[jextract/jni] update docs about throwing and optional init and add tests

Samples/SwiftJavaExtractJNISampleApp/Sources/MySwiftLibrary/MySwiftClass.swift

@@ -59,6 +59,14 @@ public class MySwiftClass {
     self.y = 5
   }
 
+  convenience public init(throwing: Bool) throws {
+    if throwing {
+      throw MySwiftError.swiftError
+    } else {
+      self.init()
+    }
+  }
+
   deinit {
     p("deinit called!")
   }

Samples/SwiftJavaExtractJNISampleApp/Sources/MySwiftLibrary/MySwiftStruct.swift

@@ -21,6 +21,14 @@ public struct MySwiftStruct {
     self.len = len
   }
 
+  public init?(doInit: Bool) {
+      if doInit {
+        self.init(cap: 10, len: 10)
+      } else {
+        return nil
+      }
+    }
+
   public func getCapacity() -> Int64 {
     self.cap
   }

Samples/SwiftJavaExtractJNISampleApp/src/test/java/com/example/swift/MySwiftClassTest.java

@@ -40,6 +40,17 @@ void init_withParameters() {
         }
     }
 
+    @Test
+    void init_throwing() {
+        try (var arena = SwiftArena.ofConfined()) {
+            Exception exception = assertThrows(Exception.class, () -> MySwiftClass.init(true, arena));
+            assertEquals("swiftError", exception.getMessage());
+
+            MySwiftClass c = assertDoesNotThrow(() -> MySwiftClass.init(false, arena));
+            assertNotNull(c);
+        }
+    }
+
     @Test
     void sum() {
         try (var arena = SwiftArena.ofConfined()) {

Samples/SwiftJavaExtractJNISampleApp/src/test/java/com/example/swift/MySwiftStructTest.java

@@ -18,6 +18,8 @@
 import org.swift.swiftkit.core.ConfinedSwiftMemorySession;
 import org.swift.swiftkit.core.SwiftArena;
 
+import java.util.Optional;
+
 import static org.junit.jupiter.api.Assertions.*;
 
 public class MySwiftStructTest {
@@ -30,6 +32,17 @@ void init() {
         }
     }
 
+    @Test
+    void init_optional() {
+        try (var arena = SwiftArena.ofConfined()) {
+            assertEquals(Optional.empty(), MySwiftStruct.init(false, arena));
+
+            Optional<MySwiftStruct> optionalStruct = MySwiftStruct.init(true, arena);
+            assertTrue(optionalStruct.isPresent());
+            assertEquals(10, optionalStruct.get().getLen());
+        }
+    }
+
     @Test
     void getAndSetLen() {
         try (var arena = SwiftArena.ofConfined()) {

Sources/SwiftJavaDocumentation/Documentation.docc/SupportedFeatures.md

@@ -47,7 +47,7 @@ SwiftJava's `swift-java jextract` tool automates generating Java bindings from S
 | Swift Feature                                                                        | FFM      | JNI |
 |--------------------------------------------------------------------------------------|----------|-----|
 | Initializers: `class`, `struct`                                                      | ✅        | ✅   |
-| Optional Initializers / Throwing Initializers                                        | ❌        | ❌   |
+| Optional Initializers / Throwing Initializers                                        | ❌        | ✅   |
 | Deinitializers:  `class`, `struct`                                                   | ✅        | ✅   |
 | `enum`                                                                               | ❌        | ✅   |
 | `actor`                                                                              | ❌        | ❌   |