Use StringJoiner where possible to simplify String joining

spring-messaging/src/main/java/org/springframework/messaging/handler/AbstractMessageCondition.java

@@ -18,6 +18,7 @@ package org.springframework.messaging.handler;
 
 import java.util.Collection;
 import java.util.Iterator;
+import java.util.StringJoiner;
 
 import org.springframework.lang.Nullable;
 
@@ -51,16 +52,11 @@ public abstract class AbstractMessageCondition<T extends AbstractMessageConditio
 
 	@Override
 	public String toString() {
-		StringBuilder builder = new StringBuilder("[");
-		for (Iterator<?> iterator = getContent().iterator(); iterator.hasNext();) {
-			Object expression = iterator.next();
-			builder.append(expression.toString());
-			if (iterator.hasNext()) {
-				builder.append(getToStringInfix());
-			}
+		StringJoiner joiner = new StringJoiner(getToStringInfix(), "[", "]");
+		for (Object expression : getContent()) {
+			joiner.add(expression.toString());
 		}
-		builder.append("]");
-		return builder.toString();
+		return joiner.toString();
 	}