JoinLineProcessor.getSpacesToAdd: properly handle end-of-file

Fixes EA-213710 - [JoinLines] IOOBE: ImmutableText.outOfRange

GitOrigin-RevId: 13ab2f27bbcf5457671331c8b41e5897d9d82255

java/java-tests/testData/codeInsight/joinLines/AtEOF.java

@@ -0,0 +1,5 @@
+class Foo {
+    void test() {
+    }
+    // this test must have a single newline at the end-of-file
+<caret>}

java/java-tests/testData/codeInsight/joinLines/AtEOF_after.java

@@ -0,0 +1,5 @@
+class Foo {
+    void test() {
+    }
+    // this test must have a single newline at the end-of-file
+}

java/java-tests/testSrc/com/intellij/java/codeInsight/JoinLinesTest.java

@@ -16,13 +16,11 @@
 package com.intellij.java.codeInsight;
 
 import com.intellij.JavaTestUtil;
-import com.intellij.application.options.CodeStyle;
 import com.intellij.ide.DataManager;
 import com.intellij.lang.java.JavaLanguage;
 import com.intellij.openapi.actionSystem.IdeActions;
 import com.intellij.openapi.editor.actionSystem.EditorActionHandler;
 import com.intellij.openapi.editor.actionSystem.EditorActionManager;
-import com.intellij.psi.codeStyle.CodeStyleSettings;
 import com.intellij.psi.codeStyle.CodeStyleSettingsManager;
 import com.intellij.psi.codeStyle.CommonCodeStyleSettings;
 import com.intellij.testFramework.LightJavaCodeInsightTestCase;
@@ -64,6 +62,7 @@ public class JoinLinesTest extends LightJavaCodeInsightTestCase {
   public void testIfChainNoBraces() { doTest(); }
   public void testIfChainElse() { doTest(); }
   public void testIfChainSelection() { doTest(); }
+  public void testAtEOF() { doTest(); }
 
   public void testSCR3493() {
     CommonCodeStyleSettings settings = getJavaSettings();

platform/lang-impl/src/com/intellij/codeInsight/editorActions/JoinLinesHandler.java

@@ -340,7 +340,7 @@ public class JoinLinesHandler extends EditorActionHandler {
         RangeMarker marker = markers.get(i);
         if (!marker.isValid()) continue;
         int end = StringUtil.skipWhitespaceForward(text, marker.getStartOffset());
-        int spacesToCreate = text.charAt(end) == '\n' ? 0 :
+        int spacesToCreate = end == text.length() || text.charAt(end) == '\n' ? 0 :
                              model == null ? 1 : formatter.getSpacingForBlockAtOffset(model, end);
         spacesToAdd[i] = spacesToCreate < 0 ? 1 : spacesToCreate;
       }