From 9cacda725dcfe5bf30cbb98ee56b04cce055d709 Mon Sep 17 00:00:00 2001
From: Michael Williamson <mike@zwobble.org>
Date: Sat, 8 Aug 2026 21:35:17 +0100
Subject: Add type parameters to Java interface declaration

---
 .../generators/javatypes/JavaTypesGenerator.java   |  2 ++
 .../compiler/output/lang/java/JavaWriter.java      | 15 +++++++++
 .../lang/java/ast/JavaInterfaceDeclaration.java    | 38 ++++++++++++++++------
 .../output/lang/java/ast/JavaTypeParameter.java    | 36 ++++++++++++++++++++
 4 files changed, 81 insertions(+), 10 deletions(-)
 create mode 100644 src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaTypeParameter.java

(limited to 'src/main/java/org/zwobble')

diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
index cd311d9..33072d7 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/generators/javatypes/JavaTypesGenerator.java
@@ -477,6 +477,7 @@ public class JavaTypesGenerator implements Generator {
         return new JavaInterfaceDeclaration(
             generateTypeRef(sumDefinition.type(), context),
             JavaInterfaceOpenness.SEALED,
+            List.of(),
             sumDefinition.variants().stream()
                 .map(variant -> generateTypeRef(variant.type(), context))
                 .toList(),
@@ -493,6 +494,7 @@ public class JavaTypesGenerator implements Generator {
             generateBuilderTypeRef(sumDefinition.type()),
             JavaInterfaceOpenness.OPEN,
             List.of(),
+            List.of(),
             List.of(
                 new JavaMethodDeclaration(
                     BUILD_METHOD_NAME,
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 60998aa..3d57b82 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
@@ -149,6 +149,21 @@ public class JavaWriter implements AutoCloseable {
         }
         this.writer.write("interface ");
         this.writeIdentifier(interfaceDeclaration.name());
+
+        if (!interfaceDeclaration.typeParameters().isEmpty()) {
+            this.writer.write("<");
+            writeWithSeparator(
+                interfaceDeclaration.typeParameters(),
+                typeParameter -> {
+                    this.writeIdentifier(typeParameter.name());
+                },
+                () -> {
+                    this.writer.write(", ");
+                }
+            );
+            this.writer.write(">");
+        }
+
         this.writer.write(" ");
 
         if (!interfaceDeclaration.permitsTypes().isEmpty()) {
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaInterfaceDeclaration.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaInterfaceDeclaration.java
index 8feec8e..867ae84 100644
--- a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaInterfaceDeclaration.java
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaInterfaceDeclaration.java
@@ -8,61 +8,79 @@ package org.zwobble.hobgoblin.compiler.output.lang.java.ast;
 public record JavaInterfaceDeclaration(
     org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef type,
     org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceOpenness openness,
+    java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter> typeParameters,
     java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef> permitsTypes,
     java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration> body,
     org.zwobble.hobgoblin.compiler.ast.DocComment docComment
 ) implements org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeDeclaration, org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration {
     public static org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder arbitrary() {
-        return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(org.zwobble.hobgoblin.compiler.output.lang.java.ast.HobgoblinNativeAst.arbitraryJavaTypeRef(), org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceOpenness.OPEN, java.util.List.of(), java.util.List.of(), org.zwobble.hobgoblin.compiler.ast.HobgoblinNativeAst.arbitraryDocComment());
+        return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(org.zwobble.hobgoblin.compiler.output.lang.java.ast.HobgoblinNativeAst.arbitraryJavaTypeRef(), org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceOpenness.OPEN, java.util.List.of(), java.util.List.of(), java.util.List.of(), org.zwobble.hobgoblin.compiler.ast.HobgoblinNativeAst.arbitraryDocComment());
     }
 
     public record Builder(
         org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef type,
         org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceOpenness openness,
+        java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter> typeParameters,
         java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef> permitsTypes,
         java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration> body,
         org.zwobble.hobgoblin.compiler.ast.DocComment docComment
     ) implements org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeDeclaration.Builder, org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration.Builder {
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration build() {
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder withType(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef type) {
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder withOpenness(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceOpenness openness) {
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
+        }
+
+        public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder withTypeParameters(java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter> typeParameters) {
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
+        }
+
+        public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder addTypeParameter(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter typeParameter) {
+            var typeParameters = new java.util.ArrayList<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter>(this.typeParameters);
+            typeParameters.add(typeParameter);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
+        }
+
+        public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder addTypeParameter(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder typeParameter) {
+            var typeParameters = new java.util.ArrayList<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter>(this.typeParameters);
+            typeParameters.add(typeParameter.build());
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder withPermitsTypes(java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef> permitsTypes) {
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder addPermitsType(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef permitsType) {
             var permitsTypes = new java.util.ArrayList<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeRef>(this.permitsTypes);
             permitsTypes.add(permitsType);
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder withBody(java.util.List<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration> body) {
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder addBodyElement(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration bodyElement) {
             var body = new java.util.ArrayList<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration>(this.body);
             body.add(bodyElement);
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder addBodyElement(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration.Builder bodyElement) {
             var body = new java.util.ArrayList<org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaClassBodyDeclaration>(this.body);
             body.add(bodyElement.build());
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder withDocComment(org.zwobble.hobgoblin.compiler.ast.DocComment docComment) {
-            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, permitsTypes, body, docComment);
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder(type, openness, typeParameters, permitsTypes, body, docComment);
         }
 
         // Custom area start: org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaInterfaceDeclaration.Builder body
diff --git a/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaTypeParameter.java b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaTypeParameter.java
new file mode 100644
index 0000000..bb6af43
--- /dev/null
+++ b/src/main/java/org/zwobble/hobgoblin/compiler/output/lang/java/ast/JavaTypeParameter.java
@@ -0,0 +1,36 @@
+// Generated by hobgoblin.
+
+package org.zwobble.hobgoblin.compiler.output.lang.java.ast;
+
+// Custom area start: org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter imports
+// Custom area end: org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter imports
+
+public record JavaTypeParameter(
+    org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaIdentifier name
+) {
+    public static org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder arbitrary() {
+        return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaIdentifier.arbitrary().build());
+    }
+
+    public record Builder(
+        org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaIdentifier name
+    ) {
+        public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter build() {
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter(name);
+        }
+
+        public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder withName(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaIdentifier name) {
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder(name);
+        }
+
+        public org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder withName(org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaIdentifier.Builder name) {
+            return new org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder(name.build());
+        }
+
+        // Custom area start: org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder body
+        // Custom area end: org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter.Builder body
+    }
+
+    // Custom area start: org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter body
+    // Custom area end: org.zwobble.hobgoblin.compiler.output.lang.java.ast.JavaTypeParameter body
+}
-- 
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