Lecture 3 – Understanding Activities and Layouts

We will discuss Android Activities and Layouts with practical examples. Understand the Activity lifecycle, XML layouts vs Kotlin/Java UI code, and types of layouts such as LinearLayout, RelativeLayout, and ConstraintLayout.

What is an Activity in Android?

An Activity represents a single screen in an Android app like a webpage in a website.
Every time you open a new screen (e.g., Login Page, Profile Page), a new Activity runs.

Example:

• MainActivity → Home screen
• ProfileActivity → User’s profile

Example Code:

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

Here, setContentView() loads the XML layout associated with this Activity.

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Activity Lifecycle

Each Activity goes through specific states from creation to destruction.
Android manages these using lifecycle callback methods.

Example:

@Override
protected void onPause() {
    super.onPause();
    Log.d("ActivityState", "App is paused");
}

Explanation:
This method triggers when the user switches to another app allowing you to save data or pause processes.

XML Layouts vs. Kotlin/Java UI Code

You can design your UI using XML layout files or programmatically in Kotlin/Java.

a) XML Layout (Recommended)

Declarative and separated from logic.

Example (activity_main.xml):

<TextView
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="Hello Android!"
    android:textSize="22sp"
    android:layout_gravity="center"/>

b) Kotlin/Java UI Code

Created dynamically during runtime.

Example (Kotlin):

val textView = TextView(this)
textView.text = "Hello Android!"
setContentView(textView)

Difference:

• XML is ideal for design separation.
• Kotlin/Java code is useful for dynamic or data-driven UI.

4. Common Layout Types in Android

Layouts define how UI elements are arranged on the screen.

a) LinearLayout

Arranges elements horizontally or vertically.

Example:

<LinearLayout
    android:orientation="vertical"
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    
    <TextView android:text="Name:"/>
    <EditText android:hint="Enter your name"/>
</LinearLayout>

Use Case: Simple stacked or row layouts.

b) RelativeLayout

Positions views relative to each other or the parent container.

Example:

<RelativeLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">
    
    <TextView
        android:id="@+id/label"
        android:text="Username"/>
    
    <EditText
        android:layout_below="@id/label"
        android:hint="Enter username"/>
</RelativeLayout>

Use Case: When precise alignment between elements is required.

c) ConstraintLayout

Most flexible and modern layout supports responsive design using constraints.

Example:

<androidx.constraintlayout.widget.ConstraintLayout
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <TextView
        android:id="@+id/helloText"
        android:text="Hello Android!"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintEnd_toEndOf="parent"/>
</androidx.constraintlayout.widget.ConstraintLayout>

Use Case: Modern UIs, adaptive screens.

5. Summary

After this lecture, students can:

• Understand what an Activity is and its lifecycle
• Differentiate between XML layouts and Kotlin/Java UI code
• Use LinearLayout, RelativeLayout, and ConstraintLayout effectively

Conclusion

Mastering Activities and Layouts helps developers structure apps properly and build responsive, dynamic UIs.
Once you understand lifecycle behavior and layout design, you’re ready to create more advanced Android interfaces.

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