Mobile Application Development Lecture 4

Android Concepts Used in SecondActivity.java

1. Package Declaration

• package com.example.lenovo.multipleactivities;
• The package declaration helps organize classes and avoid name conflicts. It follows Java naming conventions, typically using a reverse domain name structure.

2. Import Statements

• import android.app.Activity;
• import android.content.Intent;
• import android.os.Bundle;
• import android.widget.TextView;
• These imports bring necessary Android classes into the scope, such as:
  • Activity: Represents an Android screen.
  • Intent: Used for data communication between activities.
  • Bundle: Used to pass data across activity states.
  • TextView: A UI widget for displaying text.

3. Activity Class Inheritance

• public class SecondActivity extends Activity
• The SecondActivity class extends Activity, meaning it inherits properties and behaviors from the Android Activity class, making it an interactive UI screen.

4. Constant Variable Declaration

• public final static String MESSAGE_KEY = "com.example.lenovo.multipleactivities.message_key";
• This constant key is used to retrieve data from the intent. It follows best practices for defining constant values.

5. TextView Declaration

• TextView textView;
• A member variable is declared to hold a reference to the TextView UI element, which displays the received message.

6. onCreate() Method

• @Override protected void onCreate(Bundle savedInstanceState)
• This lifecycle method is overridden to initialize the activity when it is created.
• The Bundle savedInstanceState parameter is used to restore the activity’s previous state if applicable.

7. Setting Layout for the Activity

• setContentView(R.layout.activity_second);
• This line sets the user interface layout for the activity from the XML resource file activity_second.xml.

8. Intent Handling

• Intent intent = getIntent();
• String message = intent.getStringExtra(MESSAGE_KEY);
• The getIntent() method fetches the intent that started this activity, and getStringExtra() retrieves the string data passed from the previous activity.

9. UI Component Initialization

• textView = findViewById(R.id.textView);
• The findViewById() method finds the TextView element from the XML layout using its ID.

10. Displaying Retrieved Data

• textView.setText(message);
• The retrieved message is set to the TextView to display the data passed from the previous activity.

11. Best Practices in the Code

• Proper use of constants for intent keys to avoid hard-coded strings.
• Ensuring findViewById is correctly used to reference UI elements to prevent NullPointerException.
• Code follows standard Android lifecycle and structure.

12. XML Layout Dependency

• The code assumes the existence of an activity_second.xml file in the res/layout folder containing a TextView with the ID textView.

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Key Takeaways

1. Always declare constant keys for passing data between activities.
2. Use Intent to communicate between different screens.
3. Handle NullPointerException by verifying the existence of UI elements.
4. Follow Android’s activity lifecycle methods like onCreate().
5. Always set the correct layout using setContentView().
6. Use descriptive names for variables and constants to enhance code readability.