[pigeon] Implement equals for Java data classes (#6992)

Adds implementations of `equals` and `hashCode` to Java data classes. This is frequently useful for native unit tests of plugins using Pigeon (e.g., when using a mock FlutterApi implementation to check that the expected call is being made with the right arguments).

Part of flutter/flutter#118087

packages/pigeon/CHANGELOG.md

@@ -1,5 +1,6 @@
-## NEXT
+## 20.0.2
 
+* [java] Adds `equals` and `hashCode` support for data classes.
 * [swift] Fully-qualifies types in Equatable extension test.
 
 ## 20.0.1
@@ -17,7 +18,7 @@
 
 ## 19.0.2
 
-* [kotlin] Adds the `@JvmOverloads` to the `HostApi` setUp method. This prevents the calling Java code from having to provide an empty `String` as Kotlin provides it by default  
+* [kotlin] Adds the `@JvmOverloads` to the `HostApi` setUp method. This prevents the calling Java code from having to provide an empty `String` as Kotlin provides it by default
 
 ## 19.0.1
 

packages/pigeon/example/app/android/app/src/main/java/io/flutter/plugins/Messages.java

@@ -22,6 +22,7 @@
 import java.util.Collections;
 import java.util.List;
 import java.util.Map;
+import java.util.Objects;
 
 /** Generated class from Pigeon. */
 @SuppressWarnings({"unused", "unchecked", "CodeBlock2Expr", "RedundantSuppression", "serial"})
@@ -132,6 +133,26 @@ public void setData(@NonNull Map<String, String> setterArg) {
     /** Constructor is non-public to enforce null safety; use Builder. */
     MessageData() {}
 
+    @Override
+    public boolean equals(Object o) {
+      if (this == o) {
+        return true;
+      }
+      if (o == null || getClass() != o.getClass()) {
+        return false;
+      }
+      MessageData that = (MessageData) o;
+      return Objects.equals(name, that.name)
+          && Objects.equals(description, that.description)
+          && code.equals(that.code)
+          && data.equals(that.data);
+    }
+
+    @Override
+    public int hashCode() {
+      return Objects.hash(name, description, code, data);
+    }
+
     public static final class Builder {
 
       private @Nullable String name;

packages/pigeon/lib/generator_tools.dart

@@ -13,7 +13,7 @@ import 'ast.dart';
 /// The current version of pigeon.
 ///
 /// This must match the version in pubspec.yaml.
-const String pigeonVersion = '20.0.1';
+const String pigeonVersion = '20.0.2';
 
 /// Prefix for all local variables in methods.
 ///

packages/pigeon/lib/java_generator.dart

@@ -141,6 +141,7 @@ class JavaGenerator extends StructuredGenerator<JavaOptions> {
     indent.writeln('import java.util.HashMap;');
     indent.writeln('import java.util.List;');
     indent.writeln('import java.util.Map;');
+    indent.writeln('import java.util.Objects;');
     indent.newln();
   }
 
@@ -233,6 +234,7 @@ class JavaGenerator extends StructuredGenerator<JavaOptions> {
         indent.writeln('${classDefinition.name}() {}');
         indent.newln();
       }
+      _writeEquality(indent, classDefinition);
 
       _writeClassBuilder(generatorOptions, root, indent, classDefinition);
       writeClassEncode(
@@ -282,6 +284,62 @@ class JavaGenerator extends StructuredGenerator<JavaOptions> {
     });
   }
 
+  void _writeEquality(Indent indent, Class classDefinition) {
+    // Implement equals(...).
+    indent.writeln('@Override');
+    indent.writeScoped('public boolean equals(Object o) {', '}', () {
+      indent.writeln('if (this == o) { return true; }');
+      indent.writeln(
+          'if (o == null || getClass() != o.getClass()) { return false; }');
+      indent.writeln(
+          '${classDefinition.name} that = (${classDefinition.name}) o;');
+      final Iterable<String> checks = classDefinition.fields.map(
+        (NamedType field) {
+          // Objects.equals only does pointer equality for array types.
+          if (_javaTypeIsArray(field.type)) {
+            return 'Arrays.equals(${field.name}, that.${field.name})';
+          }
+          return field.type.isNullable
+              ? 'Objects.equals(${field.name}, that.${field.name})'
+              : '${field.name}.equals(that.${field.name})';
+        },
+      );
+      indent.writeln('return ${checks.join(' && ')};');
+    });
+    indent.newln();
+
+    // Implement hashCode().
+    indent.writeln('@Override');
+    indent.writeScoped('public int hashCode() {', '}', () {
+      // As with equalty checks, arrays need special handling.
+      final Iterable<String> arrayFieldNames = classDefinition.fields
+          .where((NamedType field) => _javaTypeIsArray(field.type))
+          .map((NamedType field) => field.name);
+      final Iterable<String> nonArrayFieldNames = classDefinition.fields
+          .where((NamedType field) => !_javaTypeIsArray(field.type))
+          .map((NamedType field) => field.name);
+      final String nonArrayHashValue = nonArrayFieldNames.isNotEmpty
+          ? 'Objects.hash(${nonArrayFieldNames.join(', ')})'
+          : '0';
+
+      if (arrayFieldNames.isEmpty) {
+        // Return directly if there are no array variables, to avoid redundant
+        // variable lint warnings.
+        indent.writeln('return $nonArrayHashValue;');
+      } else {
+        const String resultVar = '${varNamePrefix}result';
+        indent.writeln('int $resultVar = $nonArrayHashValue;');
+        // Manually mix in the Arrays.hashCode values.
+        for (final String name in arrayFieldNames) {
+          indent.writeln(
+              '$resultVar = 31 * $resultVar + Arrays.hashCode($name);');
+        }
+        indent.writeln('return $resultVar;');
+      }
+    });
+    indent.newln();
+  }
+
   void _writeClassBuilder(
     JavaOptions generatorOptions,
     Root root,
@@ -1022,6 +1080,10 @@ String _javaTypeForBuiltinGenericDartType(
   }
 }
 
+bool _javaTypeIsArray(TypeDeclaration type) {
+  return _javaTypeForBuiltinDartType(type)?.endsWith('[]') ?? false;
+}
+
 String? _javaTypeForBuiltinDartType(TypeDeclaration type) {
   const Map<String, String> javaTypeForDartTypeMap = <String, String>{
     'bool': 'Boolean',