JDK-8302323 Add repeat methods to StringBuilder/StringBuffer

src/java.base/share/classes/java/lang/AbstractStringBuilder.java

@@ -29,6 +29,7 @@
 import jdk.internal.math.FloatToDecimal;
 
 import java.io.IOException;
+import java.nio.CharBuffer;
 import java.util.Arrays;
 import java.util.Spliterator;
 import java.util.stream.IntStream;
@@ -1821,4 +1822,115 @@ private final void appendChars(CharSequence s, int off, int end) {
         }
         count += end - off;
     }
+
+    private AbstractStringBuilder repeat(char c, int count) {
+        int limit = this.count + count;
+        ensureCapacityInternal(limit);
+        boolean isLatin1 = isLatin1();
+        if (isLatin1 && StringLatin1.canEncode(c)) {
+            Arrays.fill(value, this.count, limit, (byte)c);
+        } else {
+            if (isLatin1) {
+                inflate();
+            }
+            for (int index = this.count; index < limit; index++) {
+                StringUTF16.putCharSB(value, index, c);
+            }
+        }
+        this.count = limit;
+        return this;
+    }
+
+    /**
+     * Repeats {@code count} copies of the string representation of the
+     * {@code codePoint} argument to this sequence.
+     * <p>
+     * The length of this sequence increases by {@code count} times the
+     * string representation length.
+     * <p>
+     * It is usual to use {@code char} expressions for code points. For example:
+     * {@snippet lang="java":
+     * // insert 10 asterisks into the buffer
+     * sb.repeat('*', 10);
+     * }
+     *
+     * @param codePoint  code point to append
+     * @param count      number of times to copy
+     *
+     * @return  a reference to this object.
+     *
+     * @throws IllegalArgumentException if the specified {@code codePoint}
+     * is not a valid Unicode code point or if {@code count} is negative.
+     *
+     * @since 21
+     */
+    public AbstractStringBuilder repeat(int codePoint, int count) {
+        if (count < 0) {
+            throw new IllegalArgumentException("count is negative: " + count);
+        } else if (count == 0) {
+            return this;
+        }
+        if (Character.isBmpCodePoint(codePoint)) {
+            repeat((char)codePoint, count);
+        } else {
+            repeat(CharBuffer.wrap(Character.toChars(codePoint)), count);
+        }
+        return this;
+    }
+
+    /**
+     * Appends {@code count} copies of the specified {@code CharSequence} {@code cs}
+     * to this sequence.
+     * <p>
+     * The length of this sequence increases by {@code count} times the
+     * {@code CharSequence} length.
+     * <p>
+     * If {@code cs} is {@code null}, then the four characters
+     * {@code "null"} are repeated into this sequence.
+     *
+     * @param cs     a {@code CharSequence}
+     * @param count  number of times to copy
+     *
+     * @return  a reference to this object.
+     *
+     * @throws IllegalArgumentException  if {@code count} is negative
+     *
+     * @since 21
+     */
+    public AbstractStringBuilder repeat(CharSequence cs, int count) {
+        if (count < 0) {
+            throw new IllegalArgumentException("count is negative: " + count);
+        } else if (count == 0) {
+            return this;
+        } else if (count == 1) {
+            return append(cs);
+        }
+        if (cs == null) {
+            cs = "null";
+        }
+        int length = cs.length();
+        if (length == 0) {
+            return this;
+        } else if (length == 1) {
+            return repeat(cs.charAt(0), count);
+        }
+        int offset = this.count;
+        int valueLength = length << coder;
+        if ((Integer.MAX_VALUE - offset) / count < valueLength) {
+            throw new OutOfMemoryError("Required length exceeds implementation limit");
+        }
+        int total = count * length;
+        int limit = offset + total;
+        ensureCapacityInternal(limit);
+        if (cs instanceof String str) {
+            putStringAt(offset, str);
+        } else if (cs instanceof AbstractStringBuilder asb) {
+            append(asb);
+        } else {
+            appendChars(cs, 0, length);
+        }
+        String.repeatCopyRest(value, offset << coder, total << coder, length << coder);
+        this.count = limit;
+        return this;
+    }
 }

src/java.base/share/classes/java/lang/String.java

@@ -4522,12 +4522,34 @@ public String repeat(int count) {
         final int limit = len * count;
         final byte[] multiple = new byte[limit];
         System.arraycopy(value, 0, multiple, 0, len);
-        int copied = len;
+        repeatCopyRest(multiple, 0, limit, len);
+        return new String(multiple, coder);
+    }
+
+    /**
+     * Used to perform copying after the initial insertion. Copying is optimized
+     * by using power of two duplication. First pass duplicates original copy,
+     * second pass then duplicates the original and the copy yielding four copies,
+     * third pass duplicates four copies yielding eight copies, and so on.
+     * Finally, the remainder is filled in with prior copies.
+     *
+     * @implNote The technique used here is significantly faster than hand-rolled
+     * loops or special casing small numbers due to the intensive optimization
+     * done by intrinsic {@code System.arraycopy}.
+     *
+     * @param buffer    destination buffer
+     * @param offset    offset in the destination buffer
+     * @param limit     total replicated including what is already in the buffer
+     * @param copied    number of bytes that have already in the buffer
+     */
+    static void repeatCopyRest(byte[] buffer, int offset, int limit, int copied) {
+        // Initial copy is in the buffer.
         for (; copied < limit - copied; copied <<= 1) {
-            System.arraycopy(multiple, 0, multiple, copied, copied);
+            // Power of two duplicate.
+            System.arraycopy(buffer, offset, buffer, offset + copied, copied);
         }
-        System.arraycopy(multiple, 0, multiple, copied, limit - copied);
-        return new String(multiple, coder);
+        // Duplicate remainder.
+        System.arraycopy(buffer, offset, buffer, offset + copied, limit - copied);
     }
 
     ////////////////////////////////////////////////////////////////

src/java.base/share/classes/java/lang/StringBuffer.java

@@ -708,6 +708,28 @@ public synchronized StringBuffer reverse() {
         return this;
     }
 
+    /**
+     * @throws IllegalArgumentException {@inheritDoc}
+     *
+     * @since 21
+     */
+    @Override
+    public synchronized StringBuffer repeat(int codePoint, int count) {
+        super.repeat(codePoint, count);
+        return this;
+    }
+
+    /**
+     * @throws IllegalArgumentException {@inheritDoc}
+     *
+     * @since 21
+     */
+    @Override
+    public synchronized StringBuffer repeat(CharSequence cs, int count) {
+        super.repeat(cs, count);
+        return this;
+    }
+
     @Override
     @IntrinsicCandidate
     public synchronized String toString() {

src/java.base/share/classes/java/lang/StringBuilder.java

@@ -446,6 +446,28 @@ public StringBuilder reverse() {
         return this;
     }
 
+    /**
+     * @throws IllegalArgumentException {@inheritDoc}
+     *
+     * @since 21
+     */
+    @Override
+    public StringBuilder repeat(int codePoint, int count) {
+        super.repeat(codePoint, count);
+        return this;
+    }
+
+    /**
+     * @throws IllegalArgumentException {@inheritDoc}
+     *
+     * @since 21
+     */
+    @Override
+    public StringBuilder repeat(CharSequence cs, int count) {
+        super.repeat(cs, count);
+        return this;
+    }
+
     @Override
     @IntrinsicCandidate
     public String toString() {