ActivityA.java

package com.example.testSingleton;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.widget.Toast;

public class ActivityA extends Activity {
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);

        //Show the string value defined by the private constructor
        Toast.makeText(getApplicationContext(),Singleton.getInstance().getString(), Toast.LENGTH_LONG).show();

        //Change the string value and launch intent to ActivityB
        Singleton.getInstance().setString("Singleton");
        Intent intent = new Intent(getApplicationContext(),ActivityB.class);
        this.startActivity(intent);
    }
}

ActivityB.java

package com.example.testSingleton;

import android.app.Activity;
import android.os.Bundle;
import android.widget.Toast;

/**
 * Created with IntelliJ IDEA.
 * Date: 13/05/13
 * Time: 10:40
 */
public class ActivityB extends Activity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

        //Show the string value of the singleton
        Toast.makeText(getApplicationContext(),Singleton.getInstance().getString(), Toast.LENGTH_SHORT).show();
    }
}

Singleton.java

package com.example.testSingleton;

/**
 * Created with IntelliJ IDEA.
 * Date: 13/05/13
 * Time: 10:36
 */
public class Singleton {
    private static Singleton mInstance = null;

    private String mString;

    private Singleton(){
        mString = "Hello";
    }

    public static Singleton getInstance(){
        if(mInstance == null)
        {
            mInstance = new Singleton();
        }
        return mInstance;
    }

    public String getString(){
        return this.mString;
    }

    public void setString(String value){
        mString = value;
    }
}