TheInvokingActivityhas a button "Invoke Next Activity" which when clicked explicitly calls theInvokedActivityclass. The relevant part of the code is here:

        Button invokingButton = (Button)findViewById(R.id.invokebutton);        invokingButton.setOnClickListener(new OnClickListener() {                public void onClick(View v) {        Intent explicitIntent = new Intent(InvokingActivity.this,InvokedActivity.class);        startActivity(explicitIntent);        }        });

The layout forInvokingActivityis defined in/res/main.xml:

Here are our java code,InvokingActivity.java:

package com.bogotobogo.explicitintent;import android.app.Activity;import android.content.Intent;import android.os.Bundle;import android.view.View;import android.view.View.OnClickListener;import android.widget.Button;public class InvokingActivity extends Activity {@Override    public void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.main);                Button invokingButton = (Button)findViewById(R.id.invokebutton);        invokingButton.setOnClickListener(new OnClickListener() {                public void onClick(View v) {        Intent explicitIntent = new Intent(InvokingActivity.this,InvokedActivity.class);        startActivity(explicitIntent);        }        });    }}

andInvokedActivity.java:

package com.bogotobogo.explicitintent;import android.app.Activity;import android.os.Bundle;public class InvokedActivity extends Activity {@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);setContentView(R.layout.invokedactivity);}}

Files used in this Explicit Intent example,ExplicitIntent.zip

In the next section, we will see how to work with implicit intents which also needs us to understandintent-filters.

In the previous section, we learned how to useExplicit Intentsto invoke activities through a very simple example. Now, we will move on to a more interesting concept ofImplicit IntentsandIntent Filters.

As described earlier, animplicit intentdoesnotname atarget componentthat should act upon the intent. Android resolves as to which component is best suited to respond to anImplicit Intent. How does this happen?

Basically, anIntent objecthas the following information (among other things like Component name, extras and flags) which is of interest for implicit intents:

• Action
• Category
• Data

So, Android compares the three (action, category and data) to something calledIntent Filtersthat are declared by probable target components who are willing to accept Implicit Intent calls. i.e. Intent Filters are the way of any component toadvertiseits own capabilities to the Android system. This is done declaratively in the AndroidManifest.xml file.

So here are some important points to remember:

1. Implicit Intents do not specify a target component.
2. Components willing to receive implicit intents have to declare their ability to handle a specific intent by declaring intent filters.
3. A component can declare any number of Intent Filters.
4. There can be more than one component that declares the same Intent Filters and hence can respond to the same implicit intent. In that case, the user is presented both the component options and he can choose which one he wants to continue with.
5. We can set priorities for the intent filters to ensure the order of responses.

There are 3 tests conducted in order to match an intent with intent filters:

1. Action Test
2. Category Test
3. Data Test

Finally, we'll look at declaring an implicit intent in one activity which will invoke one of the native activities of the platform by matching the intent filters declared by the same.

TheImplicitIntentActivity creates an implicit intent objectcontacts. This intent object's component is not set. However, the action is set toandroid.content.intent.ACTION_VIEWand the data's URI is set toPeople.CONTENT_URI.

Such an intent matches with the intent filter declared by the view contacts native activity.

So, when we run this application, it displays the native UI for viewing the existing contacts on the phone!

Here is the relevant piece of code for the same:

 Button viewContacts = (Button)findViewById(R.id.ViewContacts); viewContacts.setOnClickListener(new OnClickListener() { public void onClick(View v) { Intent contacts = new Intent(); contacts.setAction(android.content.Intent.ACTION_VIEW); contacts.setData(People.CONTENT_URI); startActivity(contacts); } }); 

In this manner many of the native applications can be seamlessly invoked as one of the activities in our applications through implicit intents.

Here are our Java code,ImplicitIntent.java:

package com.bogotobogo.implicitintent; import android.app.Activity; import android.content.ActivityNotFoundException; import android.content.Intent; import android.os.Bundle; import android.provider.Contacts.People; import android.util.Log; import android.view.View; import android.view.View.OnClickListener; import android.widget.Button; public class ImplicitIntent extends Activity { /** Called when the activity is first created. */ @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); ViewContacts(); } private void ViewContacts() { try { Button viewContacts = (Button)findViewById(R.id.ViewContacts); viewContacts.setOnClickListener(new OnClickListener() { public void onClick(View v) { Intent contacts = new Intent(); contacts.setAction(android.content.Intent.ACTION_VIEW); contacts.setData(People.CONTENT_URI); startActivity(contacts); } });  }catch (ActivityNotFoundException anfe) { Log.e("ViewContacts","Viewing of Contacts failed", anfe); } } } 

Files used in this Implicit Intent example,ImplicitIntent.zip

In this example, we'll have two fields for the latitude and longitude, and a button asking a map for the location.

Here is the layout:

<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"android:orientation="vertical"android:layout_width="fill_parent"android:layout_height="fill_parent"><TableLayoutandroid:layout_width="fill_parent" android:layout_height="wrap_content"android:stretchColumns="1,2"><TableRow><TextViewandroid:layout_width="wrap_content" android:layout_height="wrap_content"android:paddingLeft="2dip"android:paddingRight="4dip"android:text="Location:"/><EditText android:id="@+id/lat"android:layout_width="fill_parent" android:layout_height="wrap_content"android:cursorVisible="true"android:editable="true"android:singleLine="true"android:layout_weight="1"/><EditText android:id="@+id/lon"android:layout_width="fill_parent" android:layout_height="wrap_content"android:cursorVisible="true"android:editable="true"android:singleLine="true"android:layout_weight="1"/></TableRow></TableLayout><Button android:id="@+id/map"android:layout_width="fill_parent" android:layout_height="wrap_content"android:text="Show me the map!"/></LinearLayout>

Our Java code:

package com.bogotobogo.Launch;import android.app.Activity;import android.content.Intent;import android.net.Uri;import android.os.Bundle;import android.view.View;import android.widget.Button;import android.widget.EditText;public class Launch extends Activity {private EditText lat;private EditText lon;@Overridepublic void onCreate(Bundle icicle) {super.onCreate(icicle);setContentView(R.layout.main);Button btn=(Button)findViewById(R.id.map);lat=(EditText)findViewById(R.id.lat);lon=(EditText)findViewById(R.id.lon);btn.setOnClickListener(new View.OnClickListener() {public void onClick(View view) {String _lat=lat.getText().toString();String _lon=lon.getText().toString();Uri uri=Uri.parse("geo:"+_lat+","+_lon);startActivity(new Intent(Intent.ACTION_VIEW, uri));}});}}

The button'sOnClickListenertakes the latitude and longitude, put them into geo scheme Uri.

Uri uri=Uri.parse("geo:"+_lat+","+_lon);

Then, starts the activity after creating an intent requesting to view this Uri (ACTION_VIEW)

startActivity(new Intent(Intent.ACTION_VIEW, uri));

In this section, we'll have a tab browser using an Intent. Each tab will launch its own browser Activity. Actually Android's tab-management framework the Activity's UI into each tab.

Here is the source for our main activity which is hosting theTabView, IntentTab.java:

package com.bogotobogo.intenttab;import android.app.TabActivity;import android.content.Intent;import android.os.Bundle;import android.widget.TabHost;public class IntentTab extends TabActivity {@Overridepublic void onCreate(Bundle savedInstanceState) {super.onCreate(savedInstanceState);TabHost host=getTabHost();host.addTab(host.newTabSpec("one").setIndicator("BoGoToBoGo").setContent(new Intent(this, BoGoBrowser.class)));host.addTab(host.newTabSpec("two").setIndicator("Android").setContent(new Intent(this, AndroidBrowser.class)));}}

Here, we are usingTabActivityas the base class, and so we don't have to use our own layout for the view sinceTabActivitysupplies it for us. So, we just get access to theTabHostand add two tabs. Each tab specifies an Intent that directly refers to another class: BoGoBrowser and AndroidBrowser, respectively.

Other sources we need are:

BoGoBrowser.java:

package com.bogotobogo.intenttab;import android.app.Activity;import android.os.Bundle;import android.webkit.WebView;public class BoGoBrowser extends Activity {WebView browser;@Overridepublic void onCreate(Bundle icicle) {super.onCreate(icicle);browser=new WebView(this);setContentView(browser);browser.loadUrl("http://bogotobogo.com");}}

AdroidBrowser.java:

package com.bogotobogo.intenttab;import android.app.Activity;import android.os.Bundle;import android.webkit.WebView;public class AndroidBrowser extends Activity {WebView browser;@Overridepublic void onCreate(Bundle icicle) {super.onCreate(icicle);browser=new WebView(this);setContentView(browser);browser.loadUrl("http://www.android.com/");}}

One more thing, we need to add the following lines toAndroidManifest.xml.

<activity android:name=".BoGoBrowser" /> <activity android:name=".AndroidBrowser" /> <uses-permission android:name="android.permission.INTERNET" /> 

So, the manifest file should look like this:

<?xml version="1.0" encoding="utf-8"?><manifest xmlns:android="http://schemas.android.com/apk/res/android"      package="com.bogotobogo.intenttab"      android:versionCode="1"      android:versionName="1.0">    <application android:icon="@drawable/icon" android:label="@string/app_name"><activity android:name=".IntentTab"                  android:label="@string/app_name">            <intent-filter>                <action android:name="android.intent.action.MAIN" />                <category android:name="android.intent.category.LAUNCHER" />            </intent-filter>        </activity><activity android:name=".BoGoBrowser" /> <activity android:name=".AndroidBrowser" />     </application>    <uses-sdk android:minSdkVersion="3" />    <uses-permission android:name="android.permission.INTERNET" /> </manifest>