Android HTTP 101

Manifest

Right below the <manifest /> tag:

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

build.gradle

Add to dependencies in the build.gradle inside "app"; your gradle dependencies should look something like this (do not copy this file or your app WILL break, this is just an example):

dependencies {
    //[...] Other libraries here
    implementation 'com.google.code.gson:gson:2.7.+'
    implementation 'com.android.volley:volley:1.0.+'
    implementation 'com.squareup.picasso:picasso:2.71828'
    //[...] More libraries here
}

Request Code

//Imports
import com.android.volley.Request;
import com.android.volley.RequestQueue;
import com.android.volley.Response;
import com.android.volley.VolleyError;
import com.android.volley.toolbox.StringRequest;
import com.android.volley.toolbox.Volley;
import com.google.gson.Gson;

//Request Code
    RequestQueue queue = Volley.newRequestQueue(this);
    
    StringRequest example = new StringRequest(Request.Method.POST, //Make sure to use .GET if it's  a get request.
        checkAppIDURL,
        new Response.Listener<String>() //This is an in-lined function that waits for a response from the server running asynchronously- you do not need to set up the async yourself.
        {
            @Override
            public void onResponse(String response)
            {
                if (StringUtils.isNumeric(response))
                {
                    display.setText("Successfully recieved number.");
                }
                else
                {
                    display.setText("Fatal app error.");
                }
            }

        },
        new Response.ErrorListener() //This is an in-lined function that handles CONNECTION errors. Errors from the SERVER (i.e. wrong username/password, et cetera) should still be handled by Response.Listener.
        {
            @Override
            public void onErrorResponse(VolleyError error)
            {
                display.setVisibility(View.GONE);
                Toast.makeText(ExampleActivity.this, "Connection error. Try again later.", Toast.LENGTH_SHORT).show();
            }
        }
    )
    {
        // POST REQUEST ONLY
        @Override
        public byte[] getBody()
        // To set the POST request body, we need to override the "getBody" function. String.getBytes() usually works here.
        // You should use it in conjunction with the Google Gson library to easily convert classes to JSON.
        {
            Gson gson = new Gson();
            String json = gson.toJson(exampleObject);
            return json.getBytes();
        }
    };
    
    queue.add(example);

Tips on using Gson

A simple Gson tutorial exists here: https://www.tutorialspoint.com/gson/gson_first_application.htm

Try to match the names between the JSON file and the Java class, i.e.

"MyKey": "value"

...means that the java property should be...

String MyKey;

If the JSON has an invalid character in it, i.e. something like this:

"My 😂😂😂    Key": "value"

You can let GSON know by using an annotation.

@SerializedName("My 😂😂😂    Key") String key;

Of course, if you're using emoji in your keys I would recommend you re-evaluate your life choices but you get the idea.

Using Picasso

    ImageView image = (ImageView)findViewById(R.id.imageView);
    String url = "https://upload.wikimedia.org/wikipedia/en/a/a9/Example.jpg";
    
    Picasso.
    get().
    load(url). // URL to Image
    fit(). // Fit in frame
    centerCrop(). // Crop until image is centered
    into(image); // Put result in imageView

Licensing Note

This document and all code are under the UNLICENSE.

donn/AndroidHTTP101.md