Activity

What Is an Activity and How to Add One Manually

1. What Is an Activity?

An Activity is a component in an Android app that provides a user interface and is mainly used for interacting with the user.

Key Point	Description
Core Responsibility	Hosts the UI and handles user interactions (taps, input, etc.)
Quantity	An app can contain zero or more activities (apps without any activity can still run, e.g., background-only service apps)
Nature	Each activity corresponds to a full-screen window or a single screen

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2. Creating an Activity Manually (Without Templates)

2.1 Create the Project

• Create a new project and name it ActivityDemo
• Choose the Add No Activity template (no activity will be generated automatically)
• The project path must not contain Chinese characters
• An internet connection is required during creation (to download Gradle dependencies)

If you are offline, you will see this error:

Gradle 'ActivityDemo' project refresh failed
Error:Cause: dl.google.comCode language: JavaScript (javascript)

Without Gradle loaded, the project won’t be split into packages automatically, and you won’t be able to add activities normally.

2.2 Create the Activity Class

Right-click java/cn.bamn.www.activitydemo/ → New → Java Class

Name it FirstActivity and extend AppCompatActivity:

package cn.bamn.www.activitydemo;

import android.os.Bundle;
import androidx.appcompat.app.AppCompatActivity;

public class FirstActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
    }
}Code language: JavaScript (javascript)

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3. Creating a Layout File

3.1 Create a New Layout

Right-click res/layout → New → Layout Resource File

Parameter	Value
File name	first_layout
Root element	LinearLayout

3.2 Write the Layout Code

Open first_layout.xml and add a button:

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical">

    <Button
        android:id="@+id/button"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Join FoxDevelop" />

</LinearLayout>Code language: HTML, XML (xml)

3.3 Key Attribute Explanation

Attribute	Description
android:id="@+id/button"	Defines a unique ID for the button. The + means this ID is being defined for the first time in this XML, not referenced from existing resources
android:layout_width="match_parent"	Width fills the parent container
android:layout_height="wrap_content"	Height adjusts to fit the content
android:text	Text displayed on the button

+id vs no +:

• @+id/xxx → Define a new ID (used when creating for the first time)
• @id/xxx → Reference an existing ID (e.g., when referencing between layouts)

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4. Loading the Layout in the Activity

Go back to FirstActivity and call setContentView() inside onCreate():

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.first_layout);
}Code language: CSS (css)

Code	Purpose
setContentView()	Binds the XML layout file to the activity
R.layout.first_layout	References the res/layout/first_layout.xml layout

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5. Registering the Activity in AndroidManifest.xml

Open AndroidManifest.xml and register it inside the <application> tag:

<activity android:name=".FirstActivity">
    <intent-filter>
        <action android:name="android.intent.action.MAIN" />
        <category android:name="android.intent.category.LAUNCHER" />
    </intent-filter>
</activity>Code language: HTML, XML (xml)

Configuration	Purpose
android:name=".FirstActivity"	Registers the activity (. stands for the current package name)
MAIN + LAUNCHER	Sets it as the launch entry point; opened first when the app icon is tapped

An unregistered activity cannot be used; the system will throw an ActivityNotFoundException.

Activity