I think I finished it...

demo/.java-version

@@ -0,0 +1 @@
+21

demo/Nest.toml

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+dependencies = []
+
+[workspace]
+default_package = "main"
+
+[meta]
+name = "demo"
+version = "0.1.0"

demo/src/main/Prey.toml

@@ -0,0 +1,6 @@
+[package]
+entry_point = ""
+
+[meta]
+name = "main"
+version = "0.1.0"

demo/src/main/java/Main.java

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+public class Main {
+
+    public static void main(String[] args) {
+        System.out.println("Hello, world!");
+    }
+
+    public static int add(int a, int b) {
+        return a + b;
+    }
+}

demo/src/test/Prey.toml

@@ -0,0 +1,6 @@
+[package]
+entry_point = ""
+
+[meta]
+name = "test"
+version = "0.1.0"

demo/src/test/java/MainTest.java

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+public class MainTest {
+
+    public static void main(String[] args) {
+        testAdd();
+    }
+
+    public static void testAdd() {
+        assert Main.add(2, 2) == 4;
+    }
+}