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8364317: Explicitly document some assumptions of StringUTF16 #26541

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8364317: Explicitly document some assumptions of StringUTF16 #26541

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933557e
Miscellaneous docs and checks for UTF16
liach Jul 28, 2025
1b052e9
Add paragraph for endianness and layout
liach Jul 30, 2025
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12 changes: 12 additions & 0 deletions src/java.base/share/classes/java/lang/StringUTF16.java
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Original file line number Diff line number Diff line change
Expand Up @@ -40,6 +40,15 @@

import static java.lang.String.UTF16;

/// UTF16 String operations.
///
/// UTF16 byte arrays have the identical layout as char arrays. They share the
/// same base offset and scale, and for each two-byte unit interpreted as a char,
/// it has the same endianness as a char, which is the platform endianness.
/// This is ensured in the static initializer of StringUTF16.
///
/// All indices and sizes for byte arrays carrying UTF16 data are in number of
/// chars instead of number of bytes.
final class StringUTF16 {

// Return a new byte array for a UTF16-coded string for len chars
Expand Down Expand Up @@ -1635,6 +1644,9 @@ public static int lastIndexOfLatin1(byte[] src, int srcCount,
private static final int HI_BYTE_SHIFT;
private static final int LO_BYTE_SHIFT;
static {
// Assumptions for StringUTF16 operations. Present in `LibraryCallKit::inline_string_char_access` too.
assert Unsafe.ARRAY_CHAR_BASE_OFFSET == Unsafe.ARRAY_BYTE_BASE_OFFSET : "sanity: byte[] and char[] bases agree";
assert Unsafe.ARRAY_CHAR_INDEX_SCALE == Unsafe.ARRAY_BYTE_INDEX_SCALE * 2 : "sanity: byte[] and char[] scales agree";
if (Unsafe.getUnsafe().isBigEndian()) {
HI_BYTE_SHIFT = 8;
LO_BYTE_SHIFT = 0;
Expand Down