sometimes we have to make a unique identifier for detecting each device. this class provides a UUID combined with Time in Millisec to generate a unique identifier.

UUIDHelper.java

package com.graymind;

import android.content.Context;
import java.io.IOException;
import java.util.UUID;

public class UUIDHelper {
    private static final String TAG = "UUIDHelper";
    private static final String PATH_ON_INTERNAL_STORAGE = "/Android/data/com.graymind/";
    private static final String FILE_NAME = ".uuid_content";
    public static final String UNIQUE_ID_KEY = "unique_id_value";

    private Context mContext;
    private String mUniqueID = "";

    public UUIDHelper(Context context){
        this.mContext = context;
    }

    public void checkForUUID(){
        String uuidFromPreference = getUniqueIDFromPreference();
        if (uuidFromPreference.equalsIgnoreCase("")){
            if (StorageHelper.isFileExist(getUniqueIDContainerFile())){
                String fileContent = "";
                try {
                    fileContent = StorageHelper.readFromFile(getUniqueIDContainerFile());
                }catch (IOException e){
                    e.printStackTrace();
                }
                if (!fileContent.equalsIgnoreCase("")){
                    mUniqueID = fileContent;
                    saveUniqueIDToPreference();
                }else{
                    mUniqueID = getCombinedUniqueID();
                    saveUniqueIDToPreference();
                    saveUniqueIDToFile();
                }
            }
        }
    }

    /**
     *  generate random app unique id from UUID class and finally combine them
     */
    private String getNewUUID(){
        return UUID.randomUUID().toString();
    }

    private long getCurrentMillis(){
        return System.currentTimeMillis();
    }

    private String getCombinedUniqueID(){
        return getNewUUID() + "_" + getCurrentMillis();
    }

    /**
     * store & restore in SharedPreference
     */
    private void saveUniqueIDToPreference(){
        SharedPreferenceHelper.getInstance(mContext).storeStringValue(UNIQUE_ID_KEY, mUniqueID);
    }

    private String getUniqueIDFromPreference(){
        return SharedPreferenceHelper.getInstance(mContext).restoreStringValue(UNIQUE_ID_KEY);
    }

    /**
     * store & restore in Internal Storage
     */
    private void saveUniqueIDToFile(){
        StorageHelper.createIfNotExist(getUniqueIDContainerFolder());
        try {
            StorageHelper.writeToFile(getUniqueIDContainerFile(), mUniqueID);
        }catch (IOException io){
            io.printStackTrace();
        }
    }

    private String getUniqueIDFromFile(){
        String content = "";
        try {
            content = StorageHelper.readFromFile(getUniqueIDContainerFile());
        }catch (IOException io){
            io.printStackTrace();
        }
        return content;
    }

    private String getUniqueIDContainerFolder(){
        return StorageHelper.getInternalStoragePath() + PATH_ON_INTERNAL_STORAGE;
    }

    private String getUniqueIDContainerFile(){
        return getUniqueIDContainerFolder() + FILE_NAME;
    }

}