How to call explicitly and implicitly an Activity on Android?

Question

I would like to know how and when to call explicitly and implicitly an Activity(activity)?

If I want to call an Activity(activity), an internal screen of my app and do a standard processing, how to call the function of the camera or gps, what context should I use, explicit or implicit call?

If I want to call a native feature of Android, or third (from another application), what context should I use, explicit or implicit call?

Answer 1

1 - Implicitly:

In the AndroidManifest.xml declare a intent filter for your activity:

<activity
    android:name="com.example.counter.MainActivity"
    android:label="@string/app_name" >
    <intent-filter>
        <action android:name="com.example.counter.MainAction" />
        <category android:name="android.intent.category.DEFAULT" />
    </intent-filter>
<activity>

Then you call yours Activity:

Intent i=new Intent ("com.example.counter.MainAction");
startActivity(i);

2 - Explicitly.

You use the same way as the previous one but in the intent you need to pass the parameters to activity current and the next Activity.

Intent i=new Intent (this,MainActivity.class);
startActivity(i);

Answer 2

To call your Activity explicitly, you will make use of the Intent:

Intent is a message object that can be used to request an action from another application component.

Knob Próximo onscreen activity_nome.xml:

<Button
        android:id="@+id/btn_prox"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_alignParentEnd="true"
        android:layout_below="@+id/edt_nome"
        android:layout_marginTop="10dp"
        android:text="@string/btn_prox"
        style="?android:attr/buttonBarButtonStyle"
        android:onClick="next" />

Method next in the Archive NomeActivity.java, in that way:

public void next(View view) {

Intent intent = new Intent(this, EmailActivity.class);
startActivity(intent); 

}

See above, imagining you have a button next(método next), as in a login screen, when you click on it you will be sent to another of it through the Explicit Invocation.

• Explicit intentions specify the component to start with the name (the fully qualified class name). Normally, a explicit intention to start a component in the application itself because you know the class name of the activity or service you want start. For example, start a new activity in response to a user action or start a service to download a file in background.

---

To call your Activity implicitly:

Modify your Activity .EmailActivity in your file AndroidManifest.xml:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="br.com.intents">

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme">

        <activity android:name=".NomeActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

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

    </application>

</manifest>

---

Modify by adding the intent-filter:

                <activity android:name=".EmailActivity">

                <intent-filter>
                <action android:name="br.com.intents.android.intent.action.EMAIL" />
                <category android:name="android.intent.category.DEFAULT" />
                </intent-filter>

                </activity>

In the method next, in the Archive NomeActivity.java, modify it like this:

 public void next(View view) {

    Intent intent = new Intent("br.com.intents.android.intent.action.EMAIL");
    startActivity(intent); 

    }

• Implicit intentions do not name any specific component, but declare a general action to be taken, which allows a component from another application treat it. For example: if you want to display to user a location on a map, can use an implicit intention to request another capable application to display a location specified on the map.

---

When explicitly invoke:

Fixed call from an Activity, simple, used by default.

When implicitly invoke:

When you want to call one activity who knows she can be replaced by another activity(flexibility) or some resource native android or third party service.

See the Android Developer guide: https://developer.android.com/guide/components/intents-filters.html