This is an Android library that will create a preferences file to store a unique id. The unique id gets generated based on the system time, and then is hashed. The id will persist until the app data is cleared or the app is uninstalled. Using the UniqueId.getId() method, the id will be either generated or simply returned if it already exists.

Example implementation:

package com.example.exampleactivity;

import com.feigdev.uniqueid.UniqueId;

import android.app.Activity;
import android.os.Bundle;
import android.util.Log;

public class ExampleActivity extends Activity {
    private static String TAG = "ExampleActivity";

    /** Called when the activity  is first created. */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        getId();
    }
    
    public void getId(){
        UniqueId id = new UniqueId(ExampleActivity.this);
        Log.d(TAG,id.getId());
        id.generateId("user","pass");
        Log.d(TAG,id.getId());
    }
}