[PATCH] Optimization of AbstractStringBuilder.ensureCapacityInternal()

[Roman Leventov]

AbstractStringBuilder.ensureCapacityInternal() doesn't need to copy the
whole underlying array, only the part until the current count.

diff --git
a/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
b/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
--- a/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
+++ b/src/java.base/share/classes/java/lang/AbstractStringBuilder.java
@@ -140,11 +140,12 @@
      * overflow, this method throws {@code OutOfMemoryError}.
      */
     private void ensureCapacityInternal(int minimumCapacity) {
+        byte[] oldValue = value;
         // overflow-conscious code
-        int oldCapacity = value.length >> coder;
+        int oldCapacity = oldValue.length >> coder;
         if (minimumCapacity - oldCapacity > 0) {
-            value = Arrays.copyOf(value,
-                    newCapacity(minimumCapacity) << coder);
+            value = new byte[newCapacity(minimumCapacity) << coder];
+            System.arraycopy(oldValue, 0, value, 0, count << coder);
         }
     }