1. Android (Java/Kotlin)mdthv.cn
   Java示例 (简单的Android Activity):

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

public class MainActivity extends AppCompatActivity {

@Override  
protected void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.activity_main);  
}  

}
Kotlin示例 (与上面的Java代码功能相同):

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

class MainActivity : AppCompatActivity() {

override fun onCreate(savedInstanceState: Bundle?) {  
    super.onCreate(savedInstanceState)  
    setContentView(R.layout.activity_main)  
}  

}
2. iOS (Swift/Objective-C)
Swift示例 (简单的iOS ViewController):

swift
import UIKit

class ViewController: UIViewController {

override func viewDidLoad() {  
    super.viewDidLoad()  
    // Do any additional setup after loading the view.  
}  

}
Objective-C示例 (与上面的Swift代码功能相同):

objc
#import “ViewController.h”

@interface ViewController ()

@end

@implementation ViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view.
  }

@end
3. Flutter (Dart)
Dart示例 (Flutter中的简单页面):

dart
import ‘package:flutter/material.dart’;

void main() {
runApp(MyApp());
}

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: ‘Flutter Demo’,
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: Scaffold(
appBar: AppBar(
title: Text(‘Welcome to Flutter’),
),
body: Center(
child: Text(‘Hello, Flutter!’),
),
),
);
}
}
4. Web (JavaScript/React/Vue/Angular)
由于Web App的代码较长且涉及多个文件，这里仅提供一个简单的React组件示例：

React示例 (简单的React组件):

jsx
import React from ‘react’;

function HelloWorld() {
return

Hello, World!

;
}

export default HelloWorld;
这些只是最基础的代码示例，真正的App开发会涉及更多的细节和复杂性。如果你需要关于某个特定平台或技术的更深入的指导，请告诉我！由于一个完整的App通常涉及多个层面（如前端、后端、数据库等），并且不同的平台（如iOS、Android、Web等）有不同的开发语言和工具，这里我将为你提供几个简单的示例，展示如何用不同语言创建一个简单的App的部分代码。

1. Android (Java/Kotlin)
   Java示例 (简单的Android Activity):

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

public class MainActivity extends AppCompatActivity {

@Override  
protected void onCreate(Bundle savedInstanceState) {  
    super.onCreate(savedInstanceState);  
    setContentView(R.layout.activity_main);  
}  

}
Kotlin示例 (与上面的Java代码功能相同):

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

class MainActivity : AppCompatActivity() {

override fun onCreate(savedInstanceState: Bundle?) {  
    super.onCreate(savedInstanceState)  
    setContentView(R.layout.activity_main)  
}  

}
2. iOS (Swift/Objective-C)
Swift示例 (简单的iOS ViewController):

swift
import UIKit

class ViewController: UIViewController {

override func viewDidLoad() {  
    super.viewDidLoad()  
    // Do any additional setup after loading the view.  
}  

}
Objective-C示例 (与上面的Swift代码功能相同):

objc
#import “ViewController.h”

@interface ViewController ()

@end

@implementation ViewController

• (void)viewDidLoad {
  [super viewDidLoad];
  // Do any additional setup after loading the view.
  }

@end
3. Flutter (Dart)
Dart示例 (Flutter中的简单页面):

dart
import ‘package:flutter/material.dart’;

void main() {
runApp(MyApp());
}

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
title: ‘Flutter Demo’,
theme: ThemeData(
primarySwatch: Colors.blue,
),
home: Scaffold(
appBar: AppBar(
title: Text(‘Welcome to Flutter’),
),
body: Center(
child: Text(‘Hello, Flutter!’),
),
),
);
}
}
4. Web (JavaScript/React/Vue/Angular)
由于Web App的代码较长且涉及多个文件，这里仅提供一个简单的React组件示例：

React示例 (简单的React组件):

jsx
import React from ‘react’;

function HelloWorld() {
return

Hello, World!

;
}

export default HelloWorld;
这些只是最基础的代码示例，真正的App开发会涉及更多的细节和复杂性。如果你需要关于某个特定平台或技术的更深入的指导，请告诉我！