Add Java Hello World demo project

.gitignore

@@ -213,4 +213,11 @@ marimo/_lsp/
 __marimo__/
 
 # Streamlit
-.streamlit/secrets.toml
+.streamlit/secrets.toml
+
+# Java / Maven
+*.class
+*.jar
+*.war
+*.ear
+java-hello-world/target/

java-hello-world/pom.xml

@@ -0,0 +1,52 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<project xmlns="http://maven.apache.org/POM/4.0.0"
+         xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
+         xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/xsd/maven-4.0.0.xsd">
+    <modelVersion>4.0.0</modelVersion>
+
+    <groupId>com.example</groupId>
+    <artifactId>java-hello-world</artifactId>
+    <version>1.0.0</version>
+    <packaging>jar</packaging>
+
+    <name>Java Hello World</name>
+    <description>A simple Java Hello World demo project</description>
+
+    <properties>
+        <maven.compiler.source>17</maven.compiler.source>
+        <maven.compiler.target>17</maven.compiler.target>
+        <project.build.sourceEncoding>UTF-8</project.build.sourceEncoding>
+        <junit.version>5.10.2</junit.version>
+    </properties>
+
+    <dependencies>
+        <dependency>
+            <groupId>org.junit.jupiter</groupId>
+            <artifactId>junit-jupiter</artifactId>
+            <version>${junit.version}</version>
+            <scope>test</scope>
+        </dependency>
+    </dependencies>
+
+    <build>
+        <plugins>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-surefire-plugin</artifactId>
+                <version>3.2.5</version>
+            </plugin>
+            <plugin>
+                <groupId>org.apache.maven.plugins</groupId>
+                <artifactId>maven-jar-plugin</artifactId>
+                <version>3.3.0</version>
+                <configuration>
+                    <archive>
+                        <manifest>
+                            <mainClass>com.example.HelloWorld</mainClass>
+                        </manifest>
+                    </archive>
+                </configuration>
+            </plugin>
+        </plugins>
+    </build>
+</project>

java-hello-world/src/main/java/com/example/HelloWorld.java

@@ -0,0 +1,15 @@
+package com.example;
+
+public class HelloWorld {
+
+    public static String greet(String name) {
+        if (name == null || name.isBlank()) {
+            name = "World";
+        }
+        return "Hello, " + name + "!";
+    }
+
+    public static void main(String[] args) {
+        System.out.println(greet("World"));
+    }
+}

java-hello-world/src/test/java/com/example/HelloWorldTest.java

@@ -0,0 +1,27 @@
+package com.example;
+
+import org.junit.jupiter.api.Test;
+import static org.junit.jupiter.api.Assertions.assertEquals;
+
+class HelloWorldTest {
+
+    @Test
+    void greetWorld() {
+        assertEquals("Hello, World!", HelloWorld.greet("World"));
+    }
+
+    @Test
+    void greetCustomName() {
+        assertEquals("Hello, Java!", HelloWorld.greet("Java"));
+    }
+
+    @Test
+    void greetNullFallsBackToWorld() {
+        assertEquals("Hello, World!", HelloWorld.greet(null));
+    }
+
+    @Test
+    void greetBlankFallsBackToWorld() {
+        assertEquals("Hello, World!", HelloWorld.greet("  "));
+    }
+}