Activities & Intents (Java)

An Activity is a single, focused screen in your app. An Intent is a message object you use to start activities, services, or deliver broadcasts. Together, they drive navigation, data handoff, and interactions with other apps.

Big Picture

Explicit vs Implicit Intents

Type	Use Case	How it Resolves	Example
Explicit	Navigate inside your app	You specify the exact class	new Intent(this, DetailActivity.class)
Implicit	Ask system/other apps to handle an action	Intent resolver matches action/data to registered intent-filters	ACTION_SEND with text/plain to share text

// Explicit: open DetailActivity
Intent intent = new Intent(CurrentActivity.this, DetailActivity.class);
intent.putExtra("ITEM_ID", 42);
startActivity(intent);

// Implicit: share text
Intent share = new Intent(Intent.ACTION_SEND);
share.setType("text/plain");
share.putExtra(Intent.EXTRA_TEXT, "Hello from CodingwithSonu!");
startActivity(Intent.createChooser(share, "Share via"));

Passing Data (Extras & Bundles)

Attach key-value pairs with putExtra (or a Bundle) and read them in the target activity.

// Sender
Intent i = new Intent(this, ProfileActivity.class);
i.putExtra("name", "Sonu");
i.putExtra("age", 28);
ArrayList<String> hobbies = new ArrayList<>();
hobbies.add("Android"); hobbies.add("Biking");
i.putStringArrayListExtra("hobbies", hobbies);
startActivity(i);

// Receiver (ProfileActivity)
String name = getIntent().getStringExtra("name");
int age = getIntent().getIntExtra("age", 0);
ArrayList<String> h = getIntent().getStringArrayListExtra("hobbies");

Activity Result API (Modern way)

Instead of the old startActivityForResult(), use the Activity Result API to get results safely across configuration changes.

// In Activity (Java): register a launcher at field scope
private ActivityResultLauncher<Intent> pickContactLauncher =
    registerForActivityResult(new ActivityResultContracts.StartActivityForResult(), result -> {
        if (result.getResultCode() == Activity.RESULT_OK) {
            Intent data = result.getData();
            // TODO: read selected contact from data
        }
    });

// Launch somewhere
Intent pick = new Intent(Intent.ACTION_PICK, ContactsContract.Contacts.CONTENT_URI);
pickContactLauncher.launch(pick);

Intent-Filters (Manifest)

Components advertise what they can handle. The resolver compares an implicit intent’s action/data/category to these.

<!-- In AndroidManifest.xml -->
<activity android:name=".ShareActivity"
          android:exported="true">
  <intent-filter>
    <action android:name="android.intent.action.SEND" />
    <category android:name="android.intent.category.DEFAULT" />
    <data android:mimeType="image/*" />
  </intent-filter>
</activity>

Tasks & Back Stack (Essentials)

Each Activity belongs to a task (a stack). Pressing back pops the top Activity. Launch modes and flags control stack behavior.

Flag / Mode	Effect	Common Use
FLAG_ACTIVITY_CLEAR_TOP	Clear above target if it exists	Return to existing instance
FLAG_ACTIVITY_NEW_TASK	Start in a new task	Launch from notification/service
singleTop (manifest)	Reuse if already at top	Home screens, detail screens
singleTask (manifest)	One instance per task	Main entry points

PendingIntent (for Notifications/Alarms)

A PendingIntent wraps an intent so other processes (e.g., NotificationManager, AlarmManager) can execute it on your behalf with your app’s identity.

// Create an explicit Intent for an Activity
Intent open = new Intent(this, DetailActivity.class);
open.putExtra("from", "notification");

PendingIntent contentPI = PendingIntent.getActivity(
    this, 1001, open,
    PendingIntent.FLAG_UPDATE_CURRENT | PendingIntent.FLAG_IMMUTABLE
);

Common Patterns

1) Open details with an ID

Intent i = new Intent(this, ProductDetailActivity.class);
i.putExtra("PRODUCT_ID", productId);
startActivity(i);

2) Share an image (file provider)

Uri uri = FileProvider.getUriForFile(this, BuildConfig.APPLICATION_ID + ".fileprovider", imageFile);
Intent share = new Intent(Intent.ACTION_SEND);
share.setType("image/*");
share.putExtra(Intent.EXTRA_STREAM, uri);
share.addFlags(Intent.FLAG_GRANT_READ_URI_PERMISSION);
startActivity(Intent.createChooser(share, "Share image"));

3) Open map direction (implicit VIEW)

Uri gmmIntentUri = Uri.parse("geo:0,0?q=India Gate, New Delhi");
Intent mapIntent = new Intent(Intent.ACTION_VIEW, gmmIntentUri);
mapIntent.setPackage("com.google.android.apps.maps");
startActivity(mapIntent);

Error Handling & Safety

• Intent may not resolve: Check with resolveActivity(getPackageManager()) before startActivity().
• Large data in extras: Avoid huge bitmaps/arrays; use file URIs or a shared repository (DB).
• Security: Be careful with exported=true. Only expose what’s necessary.

Intent i = new Intent(Intent.ACTION_VIEW, Uri.parse("https://codingwithsonu.com"));
if (i.resolveActivity(getPackageManager()) != null) {
  startActivity(i);
} else {
  Toast.makeText(this, "No app can handle this action", Toast.LENGTH_SHORT).show();
}

Hands-On Exercise

1. Create SecondActivity that displays a greeting from MainActivity.
2. Add a button in MainActivity to open SecondActivity with name as extra.
3. Use the Activity Result API to let SecondActivity return a resultMessage back.

// In MainActivity
private ActivityResultLauncher<Intent> launcher =
  registerForActivityResult(new ActivityResultContracts.StartActivityForResult(), res -> {
    if (res.getResultCode() == RESULT_OK && res.getData() != null) {
      String msg = res.getData().getStringExtra("resultMessage");
      Toast.makeText(this, "From SecondActivity: " + msg, Toast.LENGTH_SHORT).show();
    }
  });

public void openSecond(View v) {
  Intent i = new Intent(this, SecondActivity.class);
  i.putExtra("name", "Sonu");
  launcher.launch(i);
}

// In SecondActivity
String name = getIntent().getStringExtra("name");
TextView tv = findViewById(R.id.tvHello);
tv.setText("Hello, " + name + "!");

public void finishWithResult(View v) {
  Intent data = new Intent();
  data.putExtra("resultMessage", "Nice to meet you.");
  setResult(RESULT_OK, data);
  finish();
}

Troubleshooting

Best Practices

• Keep Activities thin; move logic to ViewModels/helpers.
• Prefer explicit intents for in-app navigation.
• Validate external intents; never trust incoming data blindly.
• Use Activity Result API (avoid legacy request codes).
• Minimize exported components; least-privilege principle.