Question

Kind of need help understanding what this code actually outputs. Does it out put a uuid to a file?
I found it on http://android-developers.blogspot.com/2011/03/identifying-app-installations.html

    public synchronized static String id(Context context) {
        if (sID == null) {  
            File installation = new File(context.getFilesDir(), INSTALLATION);
            try {
                if (!installation.exists())
                    writeInstallationFile(installation);
                sID = readInstallationFile(installation);
            } catch (Exception e) {
                throw new RuntimeException(e);
            }
        }
        return sID;
    }

    private static String readInstallationFile(File installation) throws IOException {
        RandomAccessFile f = new RandomAccessFile(installation, "r");
        byte[] bytes = new byte[(int) f.length()];
        f.readFully(bytes);
        f.close();
        return new String(bytes);
    }

    private static void writeInstallationFile(File installation) throws IOException {
        FileOutputStream out = new FileOutputStream(installation);
        String id = UUID.randomUUID().toString();
        out.write(id.getBytes());
        out.close();
    }
}

The code exactly how it is posted in my app.

package com.UUIID;

import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import android.util.Log;
import java.io.RandomAccessFile;
import java.util.UUID;

import android.app.Activity;
import android.content.Context;
import android.os.Bundle;
import android.widget.TextView;

public class UUIDActivity extends Activity {
    /** Called when the activity is first created. */
    TextView text;
    private static final String TAG = "Installation";

    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        Log.d(TAG, "program started");
        text = (TextView) findViewById(R.id.textfield);

    }

    class Installation {

        private String sID = null;
        private static final String INSTALLATION = "INSTALLATION";

        public synchronized String id(Context context) {
            if (sID == null) {
                File installation = new File(context.getFilesDir(),
                        INSTALLATION);
                try {

                    if (!installation.exists())
                        writeInstallationFile(installation);
                    Log.d(TAG, "Inside of installation If statement");
                    sID = readInstallationFile(installation);
                } catch (Exception e) {
                    throw new RuntimeException(e);
                }
            }
            return sID;
        }

        private String readInstallationFile(File installation)
                throws IOException {
            RandomAccessFile f = new RandomAccessFile(installation, "r");
            byte[] bytes = new byte[(int) f.length()];
            f.readFully(bytes);
            Log.d(TAG, "Right before it calls f to close");
            f.close();
            return new String(bytes);
        }

        private void writeInstallationFile(File installation)
                throws IOException {
            FileOutputStream out = new FileOutputStream(installation);
            String id = UUID.randomUUID().toString();
            Log.d(TAG, "Right before the file gets written out.");
            out.write(id.getBytes());
            out.close();
        }
    }
}

Answer 1

public synchronized static String id(Context context)

returns a persistent UUID (generated by UUID.randomUUID()). In other words, it will return the same UUID every time. As @Alonso Domiguez answered, it’s probably an installation ID, based on the naming. The goal is to give each instance of the application that uses this code a unique ID.

The trick here is

if (!installation.exists())
    writeInstallationFile(installation);
sID = readInstallationFile(installation);

The function:

writeInstallationFile(installation)

generates a random UUID, and writes that the UUID to a hard-coded file. However, it will only be called once; because after the first call, !installation.exists() will always be false (because the writing of the UUID creates that file).