Use StringJoiner where possible to simplify String joining

Closes gh-23237

spring-core/src/main/java/org/springframework/core/ResolvableType.java

@@ -30,6 +30,7 @@ import java.util.Arrays;
 import java.util.Collection;
 import java.util.IdentityHashMap;
 import java.util.Map;
+import java.util.StringJoiner;
 
 import org.springframework.core.SerializableTypeWrapper.FieldTypeProvider;
 import org.springframework.core.SerializableTypeWrapper.MethodParameterTypeProvider;
@@ -1518,18 +1519,15 @@ public class ResolvableType implements Serializable {
 
 		@Override
 		public String getTypeName() {
-			StringBuilder result = new StringBuilder(this.rawType.getTypeName());
+			String typeName = this.rawType.getTypeName();
 			if (this.typeArguments.length > 0) {
-				result.append('<');
-				for (int i = 0; i < this.typeArguments.length; i++) {
-					if (i > 0) {
-						result.append(", ");
-					}
-					result.append(this.typeArguments[i].getTypeName());
+				StringJoiner result = new StringJoiner(", ", "<", ">");
+				for (Type argument : this.typeArguments) {
+					result.add(argument.getTypeName());
 				}
-				result.append('>');
+				return typeName + result.toString();
 			}
-			return result.toString();
+			return typeName;
 		}
 
 		@Override

spring-core/src/main/java/org/springframework/util/ClassUtils.java

@@ -911,14 +911,10 @@ public abstract class ClassUtils {
 		}
 		Class<?> clazz = value.getClass();
 		if (Proxy.isProxyClass(clazz)) {
-			StringBuilder result = new StringBuilder(clazz.getName());
-			result.append(" implementing ");
-			Class<?>[] ifcs = clazz.getInterfaces();
-			for (int i = 0; i < ifcs.length; i++) {
-				result.append(ifcs[i].getName());
-				if (i < ifcs.length - 1) {
-					result.append(',');
-				}
+			String prefix = clazz.getName() + " implementing ";
+			StringJoiner result = new StringJoiner(",", prefix, "");
+			for (Class<?> ifc : clazz.getInterfaces()) {
+				result.add(ifc.getName());
 			}
 			return result.toString();
 		}