1. Android (Java/Kotlin)lnkryy.cn
   Java

java
public class MainActivity extends AppCompatActivity {

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

    TextView textView = findViewById(R.id.textView);  
    textView.setText("Hello, World!");  
}  

}
Kotlin

kotlin
class MainActivity : AppCompatActivity() {

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

    val textView: TextView = findViewById(R.id.textView)  
    textView.text = "Hello, World!"  
}  

}
2. iOS (Swift)
swift
import UIKit

class ViewController: UIViewController {

override func viewDidLoad() {  
    super.viewDidLoad()  
    // Do any additional setup after loading the view.  
    let label = UILabel()  
    label.text = "Hello, World!"  
    label.frame = CGRect(x: 50, y: 50, width: 200, height: 20)  
    self.view.addSubview(label)  
}  

}
3. Web (JavaScript/React)
JavaScript (使用HTML和CSS)

HTML:

html

Hello, World!

JavaScript (内联):

html

React

jsx
import React from ‘react’;
import ReactDOM from ‘react-dom’;

function App() {
return (

);
}

ReactDOM.render(, document.getElementById(‘root’));
4. Flutter (Dart)
dart
import ‘package:flutter/material.dart’;

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: Text(‘Hello, World!’),
),
body: Center(
child: Text(‘Hello, World!’),
),
),
);
}
}
这些示例只是每个平台或框架的起点。为了创建一个完整的app，你需要考虑更多的因素，如UI设计、用户交互、数据持久化、网络请求等。由于“一个app”的代码可以非常复杂并且依赖于特定的平台（如Android、iOS、Web等）和使用的框架，我无法在这里为你提供一个完整的app代码示例。但我可以为你提供每种语言中创建简单“Hello, World!”应用的基本框架或示例。

1. Android (Java/Kotlin)
   Java

java
public class MainActivity extends AppCompatActivity {

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

    TextView textView = findViewById(R.id.textView);  
    textView.setText("Hello, World!");  
}  

}
Kotlin

kotlin
class MainActivity : AppCompatActivity() {

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

    val textView: TextView = findViewById(R.id.textView)  
    textView.text = "Hello, World!"  
}  

}
2. iOS (Swift)
swift
import UIKit

class ViewController: UIViewController {

override func viewDidLoad() {  
    super.viewDidLoad()  
    // Do any additional setup after loading the view.  
    let label = UILabel()  
    label.text = "Hello, World!"  
    label.frame = CGRect(x: 50, y: 50, width: 200, height: 20)  
    self.view.addSubview(label)  
}  

}
3. Web (JavaScript/React)
JavaScript (使用HTML和CSS)

HTML:

html

Hello, World!

JavaScript (内联):

html

React

jsx
import React from ‘react’;
import ReactDOM from ‘react-dom’;

function App() {
return (

);
}

ReactDOM.render(, document.getElementById(‘root’));
4. Flutter (Dart)
dart
import ‘package:flutter/material.dart’;

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
@override
Widget build(BuildContext context) {
return MaterialApp(
home: Scaffold(
appBar: AppBar(
title: Text(‘Hello, World!’),
),
body: Center(
child: Text(‘Hello, World!’),
),
),
);
}
}
这些示例只是每个平台或框架的起点。为了创建一个完整的app，你需要考虑更多的因素，如UI设计、用户交互、数据持久化、网络请求等。