8337168: Optimize LocalDateTime.toString

Reviewed-by: liach, naoto

src/java.base/share/classes/java/time/LocalDate.java

@@ -2147,11 +2147,20 @@ public final class LocalDate
      */
     @Override
     public String toString() {
+        var buf = new StringBuilder(10);
+        formatTo(buf);
+        return buf.toString();
+    }
+
+    /**
+     * Prints the toString result to the given buf, avoiding extra string allocations.
+     * Requires extra capacity of 10 to avoid StringBuilder reallocation.
+     */
+    void formatTo(StringBuilder buf) {
         int yearValue = year;
         int monthValue = month;
         int dayValue = day;
         int absYear = Math.abs(yearValue);
-        StringBuilder buf = new StringBuilder(10);
         if (absYear < 1000) {
             if (yearValue < 0) {
                 buf.append('-');
@@ -2164,11 +2173,10 @@ public final class LocalDate
             }
             buf.append(yearValue);
         }
-        return buf.append(monthValue < 10 ? "-0" : "-")
+        buf.append(monthValue < 10 ? "-0" : "-")
            .append(monthValue)
            .append(dayValue < 10 ? "-0" : "-")
-            .append(dayValue)
+           .append(dayValue);
-            .toString();
     }
 
     //-----------------------------------------------------------------------

src/java.base/share/classes/java/time/LocalDateTime.java

@@ -1,5 +1,5 @@
 /*
- * Copyright (c) 2012, 2023, Oracle and/or its affiliates. All rights reserved.
+ * Copyright (c) 2012, 2024, Oracle and/or its affiliates. All rights reserved.
  * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
  *
  * This code is free software; you can redistribute it and/or modify it
@@ -1965,7 +1965,11 @@ public final class LocalDateTime
      */
     @Override
     public String toString() {
-        return date.toString() + 'T' + time.toString();
+        var buf = new StringBuilder(29);
+        date.formatTo(buf);
+        buf.append('T');
+        time.formatTo(buf);
+        return buf.toString();
     }
 
     //-----------------------------------------------------------------------

src/java.base/share/classes/java/time/LocalTime.java

@@ -1631,7 +1631,16 @@ public final class LocalTime
      */
     @Override
     public String toString() {
-        StringBuilder buf = new StringBuilder(18);
+        var buf = new StringBuilder(18);
+        formatTo(buf);
+        return buf.toString();
+    }
+
+    /**
+     * Prints the toString result to the given buf, avoiding extra string allocations.
+     * Requires extra capacity of 18 to avoid StringBuilder reallocation.
+     */
+    void formatTo(StringBuilder buf) {
         int hourValue = hour;
         int minuteValue = minute;
         int secondValue = second;
@@ -1657,7 +1666,6 @@ public final class LocalTime
                 buf.append(digits);
             }
         }
-        return buf.toString();
     }
 
     //-----------------------------------------------------------------------