implement interface inside java 8+ interface should create default methods (IDEA-135105)

java/java-impl/src/com/intellij/codeInsight/generation/JavaImplementMethodsHandler.java

@@ -23,6 +23,7 @@ import com.intellij.psi.PsiClass;
 import com.intellij.psi.PsiCodeFragment;
 import com.intellij.psi.PsiFile;
 import com.intellij.psi.PsiJavaFile;
+import com.intellij.psi.util.PsiUtil;
 import org.jetbrains.annotations.NotNull;
 
 /**
@@ -35,13 +36,13 @@ public class JavaImplementMethodsHandler implements LanguageCodeInsightActionHan
       return false;
     }
 
-    PsiClass aClass = OverrideImplementUtil.getContextClass(file.getProject(), editor, file, false);
+    PsiClass aClass = OverrideImplementUtil.getContextClass(file.getProject(), editor, file, PsiUtil.isLanguageLevel8OrHigher(file));
     return aClass != null && !OverrideImplementUtil.getMethodSignaturesToImplement(aClass).isEmpty();
   }
 
   @Override
   public void invoke(@NotNull final Project project, @NotNull final Editor editor, @NotNull final PsiFile file) {
-    PsiClass aClass = OverrideImplementUtil.getContextClass(project, editor, file, false);
+    PsiClass aClass = OverrideImplementUtil.getContextClass(project, editor, file, PsiUtil.isLanguageLevel8OrHigher(file));
     if (aClass == null) {
       return;
     }

java/java-impl/src/com/intellij/codeInsight/generation/OverrideImplementUtil.java

@@ -25,11 +25,14 @@ import com.intellij.ide.fileTemplates.FileTemplateManager;
 import com.intellij.ide.fileTemplates.FileTemplateUtil;
 import com.intellij.ide.fileTemplates.JavaTemplateUtil;
 import com.intellij.ide.util.MemberChooser;
+import com.intellij.idea.ActionsBundle;
 import com.intellij.lang.java.JavaLanguage;
 import com.intellij.openapi.actionSystem.KeyboardShortcut;
+import com.intellij.openapi.actionSystem.Presentation;
 import com.intellij.openapi.actionSystem.Shortcut;
 import com.intellij.openapi.application.ApplicationManager;
 import com.intellij.openapi.application.Result;
+import com.intellij.openapi.command.CommandProcessor;
 import com.intellij.openapi.command.WriteCommandAction;
 import com.intellij.openapi.diagnostic.Logger;
 import com.intellij.openapi.editor.Editor;
@@ -363,8 +366,13 @@ public class OverrideImplementUtil extends OverrideImplementExploreUtil {
                                      final PsiClass targetClass,
                                      final FileTemplate template) throws IncorrectOperationException {
     if (targetClass.isInterface()) {
-      final PsiCodeBlock body = result.getBody();
-      if (body != null) body.delete();
+      if (isImplementInterfaceInJava8Interface(targetClass)) {
+        PsiUtil.setModifierProperty(result, PsiModifier.DEFAULT, true);
+      }
+      else {
+        final PsiCodeBlock body = result.getBody();
+        if (body != null) body.delete();
+      }
     }
     FileType fileType = FileTypeManager.getInstance().getFileTypeByExtension(template.getExtension());
     PsiType returnType = result.getReturnType();
@@ -410,6 +418,16 @@ public class OverrideImplementUtil extends OverrideImplementExploreUtil {
     }
   }
 
+  private static boolean isImplementInterfaceInJava8Interface(PsiClass targetClass) {
+    if (!PsiUtil.isLanguageLevel8OrHigher(targetClass)){
+      return false;
+    }
+    final String implementMethodsName = ActionsBundle.message("action.ImplementMethods.text");
+    final Presentation presentation = new Presentation();
+    presentation.setText(implementMethodsName);
+    return presentation.getText().equals(CommandProcessor.getInstance().getCurrentCommandName());
+  }
+
   public static void chooseAndOverrideMethods(Project project, Editor editor, PsiClass aClass){
     FeatureUsageTracker.getInstance().triggerFeatureUsed(ProductivityFeatureNames.CODEASSISTS_OVERRIDE_IMPLEMENT);
     chooseAndOverrideOrImplementMethods(project, editor, aClass, false);

java/java-tests/testSrc/com/intellij/codeInsight/OverrideImplementTest.groovy

@@ -16,7 +16,10 @@
 package com.intellij.codeInsight;
 
 import com.intellij.JavaTestUtil;
-import com.intellij.codeInsight.generation.OverrideImplementUtil;
+import com.intellij.codeInsight.generation.OverrideImplementUtil
+import com.intellij.idea.ActionsBundle
+import com.intellij.openapi.actionSystem.Presentation
+import com.intellij.openapi.command.CommandProcessor;
 import com.intellij.psi.PsiClass;
 import com.intellij.psi.util.PsiTreeUtil;
 import com.intellij.testFramework.fixtures.LightCodeInsightFixtureTestCase;
@@ -38,6 +41,41 @@ public class OverrideImplementTest extends LightCodeInsightFixtureTestCase {
   public void testSkipUnknownAnnotations() { doTest(true); }
 
   public void testOverrideInInterface() { doTest(false); }
+  public void testImplementInInterface() {
+    myFixture.addClass """
+interface A {
+    void foo();
+}
+"""
+    def file = myFixture.addClass("""
+interface B extends A {
+    <caret>
+}
+""").containingFile.virtualFile
+    myFixture.configureFromExistingVirtualFile(file)
+
+    def implementMethodsName = ActionsBundle.message("action.ImplementMethods.text")
+    final Presentation presentation = new Presentation();
+    presentation.setText(implementMethodsName);
+
+    CommandProcessor.instance.executeCommand(project, new Runnable() {
+      @Override
+      void run() {
+        invokeAction(true)
+      }
+    }, presentation.getText(), null)
+    
+
+    myFixture.checkResult """
+interface B extends A {
+    @Override
+    default void foo() {
+        <caret>
+    }
+}
+"""
+
+  }
   public void testMultipleInheritedThrows() {doTest(false);}
 
   public void "test overriding overloaded method"() {