How to make my Android app appear in the share list of another specific app

- Add below code into your Project AndroidManifest.xml file in Specific Activity.

     <activity
                android:name=".MainActivity"
                android:theme="@style/Theme.AppCompat.Light.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.SEND" />
                <category android:name="android.intent.category.DEFAULT" />
                <data android:mimeType="text/plain" />
            </intent-filter>
     </activity>

- Add the following line of code into your project specific Activity.

    Intent intent = getIntent();
    String action = intent.getAction();
    String type = intent.getType();
    if ("android.intent.action.SEND".equals(action) && type != null && "text/plain".equals(type)) {
                Log.println(Log.ASSERT,"shareablTextExtra",intent.getStringExtra("android.intent.extra.TEXT"));
            }

In order to do this, you need to know the Intent that the application is creating and create an IntentFilter that will add your application to the specific list.

Receiving an Implicit Intent on Intents and Filters (Android Developers)

The application probably uses a specific action name that you could hook to.

<intent-filter . . . >
    <action android:name="com.example.project.SHOW_CURRENT" />
    <action android:name="com.example.project.SHOW_RECENT" />
    <action android:name="com.example.project.SHOW_PENDING" />
    . . .
</intent-filter>

Or it could be looking for applications accepting a certain type of file.

<intent-filter . . . >
    <data android:mimeType="video/mpeg" android:scheme="http" . . . /> 
    <data android:mimeType="audio/mpeg" android:scheme="http" . . . />
    . . .
</intent-filter>

The name of the application and what it is sharing would help me give a more specific response.

add this to your mainefist file

<activity android:name=".ShareActivity">
<intent-filter
    android:label="Share with my app">
    <action android:name="android.intent.action.SEND" />
    <category android:name="android.intent.category.DEFAULT" />
</intent-filter>

this link may help you

Add this code in the activity you want opened first when sharing a content from outside the app, call this method in onCreate()

private void onSharedIntent() {
    Intent receiverdIntent = getIntent();
    String receivedAction = receiverdIntent.getAction();
    String receivedType = receiverdIntent.getType();

    if (receivedAction.equals(Intent.ACTION_SEND)) {

        // check mime type 
        if (receivedType.startsWith("text/")) {

            String receivedText = receiverdIntent
                    .getStringExtra(Intent.EXTRA_TEXT);
            if (receivedText != null) {
                //do your stuff
            }
        }

        else if (receivedType.startsWith("image/")) {

            Uri receiveUri = (Uri) receiverdIntent
                    .getParcelableExtra(Intent.EXTRA_STREAM);

            if (receiveUri != null) {
                //do your stuff
                fileUri = receiveUri;// save to your own Uri object

                Log.e(TAG,receiveUri.toString());
            }
        }

    } else if (receivedAction.equals(Intent.ACTION_MAIN)) {

        Log.e(TAG, "onSharedIntent: nothing shared" );
    }
}

Add this in Manifest,

 <activity
            android:name="your-package-name.YourActivity">
            <intent-filter>
                <action android:name="android.intent.action.SEND" /> 

                <category android:name="android.intent.category.DEFAULT" />      
                <data android:mimeType="image/*" />
                <data android:mimeType="text/*" />
            </intent-filter>
        </activity>