java2swift: Add Support for Enum Cases (#76)

* Add Support for Enum Cases

* Fix building

* Remove print

* Update enum values to the new setup

* Add new initializer from enum case

* not throws or optional

* Remove unnecessary addition of java.lang.Object mapping

---------

Co-authored-by: Doug Gregor <dgregor@apple.com>

Author: jrosen081

Sources/Java2SwiftLib/JavaTranslator.swift

@@ -234,11 +234,16 @@ extension JavaTranslator {
 
     // Fields
     var staticFields: [Field] = []
+    var enumConstants: [Field] = []
     members.append(
       contentsOf: javaClass.getFields().compactMap {
         $0.flatMap { field in
           if field.isStatic {
             staticFields.append(field)
+
+            if field.isEnumConstant() {
+              enumConstants.append(field)
+            }
             return nil
           }
 
@@ -252,6 +257,13 @@ extension JavaTranslator {
       }
     )
 
+    if !enumConstants.isEmpty {
+      let enumName = "\(swiftTypeName)Cases"
+      members.append(
+        contentsOf: translateToEnumValue(name: enumName, enumFields: enumConstants)
+      )
+    }
+
     // Constructors
     members.append(
       contentsOf: javaClass.getConstructors().compactMap {
@@ -447,6 +459,50 @@ extension JavaTranslator {
       """
   }
 
+  package func translateToEnumValue(name: String, enumFields: [Field]) -> [DeclSyntax] {
+    let extensionSyntax: DeclSyntax = """
+      public enum \(raw: name): Equatable {
+        \(raw: enumFields.map { "case \($0.getName())" }.joined(separator: "\n"))
+      }
+    """
+
+    let mappingSyntax: DeclSyntax = """
+      public var enumValue: \(raw: name)? {
+        let classObj = self.javaClass
+        \(raw: enumFields.map {
+          // The equals method takes a java object, so we need to cast it here
+          """
+          if self.equals(classObj.\($0.getName())?.as(JavaObject.self)) {
+                return \(name).\($0.getName())
+          }
+          """
+        }.joined(separator: " else ")) else {
+          return nil
+        }
+      }
+    """
+
+    let initSyntax: DeclSyntax = """
+    public init(_ enumValue: \(raw: name), environment: JNIEnvironment) {
+      let classObj = try! JavaClass<Self>(in: environment)
+      switch enumValue {
+    \(raw: enumFields.map {
+      return """
+          case .\($0.getName()):
+            if let \($0.getName()) = classObj.\($0.getName()) {
+              self = \($0.getName())
+            } else {
+              fatalError("Enum value \($0.getName()) was unexpectedly nil, please re-run Java2Swift on the most updated Java class") 
+            }
+      """
+    }.joined(separator: "\n"))
+      }
+    }
+    """
+
+    return [extensionSyntax, mappingSyntax, initSyntax]
+  }
+
   // Translate a Java parameter list into Swift parameters.
   private func translateParameters(_ parameters: [Parameter?]) throws -> [FunctionParameterSyntax] {
     return try parameters.compactMap { javaParameter in

Tests/Java2SwiftTests/Java2SwiftTests.swift

@@ -24,6 +24,11 @@ var jvm: JavaVirtualMachine {
   }
 }
 
+@JavaClass("java.time.Month")
+public struct JavaMonth {
+
+}
+
 class Java2SwiftTests: XCTestCase {
   func testJavaLangObjectMapping() throws {
     try assertTranslatedClass(
@@ -47,6 +52,36 @@ class Java2SwiftTests: XCTestCase {
     )
   }
 
+  func testEnum() async throws {
+    try assertTranslatedClass(
+      JavaMonth.self,
+      swiftTypeName: "Month",
+      expectedChunks: [
+        "import JavaKit",
+        "enum MonthCases: Equatable",
+        "case APRIL",
+        "public var enumValue: MonthCases?",
+        """
+            } else if self.equals(classObj.APRIL?.as(JavaObject.self)) {
+              return MonthCases.APRIL
+            }
+        """,
+        "public init(_ enumValue: MonthCases, environment: JNIEnvironment) {",
+        """
+              case .APRIL:
+                if let APRIL = classObj.APRIL {
+                  self = APRIL
+                } else {
+                  fatalError("Enum value APRIL was unexpectedly nil, please re-run Java2Swift on the most updated Java class")
+                }
+        """,
+        """
+          @JavaStaticField
+          public var APRIL: Month?
+        """
+      ])
+  }
+
   func testGenericCollections() throws {
     try assertTranslatedClass(
       MyArrayList<JavaObject>.self,