activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_centerHorizontal="true"
        android:layout_centerVertical="true"
        android:text="TextView" />
</RelativeLayout>

mainactivity.java

package com.example.miccy.myapplication;

import android.os.AsyncTask;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;

import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;

public class MainActivity extends AppCompatActivity {

    public class WeatherAPI extends AsyncTask<String, Void, String>{


        @Override
        protected String doInBackground(String... apiUrl) {

            URL url;
            HttpURLConnection httpURLConnection = null;
            InputStream is;
            InputStreamReader isr;
            int data;
            String result = "";

            try{
                url = new URL(apiUrl[0]);
                httpURLConnection = (HttpURLConnection) url.openConnection();
                is = httpURLConnection.getInputStream();
                isr = new InputStreamReader(is);
                data = isr.read();
                while(data!=-1){
                    result += (char) data;
                    isr.read();
                }
                return result;


            }
            catch(Exception e){
                e.printStackTrace();
            }

            return null;
        }
    }

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

        WeatherAPI objectapi = new WeatherAPI();
        String finalCode = "";
        TextView textView = (TextView) findViewById(R.id.textView);

        try{
            finalCode = objectapi.execute("http://api.openweathermap.org/data/2.5/weather?q=London&APPID=ef10661606adcfb9eb4b4808b44d07c2").get();
            Log.i("API:",finalCode);
            textView.setText(finalCode);
        }
        catch (Exception e){
            e.printStackTrace();
        }



    }
}