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-expression/src/test/java/org/springframework/expression/spel/AbstractExpressionTests.java

@@ -245,22 +245,22 @@ public abstract class AbstractExpressionTests {
 					sb.append("int[").append(l.length).append("]{");
 					for (int j = 0; j < l.length; j++) {
 						if (j > 0) {
-							sb.append(",");
+							sb.append(',');
 						}
 						sb.append(stringValueOf(l[j]));
 					}
-					sb.append("}");
+					sb.append('}');
 				}
 				else if (primitiveType == Long.TYPE) {
 					long[] l = (long[]) value;
 					sb.append("long[").append(l.length).append("]{");
 					for (int j = 0; j < l.length; j++) {
 						if (j > 0) {
-							sb.append(",");
+							sb.append(',');
 						}
 						sb.append(stringValueOf(l[j]));
 					}
-					sb.append("}");
+					sb.append('}');
 				}
 				else {
 					throw new RuntimeException("Please implement support for type " + primitiveType.getName() +
@@ -272,32 +272,32 @@ public abstract class AbstractExpressionTests {
 				if (!isNested) {
 					sb.append(value.getClass().getComponentType().getName());
 				}
-				sb.append("[").append(l.size()).append("]{");
+				sb.append('[').append(l.size()).append("]{");
 				int i = 0;
 				for (Object object : l) {
 					if (i > 0) {
-						sb.append(",");
+						sb.append(',');
 					}
 					i++;
 					sb.append(stringValueOf(object, true));
 				}
-				sb.append("}");
+				sb.append('}');
 			}
 			else {
 				List<Object> l = Arrays.asList((Object[]) value);
 				if (!isNested) {
 					sb.append(value.getClass().getComponentType().getName());
 				}
-				sb.append("[").append(l.size()).append("]{");
+				sb.append('[').append(l.size()).append("]{");
 				int i = 0;
 				for (Object object : l) {
 					if (i > 0) {
-						sb.append(",");
+						sb.append(',');
 					}
 					i++;
 					sb.append(stringValueOf(object));
 				}
-				sb.append("}");
+				sb.append('}');
 			}
 			return sb.toString();
 		}

spring-expression/src/test/java/org/springframework/expression/spel/SpelCompilationCoverageTests.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.
@@ -5111,21 +5111,21 @@ public class SpelCompilationCoverageTests extends AbstractExpressionTests {
 			List<?> ls = (List<?>) object;
 			for (Object l: ls) {
 				s.append(l);
-				s.append(" ");
+				s.append(' ');
 			}
 		}
 		else if (object instanceof Object[]) {
 			Object[] os = (Object[]) object;
 			for (Object o: os) {
 				s.append(o);
-				s.append(" ");
+				s.append(' ');
 			}
 		}
 		else if (object instanceof int[]) {
 			int[] is = (int[]) object;
 			for (int i: is) {
 				s.append(i);
-				s.append(" ");
+				s.append(' ');
 			}
 		}
 		else {
@@ -5931,9 +5931,9 @@ public class SpelCompilationCoverageTests extends AbstractExpressionTests {
 		public Obj3(String s, Float f, int... ints) {
 			StringBuilder b = new StringBuilder();
 			b.append(s);
-			b.append(":");
+			b.append(':');
 			b.append(Float.toString(f));
-			b.append(":");
+			b.append(':');
 			for (int param: ints) {
 				b.append(Integer.toString(param));
 			}