequest that you have to add to volley’s Volley.newRequestQueue.

String url = "http://httpbin.org/get";
 
//request a string response
StringRequest stringRequest = new StringRequest(Request.Method.GET, url, new Response.Listener<String>() {
    @Override
    public void onResponse(String response) {
 
        //now handle the response
        Toast.makeText(MainActivity.this, response, Toast.LENGTH_SHORT).show();
 
    }
}, new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {
         
        //handle the error
        Toast.makeText(MainActivity.this, "An error occurred", Toast.LENGTH_SHORT).show();
        error.printStackTrace();
 
    }
});
 
// Add the request to the queue
Volley.newRequestQueue(this).add(stringRequest);

Now that was just the simplest implementation. You can do also other requests:

• StringRequest
• JSONArrayRequest
• JSONObjectRequest
• ImageRequest

JsonRequest

When you want to make a POST-request you usually want to pass some data, too. For that we are using one of the JSONRequests.

String url = "http://httpbin.org/post";

//create post data as JSONObject - if your are using JSONArrayRequest use obviously an JSONArray :)
JSONObject jsonBody = new JSONObject("{\"message\": \"Hello\"}");
 
//request a json object response
JsonObjectRequest jsonRequest = new JsonObjectRequest(Request.Method.POST, url, jsonBody, new Response.Listener<JSONObject>() {
    @Override
    public void onResponse(JSONObject response) {
 
        //now handle the response
        Toast.makeText(MainActivity.this, response, Toast.LENGTH_SHORT).show();
 
    }
}, new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {
         
        //handle the error
        Toast.makeText(MainActivity.this, "An error occurred", Toast.LENGTH_SHORT).show();
        error.printStackTrace();
 
    }
});
 
// Add the request to the queue
Volley.newRequestQueue(this).add(jsonRequest);

When you don’t want to pass data on the request, you can just replace jsonBody with null:

JsonObjectRequest jsonRequest = new JsonObjectRequest(Request.Method.POST, url, null, new Response.Listener<JSONObject>() {
...
}

ImageRequest

Sometimes you may want to load images from the web and display it in an ImageView or process it. For that you could use volley’s ImageRequest

String url = "http://www.underconsideration.com/brandnew/archives/google_2015_logo_detail.png";

//you could use an image view to display the resulting image
imageView = (ImageView) findViewById(R.id.image);
 
ImageRequest imageRequest = new ImageRequest(url, new Response.Listener<Bitmap>() {
    @Override
    public void onResponse(Bitmap response) {
        
        //now handle the response
        imageView.setImageBitmap(response);

    }
},  0,  //maxWidth if 0 it's going to be ignored
    0,  //maxHeight if 0 it's going to be ignored
    ImageView.ScaleType.FIT_XY, //ScaleType
    Bitmap.Config.ARGB_8888, //just leave it. Fits for the most
    new Response.ErrorListener() {
    @Override
    public void onErrorResponse(VolleyError error) {

        //handle the error
        Toast.makeText(MainActivity.this, "An error occurred", Toast.LENGTH_SHORT).show();

        error.printStackTrace();
    }
});

// Add the request to the queue
Volley.newRequestQueue(this).add(imageRequest);

But in fact I would recommend to use an image loading library like glide or picasso. They are a lot easier to implement and to use and they do have other helpful image related features as well.