Support interface implementation in SourceCodeWriter

Closes gh-1617

initializr-generator/src/main/java/io/spring/initializr/generator/language/TypeDeclaration.java

@@ -16,10 +16,15 @@
 
 package io.spring.initializr.generator.language;
 
+import java.util.Collection;
+import java.util.Collections;
+import java.util.List;
+
 /**
  * A type declared in a {@link CompilationUnit}.
  *
  * @author Andy Wilkinson
+ * @author Moritz Halbritter
  */
 public class TypeDeclaration implements Annotatable {
 
@@ -29,14 +34,40 @@ public class TypeDeclaration implements Annotatable {
 
 	private String extendedClassName;
 
+	private List<String> implementsClassNames = Collections.emptyList();
+
+	/**
+	 * Creates a new instance.
+	 * @param name the type name
+	 */
 	public TypeDeclaration(String name) {
 		this.name = name;
 	}
 
+	/**
+	 * Extend the class with the given name.
+	 * @param name the name of the class to extend
+	 */
 	public void extend(String name) {
 		this.extendedClassName = name;
 	}
 
+	/**
+	 * Implement the given interfaces.
+	 * @param names the names of the interfaces to implement
+	 */
+	public void implement(Collection<String> names) {
+		this.implementsClassNames = List.copyOf(names);
+	}
+
+	/**
+	 * Implement the given interfaces.
+	 * @param names the names of the interfaces to implement
+	 */
+	public void implement(String... names) {
+		this.implementsClassNames = List.of(names);
+	}
+
 	@Override
 	public AnnotationContainer annotations() {
 		return this.annotations;
@@ -50,4 +81,8 @@ public String getExtends() {
 		return this.extendedClassName;
 	}
 
+	public List<String> getImplements() {
+		return this.implementsClassNames;
+	}
+
 }

initializr-generator/src/main/java/io/spring/initializr/generator/language/groovy/GroovySourceCodeWriter.java

@@ -48,6 +48,8 @@
 import io.spring.initializr.generator.language.SourceCodeWriter;
 import io.spring.initializr.generator.language.SourceStructure;
 
+import org.springframework.util.CollectionUtils;
+
 /**
  * A {@link SourceCodeWriter} that writes {@link SourceCode} in Groovy.
  *
@@ -125,6 +127,9 @@ private void writeTo(SourceStructure structure, GroovyCompilationUnit compilatio
 				if (type.getExtends() != null) {
 					writer.print(" extends " + getUnqualifiedName(type.getExtends()));
 				}
+				if (!CollectionUtils.isEmpty(type.getImplements())) {
+					writeImplements(type, writer);
+				}
 				writer.println(" {");
 				writer.println();
 				List<GroovyFieldDeclaration> fieldDeclarations = type.getFieldDeclarations();
@@ -148,6 +153,18 @@ private void writeTo(SourceStructure structure, GroovyCompilationUnit compilatio
 		}
 	}
 
+	private void writeImplements(GroovyTypeDeclaration type, IndentingWriter writer) {
+		writer.print(" implements ");
+		Iterator<String> iterator = type.getImplements().iterator();
+		while (iterator.hasNext()) {
+			String name = iterator.next();
+			writer.print(getUnqualifiedName(name));
+			if (iterator.hasNext()) {
+				writer.print(", ");
+			}
+		}
+	}
+
 	private void writeAnnotations(IndentingWriter writer, Annotatable annotatable, Runnable separator) {
 		annotatable.annotations().values().forEach((annotation) -> {
 			annotation.write(writer, FORMATTING_OPTIONS);
@@ -216,6 +233,7 @@ private Set<String> determineImports(GroovyCompilationUnit compilationUnit) {
 		List<String> imports = new ArrayList<>();
 		for (GroovyTypeDeclaration typeDeclaration : compilationUnit.getTypeDeclarations()) {
 			imports.add(typeDeclaration.getExtends());
+			imports.addAll(typeDeclaration.getImplements());
 			imports.addAll(appendImports(typeDeclaration.annotations().values(), Annotation::getImports));
 			for (GroovyFieldDeclaration fieldDeclaration : typeDeclaration.getFieldDeclarations()) {
 				imports.add(fieldDeclaration.getReturnType());

initializr-generator/src/main/java/io/spring/initializr/generator/language/java/JavaSourceCodeWriter.java

@@ -45,11 +45,14 @@
 import io.spring.initializr.generator.language.SourceCodeWriter;
 import io.spring.initializr.generator.language.SourceStructure;
 
+import org.springframework.util.CollectionUtils;
+
 /**
  * A {@link SourceCodeWriter} that writes {@link SourceCode} in Java.
  *
  * @author Andy Wilkinson
  * @author Matt Berteaux
+ * @author Moritz Halbritter
  */
 public class JavaSourceCodeWriter implements SourceCodeWriter<JavaSourceCode> {
 
@@ -122,6 +125,9 @@ private void writeTo(SourceStructure structure, JavaCompilationUnit compilationU
 				if (type.getExtends() != null) {
 					writer.print(" extends " + getUnqualifiedName(type.getExtends()));
 				}
+				if (!CollectionUtils.isEmpty(type.getImplements())) {
+					writeImplements(type, writer);
+				}
 				writer.println(" {");
 				writer.println();
 				List<JavaFieldDeclaration> fieldDeclarations = type.getFieldDeclarations();
@@ -145,6 +151,18 @@ private void writeTo(SourceStructure structure, JavaCompilationUnit compilationU
 		}
 	}
 
+	private void writeImplements(JavaTypeDeclaration type, IndentingWriter writer) {
+		writer.print(" implements ");
+		Iterator<String> iterator = type.getImplements().iterator();
+		while (iterator.hasNext()) {
+			String name = iterator.next();
+			writer.print(getUnqualifiedName(name));
+			if (iterator.hasNext()) {
+				writer.print(", ");
+			}
+		}
+	}
+
 	private void writeAnnotations(IndentingWriter writer, Annotatable annotatable, Runnable separator) {
 		annotatable.annotations().values().forEach((annotation) -> {
 			annotation.write(writer, CodeBlock.JAVA_FORMATTING_OPTIONS);
@@ -213,7 +231,7 @@ private Set<String> determineImports(JavaCompilationUnit compilationUnit) {
 		List<String> imports = new ArrayList<>();
 		for (JavaTypeDeclaration typeDeclaration : compilationUnit.getTypeDeclarations()) {
 			imports.add(typeDeclaration.getExtends());
-
+			imports.addAll(typeDeclaration.getImplements());
 			imports.addAll(appendImports(typeDeclaration.annotations().values(), Annotation::getImports));
 			for (JavaFieldDeclaration fieldDeclaration : typeDeclaration.getFieldDeclarations()) {
 				imports.add(fieldDeclaration.getReturnType());

initializr-generator/src/main/java/io/spring/initializr/generator/language/kotlin/KotlinSourceCodeWriter.java

@@ -46,6 +46,8 @@
 import io.spring.initializr.generator.language.SourceCodeWriter;
 import io.spring.initializr.generator.language.SourceStructure;
 
+import org.springframework.util.CollectionUtils;
+
 /**
  * A {@link SourceCodeWriter} that writes {@link SourceCode} in Kotlin.
  *
@@ -95,9 +97,14 @@ private void writeTo(SourceStructure structure, KotlinCompilationUnit compilatio
 				writeAnnotations(writer, type);
 				writeModifiers(writer, type.getModifiers());
 				writer.print("class " + type.getName());
-				if (type.getExtends() != null) {
+				boolean hasExtends = type.getExtends() != null;
+				if (hasExtends) {
 					writer.print(" : " + getUnqualifiedName(type.getExtends()) + "()");
 				}
+				if (!CollectionUtils.isEmpty(type.getImplements())) {
+					writer.print(hasExtends ? ", " : " : ");
+					writeImplements(type, writer);
+				}
 				List<KotlinPropertyDeclaration> propertyDeclarations = type.getPropertyDeclarations();
 				List<KotlinFunctionDeclaration> functionDeclarations = type.getFunctionDeclarations();
 				boolean hasDeclarations = !propertyDeclarations.isEmpty() || !functionDeclarations.isEmpty();
@@ -136,6 +143,17 @@ private void writeTo(SourceStructure structure, KotlinCompilationUnit compilatio
 		}
 	}
 
+	private void writeImplements(KotlinTypeDeclaration type, IndentingWriter writer) {
+		Iterator<String> iterator = type.getImplements().iterator();
+		while (iterator.hasNext()) {
+			String name = iterator.next();
+			writer.print(getUnqualifiedName(name));
+			if (iterator.hasNext()) {
+				writer.print(", ");
+			}
+		}
+	}
+
 	private String escapeKotlinKeywords(String packageName) {
 		return Arrays.stream(packageName.split("\\."))
 			.map((segment) -> this.language.isKeyword(segment) ? "`" + segment + "`" : segment)
@@ -240,6 +258,7 @@ private Set<String> determineImports(KotlinCompilationUnit compilationUnit) {
 		List<String> imports = new ArrayList<>();
 		for (KotlinTypeDeclaration typeDeclaration : compilationUnit.getTypeDeclarations()) {
 			imports.add(typeDeclaration.getExtends());
+			imports.addAll(typeDeclaration.getImplements());
 			imports.addAll(appendImports(typeDeclaration.annotations().values(), Annotation::getImports));
 			typeDeclaration.getPropertyDeclarations()
 				.forEach(((propertyDeclaration) -> imports.addAll(determinePropertyImports(propertyDeclaration))));

initializr-generator/src/test/java/io/spring/initializr/generator/language/TypeDeclarationTests.java

@@ -0,0 +1,37 @@
+/*
+ * Copyright 2012-2025 the original author or authors.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      https://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package io.spring.initializr.generator.language;
+
+import org.junit.jupiter.api.Test;
+
+import static org.assertj.core.api.Assertions.assertThat;
+
+/**
+ * Tests for {@link TypeDeclaration}.
+ *
+ * @author Moritz Halbritter
+ */
+class TypeDeclarationTests {
+
+	@Test
+	void implementWithVarArgs() {
+		TypeDeclaration declaration = new TypeDeclaration("Test");
+		declaration.implement("Interface1", "Interface2");
+		assertThat(declaration.getImplements()).containsExactly("Interface1", "Interface2");
+	}
+
+}

initializr-generator/src/test/java/io/spring/initializr/generator/language/groovy/GroovySourceCodeWriterTests.java

@@ -48,6 +48,7 @@
  *
  * @author Stephane Nicoll
  * @author Matt Berteaux
+ * @author Moritz Halbritter
  */
 class GroovySourceCodeWriterTests {
 
@@ -109,6 +110,30 @@ void emptyTypeDeclarationWithSuperClass() throws IOException {
 				"class Test extends TestParent {", "", "}");
 	}
 
+	@Test
+	void shouldAddImplements() throws IOException {
+		GroovySourceCode sourceCode = new GroovySourceCode();
+		GroovyCompilationUnit compilationUnit = sourceCode.createCompilationUnit("com.example", "Test");
+		GroovyTypeDeclaration test = compilationUnit.createTypeDeclaration("Test");
+		test.implement(List.of("com.example.build.Interface1", "com.example.build.Interface2"));
+		List<String> lines = writeSingleType(sourceCode, "com/example/Test.groovy");
+		assertThat(lines).containsExactly("package com.example", "", "import com.example.build.Interface1",
+				"import com.example.build.Interface2", "", "class Test implements Interface1, Interface2 {", "", "}");
+	}
+
+	@Test
+	void shouldAddExtendsAndImplements() throws IOException {
+		GroovySourceCode sourceCode = new GroovySourceCode();
+		GroovyCompilationUnit compilationUnit = sourceCode.createCompilationUnit("com.example", "Test");
+		GroovyTypeDeclaration test = compilationUnit.createTypeDeclaration("Test");
+		test.extend("com.example.build.TestParent");
+		test.implement(List.of("com.example.build.Interface1", "com.example.build.Interface2"));
+		List<String> lines = writeSingleType(sourceCode, "com/example/Test.groovy");
+		assertThat(lines).containsExactly("package com.example", "", "import com.example.build.Interface1",
+				"import com.example.build.Interface2", "import com.example.build.TestParent", "",
+				"class Test extends TestParent implements Interface1, Interface2 {", "", "}");
+	}
+
 	@Test
 	void method() throws IOException {
 		GroovySourceCode sourceCode = new GroovySourceCode();

initializr-generator/src/test/java/io/spring/initializr/generator/language/java/JavaSourceCodeWriterTests.java

@@ -48,6 +48,7 @@
  *
  * @author Andy Wilkinson
  * @author Matt Berteaux
+ * @author Moritz Halbritter
  */
 class JavaSourceCodeWriterTests {
 
@@ -108,6 +109,30 @@ void emptyTypeDeclarationWithSuperClass() throws IOException {
 				"class Test extends TestParent {", "", "}");
 	}
 
+	@Test
+	void shouldAddImplements() throws IOException {
+		JavaSourceCode sourceCode = new JavaSourceCode();
+		JavaCompilationUnit compilationUnit = sourceCode.createCompilationUnit("com.example", "Test");
+		JavaTypeDeclaration test = compilationUnit.createTypeDeclaration("Test");
+		test.implement(List.of("com.example.build.Interface1", "com.example.build.Interface2"));
+		List<String> lines = writeSingleType(sourceCode, "com/example/Test.java");
+		assertThat(lines).containsExactly("package com.example;", "", "import com.example.build.Interface1;",
+				"import com.example.build.Interface2;", "", "class Test implements Interface1, Interface2 {", "", "}");
+	}
+
+	@Test
+	void shouldAddExtendsAndImplements() throws IOException {
+		JavaSourceCode sourceCode = new JavaSourceCode();
+		JavaCompilationUnit compilationUnit = sourceCode.createCompilationUnit("com.example", "Test");
+		JavaTypeDeclaration test = compilationUnit.createTypeDeclaration("Test");
+		test.extend("com.example.build.TestParent");
+		test.implement(List.of("com.example.build.Interface1", "com.example.build.Interface2"));
+		List<String> lines = writeSingleType(sourceCode, "com/example/Test.java");
+		assertThat(lines).containsExactly("package com.example;", "", "import com.example.build.Interface1;",
+				"import com.example.build.Interface2;", "import com.example.build.TestParent;", "",
+				"class Test extends TestParent implements Interface1, Interface2 {", "", "}");
+	}
+
 	@Test
 	void method() throws IOException {
 		JavaSourceCode sourceCode = new JavaSourceCode();

initializr-generator/src/test/java/io/spring/initializr/generator/language/kotlin/KotlinSourceCodeWriterTests.java

@@ -47,6 +47,7 @@
  *
  * @author Stephane Nicoll
  * @author Matt Berteaux
+ * @author Moritz Halbritter
  */
 class KotlinSourceCodeWriterTests {
 
@@ -108,6 +109,30 @@ void emptyTypeDeclarationWithSuperClass() throws IOException {
 				"class Test : TestParent()");
 	}
 
+	@Test
+	void shouldImplementInterfaces() throws IOException {
+		KotlinSourceCode sourceCode = new KotlinSourceCode();
+		KotlinCompilationUnit compilationUnit = sourceCode.createCompilationUnit("com.example", "Test");
+		KotlinTypeDeclaration test = compilationUnit.createTypeDeclaration("Test");
+		test.implement(List.of("com.example.build.Interface1", "com.example.build.Interface2"));
+		List<String> lines = writeSingleType(sourceCode, "com/example/Test.kt");
+		assertThat(lines).containsExactly("package com.example", "", "import com.example.build.Interface1",
+				"import com.example.build.Interface2", "", "class Test : Interface1, Interface2");
+	}
+
+	@Test
+	void shouldExtendAndImplement() throws IOException {
+		KotlinSourceCode sourceCode = new KotlinSourceCode();
+		KotlinCompilationUnit compilationUnit = sourceCode.createCompilationUnit("com.example", "Test");
+		KotlinTypeDeclaration test = compilationUnit.createTypeDeclaration("Test");
+		test.extend("com.example.build.TestParent");
+		test.implement(List.of("com.example.build.Interface1", "com.example.build.Interface2"));
+		List<String> lines = writeSingleType(sourceCode, "com/example/Test.kt");
+		assertThat(lines).containsExactly("package com.example", "", "import com.example.build.Interface1",
+				"import com.example.build.Interface2", "import com.example.build.TestParent", "",
+				"class Test : TestParent(), Interface1, Interface2");
+	}
+
 	@Test
 	void function() throws IOException {
 		KotlinSourceCode sourceCode = new KotlinSourceCode();