diff --git a/runtime/Java/src/org/antlr/v4/runtime/RuleContext.java b/runtime/Java/src/org/antlr/v4/runtime/RuleContext.java
index c6eaa8ffa..b642313f3 100644
--- a/runtime/Java/src/org/antlr/v4/runtime/RuleContext.java
+++ b/runtime/Java/src/org/antlr/v4/runtime/RuleContext.java
@@ -351,7 +351,7 @@ public class RuleContext implements ParseTree.RuleNode {
 	}
 
 	public String toString(BaseRecognizer recog, RuleContext stop) {
-		StringBuffer buf = new StringBuffer();
+		StringBuilder buf = new StringBuilder();
 		RuleContext p = this;
 		buf.append("[");
 		while ( p != null && p != stop ) {