Force Activity to have Single Instances

To force a Activity to have a Single Instances, you can include android:launchMode="singleTask" under <activity> in AndroidManifest.xml.

android:launchMode determine how the activity should be launched. With android:launchMode="singleTask", The system creates the activity at the root of a new task and routes the intent to it. However, if an instance of the activity already exists, the system routes the intent to existing instance through a call to its onNewIntent() method, rather than creating a new one.

Example:

force Single Instances for Activity

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.androidlaunchmode"
    android:versionCode="1"
    android:versionName="1.0" >

    <uses-sdk
        android:minSdkVersion="8"
        android:targetSdkVersion="17" />

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.androidlaunchmode.MainActivity"
            android:label="@string/app_name" 
            android:launchMode="singleTask">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

package com.example.androidlaunchmode;

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

public class MainActivity extends Activity {
 
 TextView prompt;
 int counter;

 @Override
 protected void onCreate(Bundle savedInstanceState) {
  super.onCreate(savedInstanceState);
  //setContentView(R.layout.activity_main);
  
  prompt = new TextView(this);
  setContentView(prompt);
  
  counter = 0;
  prompt.setText("number of onNewIntent() called = " + counter);
 }

 @Override
 protected void onNewIntent(Intent intent) {
  // TODO Auto-generated method stub
  super.onNewIntent(intent);
  Toast.makeText(getApplicationContext(), 
    "onNewIntent()", 
    Toast.LENGTH_LONG).show();
  counter++;
  prompt.setText("number of onNewIntent() called = " + counter);
 }

}