This is a test class done as an assignment for as part of the tasks given by AppAmplify team to me.

test.java

import java.io.*;									// For Input-Output.
import java.security.MessageDigest;					// For encryption.
import java.security.NoSuchAlgorithmException;		// Algorithm Exception.
import java.util.Formatter;							// Formatting generated key.
import java.util.Random;							// Generating random appsflyerkey.

public class task 
{
	// appsflyerkey - private static member.
	private static String appsflyerkey;

	// alphabet - Character to create random alphanumeric string from.
	private static final String alphabet = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZ";

	public static void main(String[] args)
	{
		Console console = System.console();

		// For checking if proper input is available or not.
		boolean flag=false;
		
		while(!flag)
		{
			appsflyerkey = console.readLine("Enter appsflyerkey (or leave empty for random appsflyerkey): ");
			if (appsflyerkey.length()>0 && appsflyerkey.length()!=16)
			{
				System.out.println("Error!!! Please either enter no key or a valid key of 16 characters. Retry.");
			}
			else
			{
				// If user doesn't enter any string, a random string is generated.
				Random random = new Random();
				StringBuilder sb = new StringBuilder(16);
   				for( int i = 0; i < 16; i++ ) 
      				sb.append(alphabet.charAt(random.nextInt(alphabet.length())));
   				appsflyerkey = sb.toString();
   				System.out.println("Generating random appsflyerkey: " + appsflyerkey);
   				flag=true;
			}
		}

		// dkh generated from a substring of appsflyerkey.
		String dkh = appsflyerkey.substring(0,8);
		
		// timestamp generated at runtime.
		String timestamp = new java.text.SimpleDateFormat("ddMMyyyyhhmmss").format(new java.util.Date());

		// uid generated from appsflyerkey and timestamp.
		String uid = appsflyerkey+timestamp;
		
		// af_v generated according to decompiled code.
		String af_v = a(appsflyerkey.substring(0, 7) + (dkh + timestamp.substring(timestamp.length() - 7)));

		System.out.println("Encrypted kay i.e. 'af_v' : " + af_v);
	}

	/**
	 * Generates an encypted key using a given string.
	 * This method is based on the logic of the decompiled code. I have only renamed variable names as they were causing conflicts.
	 * @param  paramString [Given string]
	 * @return             [Encrypted key.]
	 */
	public static String a(String paramString) {
		int i = 0;
		Formatter formatter;
		try {
			MessageDigest localObject = MessageDigest.getInstance("SHA-1");
			((MessageDigest)localObject).reset();
			((MessageDigest)localObject).update(paramString.getBytes("UTF-8"));
			byte[] bytes = localObject.digest();
			formatter = new Formatter();
			int j = bytes.length;
			while (i < j) {
				formatter.format("%02x", new Object[] { Byte.valueOf(bytes[i]) });
				i += 1;
			}
			String formatString = formatter.toString();
			formatter.close();
			return (String)formatString;
		} catch (NoSuchAlgorithmException e) {
			e.printStackTrace();
			return null;
		} catch (UnsupportedEncodingException e) {
			e.printStackTrace();
		} 
		return null;
	}
}