IDEA-167088 Stream API migration: allow resulting StringBuilder usages in concatenation

java/java-impl/src/com/intellij/codeInspection/streamMigration/CollectMigration.java

@@ -618,6 +618,15 @@ class CollectMigration extends BaseStreamApiMigration {
             String name = usage.getMethodExpression().getReferenceName();
             if (usageArgs.length == 0 && ("toString".equals(name) || "length".equals(name))) return true;
           }
+          PsiElement parent = PsiUtil.skipParenthesizedExprUp(element.getParent());
+          if (parent instanceof PsiPolyadicExpression &&
+              ((PsiPolyadicExpression)parent).getOperationTokenType().equals(JavaTokenType.PLUS)) {
+            return true;
+          }
+          if (parent instanceof PsiAssignmentExpression &&
+            ((PsiAssignmentExpression)parent).getOperationTokenType().equals(JavaTokenType.PLUSEQ)) {
+            return true;
+          }
         }
         return false;
       })) {

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/streamApiMigration/afterJoiningConcat.java

@@ -0,0 +1,16 @@
+// "Replace with collect" "true"
+
+import java.util.List;
+import java.util.stream.Collectors;
+
+public class Test {
+  static String test(List<String> list) {
+    String sb;
+    System.out.println("hello");
+      sb = list.stream().filter(s -> !s.isEmpty()).map(String::trim).collect(Collectors.joining());
+    String s = "Result: ";
+    s += sb;
+    System.out.println(s);
+    return "[" + sb + "]";
+  }
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/streamApiMigration/beforeJoiningConcat.java

@@ -0,0 +1,19 @@
+// "Replace with collect" "true"
+
+import java.util.List;
+
+public class Test {
+  static String test(List<String> list) {
+    StringBuilder sb = new StringBuilder();
+    System.out.println("hello");
+    for(String s : li<caret>st) {
+      if(!s.isEmpty()) {
+        sb.append(s.trim());
+      }
+    }
+    String s = "Result: ";
+    s += sb;
+    System.out.println(s);
+    return "[" + sb + "]";
+  }
+}