Use StringBuilder.append(char) where possible

To slightly improve performance, this commit switches to
StringBuilder.append(char) instead of StringBuilder.append(String)
whenever we append a single character to a StringBuilder.

Closes gh-27098

spring-web/src/test/java/org/springframework/http/HttpRangeTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -111,7 +111,7 @@ public class HttpRangeTests {
 		// 1. At limit..
 		StringBuilder atLimit = new StringBuilder("bytes=0-0");
 		for (int i=0; i < 99; i++) {
-			atLimit.append(",").append(i).append("-").append(i + 1);
+			atLimit.append(',').append(i).append('-').append(i + 1);
 		}
 		List<HttpRange> ranges = HttpRange.parseRanges(atLimit.toString());
 		assertThat(ranges.size()).isEqualTo(100);
@@ -119,7 +119,7 @@ public class HttpRangeTests {
 		// 2. Above limit..
 		StringBuilder aboveLimit = new StringBuilder("bytes=0-0");
 		for (int i=0; i < 100; i++) {
-			aboveLimit.append(",").append(i).append("-").append(i + 1);
+			aboveLimit.append(',').append(i).append('-').append(i + 1);
 		}
 		assertThatIllegalArgumentException().isThrownBy(() ->
 				HttpRange.parseRanges(aboveLimit.toString()));

spring-web/src/test/java/org/springframework/web/multipart/support/DefaultMultipartHttpServletRequestTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2019 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -13,6 +13,7 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
+
 package org.springframework.web.multipart.support;
 
 import java.util.HashMap;
@@ -80,7 +81,7 @@ public class DefaultMultipartHttpServletRequestTests {
 		for (String key : this.queryParams.keySet()) {
 			for (String value : this.queryParams.get(key)) {
 				this.servletRequest.addParameter(key, value);
-				query.append(query.length() > 0 ? "&" : "").append(key).append("=").append(value);
+				query.append(query.length() > 0 ? "&" : "").append(key).append('=').append(value);
 			}
 		}
 		this.servletRequest.setQueryString(query.toString());

spring-web/src/test/java/org/springframework/web/util/JavaScriptUtilsTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2004-2019 the original author or authors.
+ * Copyright 2004-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -33,15 +33,15 @@ public class JavaScriptUtilsTests {
 	public void escape() {
 		StringBuilder sb = new StringBuilder();
 		sb.append('"');
-		sb.append("'");
-		sb.append("\\");
-		sb.append("/");
-		sb.append("\t");
-		sb.append("\n");
-		sb.append("\r");
-		sb.append("\f");
-		sb.append("\b");
-		sb.append("\013");
+		sb.append('\'');
+		sb.append('\\');
+		sb.append('/');
+		sb.append('\t');
+		sb.append('\n');
+		sb.append('\r');
+		sb.append('\f');
+		sb.append('\b');
+		sb.append('\013');
 		assertThat(JavaScriptUtils.javaScriptEscape(sb.toString())).isEqualTo("\\\"\\'\\\\\\/\\t\\n\\n\\f\\b\\v");
 	}
 

spring-web/src/test/java/org/springframework/web/util/pattern/PathPatternTests.java

@@ -1,5 +1,5 @@
 /*
- * Copyright 2002-2020 the original author or authors.
+ * Copyright 2002-2021 the original author or authors.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -1210,7 +1210,7 @@ public class PathPatternTests {
 	private String elementsToString(List<Element> elements) {
 		StringBuilder s = new StringBuilder();
 		for (Element element: elements) {
-			s.append("[").append(element.value()).append("]");
+			s.append('[').append(element.value()).append(']');
 		}
 		return s.toString();
 	}