Apply some IDE suggestions, add tests, set unit-test to isolated

Without Isolation, one test did change static settings 
and thus could cause flaky tests

git-svn-id: https://svn.apache.org/repos/asf/poi/trunk@1911891 13f79535-47bb-0310-9956-ffa450edef68

poi-integration/src/test/java/org/apache/poi/stress/XSSFFileHandler.java

@@ -217,6 +217,8 @@ void test() throws Exception {
         }
 
         handleExtracting(file);
+
+        handleAdditional(file);
     }
 
     @Test

poi-ooxml/src/test/java/org/apache/poi/openxml4j/TestOPCPackageFileLimit.java

@@ -22,6 +22,7 @@ Licensed to the Apache Software Foundation (ASF) under one or more
 import org.apache.poi.openxml4j.opc.OPCPackage;
 import org.apache.poi.openxml4j.util.ZipSecureFile;
 import org.junit.jupiter.api.Test;
+import org.junit.jupiter.api.parallel.Isolated;
 
 import java.io.File;
 import java.io.IOException;
@@ -30,6 +31,7 @@ Licensed to the Apache Software Foundation (ASF) under one or more
 import static org.junit.jupiter.api.Assertions.assertTrue;
 import static org.junit.jupiter.api.Assertions.fail;
 
+@Isolated // changes static values, so other tests should not run at the same time
 class TestOPCPackageFileLimit {
     @Test
     void testWithReducedFileLimit() throws InvalidFormatException {

poi-ooxml/src/test/java/org/apache/poi/xssf/eventusermodel/TestReadOnlySharedStringsTable.java

@@ -67,6 +67,12 @@ void testParse() throws Exception {
                     assertEquals(i1.getString(), rtbl.getItemAt(i).getString());
                     assertEquals(i1.getString(), rtbl2.getItemAt(i).getString());
                 }
+
+                // verify invalid indices
+                assertThrows(IllegalStateException.class,
+                        () -> rtbl.getItemAt(stbl.getUniqueCount()));
+                assertThrows(IndexOutOfBoundsException.class,
+                        () -> rtbl.getItemAt(-1));
             }
         }
     }

poi-scratchpad/src/main/java/org/apache/poi/hslf/usermodel/HSLFSlideShowImpl.java

@@ -1085,17 +1085,17 @@ private static class CountingOS extends OutputStream {
         int count;
 
         @Override
-        public void write(int b) throws IOException {
+        public void write(int b) {
             count++;
         }
 
         @Override
-        public void write(byte[] b) throws IOException {
+        public void write(byte[] b) {
             count += b.length;
         }
 
         @Override
-        public void write(byte[] b, int off, int len) throws IOException {
+        public void write(byte[] b, int off, int len) {
             count += len;
         }