From 19d9169d58ce2f2581073fd0af6e6439604f591c Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sun, 3 May 2026 11:24:39 +0100
Subject: Add tests for writing Java interface declarations

---
 .../compiler/output/lang/java/JavaWriter.java      | 18 ++++---
 .../compiler/output/lang/java/JavaWriterTests.java | 61 ++++++++++++++++++++++
 2 files changed, 71 insertions(+), 8 deletions(-)
 create mode 100644 src/test/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriterTests.java

(limited to 'src')

diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriter.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriter.java
index 664c322..59de6b7 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriter.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriter.java
@@ -27,7 +27,7 @@ public class JavaWriter implements AutoCloseable {
         writeTypeDeclaration(compilationUnit.typeDeclaration());
     }
 
-    private void writeTypeDeclaration(JavaTypeDeclaration typeDeclaration) throws IOException {
+    void writeTypeDeclaration(JavaTypeDeclaration typeDeclaration) throws IOException {
         switch (typeDeclaration) {
             case JavaInterfaceDeclaration interfaceDeclaration -> {
                 writeInterfaceDeclaration(interfaceDeclaration);
@@ -44,13 +44,15 @@ public class JavaWriter implements AutoCloseable {
         this.writer.write(interfaceDeclaration.name());
         this.writer.write(" ");
 
-        writer.write("permits ");
-        writeWithSeparator(
-            interfaceDeclaration.permitsTypes(),
-            this::writeTypeRef,
-            () -> writer.write(", ")
-        );
-        writer.write(" ");
+        if (!interfaceDeclaration.permitsTypes().isEmpty()) {
+            writer.write("permits ");
+            writeWithSeparator(
+                interfaceDeclaration.permitsTypes(),
+                this::writeTypeRef,
+                () -> writer.write(", ")
+            );
+            writer.write(" ");
+        }
 
         this.writer.write("{");
         this.writer.newLine();
diff --git a/src/test/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriterTests.java b/src/test/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriterTests.java
new file mode 100644
index 0000000..0a614f4
--- /dev/null
+++ b/src/test/java/org/zwobble/hobgoblin/compiler/output/lang/java/JavaWriterTests.java
@@ -0,0 +1,61 @@
+package org.zwobble.hobgoblin.compiler.output.lang.java;
+
+import org.junit.jupiter.api.Test;
+import org.zwobble.hobgoblin.compiler.output.CodeWriter;
+import org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration;
+import org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef;
+
+import java.io.IOException;
+import java.io.StringWriter;
+import java.util.List;
+
+import static org.zwobble.precisely.AssertThat.assertThat;
+import static org.zwobble.precisely.Matchers.equalTo;
+
+public class JavaWriterTests {
+    @Test
+    public void emptyInterfaceDeclaration() throws IOException {
+        var java = new JavaInterfaceDeclaration(
+            "Shape",
+            List.of()
+        );
+
+        var string = write(writer -> writer.writeTypeDeclaration(java));
+
+        assertThat(string, equalTo("""
+            public sealed interface Shape {
+            }
+            """
+        ));
+    }
+
+    @Test
+    public void sealedInterfaceDeclarationWithPermitsTypes() throws IOException {
+        var java = new JavaInterfaceDeclaration(
+            "Shape",
+            List.of(
+                new JavaTypeRef(List.of("abc", "def"), "One"),
+                new JavaTypeRef(List.of("abc", "def"), "Two")
+            )
+        );
+
+        var string = write(writer -> writer.writeTypeDeclaration(java));
+
+        assertThat(string, equalTo("""
+            public sealed interface Shape permits abc.def.One, abc.def.Two {
+            }
+            """
+        ));
+    }
+
+    private String write(Write write) throws IOException {
+        var stringWriter = new StringWriter();
+        var javaWriter = new JavaWriter(new CodeWriter(stringWriter));
+        write.write(javaWriter);
+        return stringWriter.toString();
+    }
+
+    private interface Write {
+        void write(JavaWriter writer) throws IOException;
+    }
+}
-- 
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