extract method: add default modifier when extracted into interface (java 8 only)

java/java-impl/src/com/intellij/refactoring/extractMethod/ExtractMethodProcessor.java

@@ -1048,6 +1048,9 @@ public class ExtractMethodProcessor implements MatchProvider {
       }
     }
 
+    if (myTargetClass.isInterface() && PsiUtil.isLanguageLevel8OrHigher(myTargetClass)) {
+      PsiUtil.setModifierProperty(newMethod, PsiModifier.DEFAULT, true);
+    }
     return (PsiMethod)myStyleManager.reformat(newMethod);
   }
 

java/java-tests/testData/refactoring/extractMethod/Method2Interface.java

@@ -0,0 +1,5 @@
+interface I {
+    default void foo () {
+        <selection>System.out.println("hello");</selection>
+    }
+}

java/java-tests/testData/refactoring/extractMethod/Method2Interface_after.java

@@ -0,0 +1,9 @@
+interface I {
+    default void foo () {
+        newMethod();
+    }
+
+    private default void newMethod() {
+        System.out.println("hello");
+    }
+}

java/java-tests/testSrc/com/intellij/refactoring/ExtractMethodTest.java

@@ -550,6 +550,10 @@ public class ExtractMethodTest extends LightCodeInsightTestCase {
     doTest();
   }
 
+  public void testMethod2Interface() throws Exception {
+    doTest();
+  }
+
   public void testParamDetection() throws Exception {
     doTest();
   }