prioritize lambdas in basic code completion (IDEA-135296)

java/java-impl/src/com/intellij/codeInsight/completion/JavaCompletionContributor.java

@@ -225,6 +225,12 @@ public class JavaCompletionContributor extends CompletionContributor {
       new TypeArgumentCompletionProvider(false, inheritors).addCompletions(parameters, new ProcessingContext(), result);
     }
 
+    if (JavaSmartCompletionContributor.LAMBDA.accepts(parameters.getPosition())) {
+      for (LookupElement element : LambdaCompletionProvider.getLambdaVariants(parameters)) {
+        result.addElement(PrioritizedLookupElement.withPriority(element, 1));
+      }
+    }
+
     PrefixMatcher matcher = result.getPrefixMatcher();
     if (JavaSmartCompletionContributor.AFTER_NEW.accepts(position)) {
       new JavaInheritorsGetter(ConstructorInsertHandler.BASIC_INSTANCE).generateVariants(parameters, matcher, inheritors);
@@ -247,10 +253,6 @@ public class JavaCompletionContributor extends CompletionContributor {
 
     JavaGenerateMemberCompletionContributor.fillCompletionVariants(parameters, result);
 
-    if (JavaSmartCompletionContributor.LAMBDA.accepts(parameters.getPosition())) {
-      new LambdaCompletionProvider().addCompletions(parameters, new ProcessingContext(), result);
-    }
-
     addAllClasses(parameters, result, inheritors);
 
     final PsiElement parent = position.getParent();

java/java-impl/src/com/intellij/codeInsight/completion/LambdaCompletionProvider.java

@@ -31,8 +31,12 @@ import com.intellij.psi.impl.source.resolve.graphInference.FunctionalInterfacePa
 import com.intellij.psi.util.PsiUtil;
 import com.intellij.util.Function;
 import com.intellij.util.ProcessingContext;
+import com.intellij.util.containers.ContainerUtil;
 import org.jetbrains.annotations.NotNull;
 
+import java.util.Collections;
+import java.util.List;
+
 /**
  * User: anna
  */
@@ -41,9 +45,14 @@ public class LambdaCompletionProvider extends CompletionProvider<CompletionParam
   protected void addCompletions(@NotNull CompletionParameters parameters,
                                 ProcessingContext context,
                                 @NotNull CompletionResultSet result) {
-    if (!PsiUtil.isLanguageLevel8OrHigher(parameters.getOriginalFile())) return;
-    final ExpectedTypeInfo[] expectedTypes = JavaSmartCompletionContributor.getExpectedTypes(parameters);
-    for (ExpectedTypeInfo expectedType : expectedTypes) {
+    result.addAllElements(getLambdaVariants(parameters));
+  }
+
+  static List<LookupElement> getLambdaVariants(@NotNull CompletionParameters parameters) {
+    if (!PsiUtil.isLanguageLevel8OrHigher(parameters.getOriginalFile())) return Collections.emptyList();
+
+    List<LookupElement> result = ContainerUtil.newArrayList();
+    for (ExpectedTypeInfo expectedType : JavaSmartCompletionContributor.getExpectedTypes(parameters)) {
       final PsiType defaultType = expectedType.getDefaultType();
       if (LambdaUtil.isFunctionalType(defaultType)) {
         final PsiType functionalInterfaceType = FunctionalInterfaceParameterizationUtil.getGroundTargetType(defaultType);
@@ -80,10 +89,11 @@ public class LambdaCompletionProvider extends CompletionProvider<CompletionParam
                 EditorModificationUtil.insertStringAtCaret(editor, " -> ");
               }
             });
-          result.addElement(builder.withAutoCompletionPolicy(AutoCompletionPolicy.NEVER_AUTOCOMPLETE));
+          result.add(builder.withAutoCompletionPolicy(AutoCompletionPolicy.NEVER_AUTOCOMPLETE));
         }
       }
     }
+    return result;
   }
 
   private static String getParamName(PsiParameter param, JavaCodeStyleManager javaCodeStyleManager, PsiElement originalPosition) {

java/java-tests/testSrc/com/intellij/codeInsight/daemon/lambda/Normal8CompletionTest.groovy

@@ -71,7 +71,7 @@ class Test {
   }
 }"""
     def items = myFixture.completeBasic()
-    assert items.find { LookupElementPresentation.renderElement(it).itemText == 'x -> {}' }
+    assert LookupElementPresentation.renderElement(items[0]).itemText == 'x -> {}'
   }
 
 }