FoldExpressionIntoStream: disable for non-string separator and automatically convert char

CharToStringIntention#stringForCharLiteral moved to PsiLiteralUtil
Fixes IDEA-195621 "Expression can be folded into Stream chain" produces red code when delimiter is char type

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

@@ -11,6 +11,7 @@ import com.intellij.psi.codeStyle.VariableKind;
 import com.intellij.psi.impl.PsiDiamondTypeUtil;
 import com.intellij.psi.tree.IElementType;
 import com.intellij.psi.util.InheritanceUtil;
+import com.intellij.psi.util.PsiLiteralUtil;
 import com.intellij.psi.util.PsiTreeUtil;
 import com.intellij.psi.util.PsiUtil;
 import com.intellij.util.ArrayUtil;
@@ -151,6 +152,10 @@ public class FoldExpressionIntoStreamInspection extends AbstractBaseJavaLocalIns
                      .pairMap(EquivalenceChecker.getCanonicalPsiEquivalence()::expressionsAreEquivalent)
                      .allMatch(Boolean.TRUE::equals)) {
         delimiter = operands[1];
+        if (!InheritanceUtil.isInheritor(delimiter.getType(), "java.lang.CharSequence") &&
+            !(delimiter instanceof PsiLiteralExpression && PsiType.CHAR.equals(delimiter.getType()))) {
+          return null;
+        }
         if (operands.length % 2 == 0) {
           rest = ArrayUtil.getLastElement(operands);
         }
@@ -272,8 +277,20 @@ public class FoldExpressionIntoStreamInspection extends AbstractBaseJavaLocalIns
       String map = (lambda == null ? "" : mapToString(elementType, myOperandType, lambda)) + myMapToString;
       return map +
              ".collect(" + CommonClassNames.JAVA_UTIL_STREAM_COLLECTORS +
-             ".joining(" + (myDelimiter == null ? "" : ct.text(myDelimiter)) + "))" +
+             ".joining(" + getDelimiterText(ct) + "))" +
              (myRest == null ? "" : "+" + ct.text(myRest));
     }
+
+    @NotNull
+    private String getDelimiterText(CommentTracker ct) {
+      if (myDelimiter == null) {
+        return "";
+      }
+      String text = ct.text(myDelimiter);
+      if (text.startsWith("'")) {
+        return PsiLiteralUtil.stringForCharLiteral(text);
+      }
+      return text;
+    }
   }
 }

java/java-psi-api/src/com/intellij/psi/util/PsiLiteralUtil.java

@@ -3,6 +3,7 @@ package com.intellij.psi.util;
 
 import com.intellij.openapi.util.text.StringUtil;
 import org.jetbrains.annotations.NonNls;
+import org.jetbrains.annotations.NotNull;
 import org.jetbrains.annotations.Nullable;
 
 public class PsiLiteralUtil {
@@ -103,4 +104,24 @@ public class PsiLiteralUtil {
     integer |= lastDigit;
     return integer;
   }
+
+  /**
+   * Converts passed character literal (like 'a') to string literal (like "a").
+   *
+   * @param charLiteral character literal to convert.
+   * @return resulting string literal
+   */
+  @NotNull
+  public static String stringForCharLiteral(@NotNull String charLiteral) {
+    if ("'\"'".equals(charLiteral)) {
+      return "\"\\\"\"";
+    }
+    else if ("'\\''".equals(charLiteral)) {
+      return "\"'\"";
+    }
+    else {
+      return '\"' + charLiteral.substring(1, charLiteral.length() - 1) +
+             '\"';
+    }
+  }
 }

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/foldIntoStream/afterConcatCharDelimiter.java

@@ -0,0 +1,7 @@
+// "Fold expression into 'String.join'" "true"
+class Test {
+  String  foo(String a, String b, String c) {
+    String s = String.join("_", a, b, c);
+    return s;
+  }
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/foldIntoStream/beforeConcatCharDelimiter.java

@@ -0,0 +1,7 @@
+// "Fold expression into 'String.join'" "true"
+class Test {
+  String  foo(String a, String b, String c) {
+    String s = a + <caret>'_' + b + '_' + c ;
+    return s;
+  }
+}

java/java-tests/testData/codeInsight/daemonCodeAnalyzer/quickFix/foldIntoStream/beforeConcatIntDelimiter.java

@@ -0,0 +1,7 @@
+// "Fold expression into 'String.join'" "false"
+class Test {
+  String  foo(String a, String b, String c) {
+    String s = a + <caret>5 + b + 5 + c ;
+    return s;
+  }
+}

plugins/IntentionPowerPak/src/com/siyeh/ipp/chartostring/CharToStringIntention.java

@@ -17,6 +17,7 @@ package com.siyeh.ipp.chartostring;
 
 import com.intellij.psi.PsiElement;
 import com.intellij.psi.PsiLiteralExpression;
+import com.intellij.psi.util.PsiLiteralUtil;
 import com.siyeh.ig.PsiReplacementUtil;
 import com.siyeh.ipp.base.Intention;
 import com.siyeh.ipp.base.PsiElementPredicate;
@@ -34,20 +35,7 @@ public class CharToStringIntention extends Intention {
   public void processIntention(@NotNull PsiElement element) {
     final PsiLiteralExpression charLiteral = (PsiLiteralExpression)element;
     final String charLiteralText = charLiteral.getText();
-    final String stringLiteral = stringForCharLiteral(charLiteralText);
+    final String stringLiteral = PsiLiteralUtil.stringForCharLiteral(charLiteralText);
     PsiReplacementUtil.replaceExpression(charLiteral, stringLiteral);
   }
-
-  private static String stringForCharLiteral(String charLiteral) {
-    if ("'\"'".equals(charLiteral)) {
-      return "\"\\\"\"";
-    }
-    else if ("'\\''".equals(charLiteral)) {
-      return "\"'\"";
-    }
-    else {
-      return '\"' + charLiteral.substring(1, charLiteral.length() - 1) +
-             '\"';
-    }
-  }
 }