String s Maximum length in Java - calling length method

Question

In Java  what is the maximum size a String object may have  referring to the length   method call   I know that length   return the size of a String as a char

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java io DataInput readUTF   and java io DataOutput writeUTF String  say that a String object is represented by two bytes of length information and the modified UTF-8 representation of every character in the string  This concludes that the length of String is limited by the number of bytes of the modified UTF-8 representation of the string when used with DataInput and DataOutput   In addition  The specification of CONSTANT Utf8 info found in the Java virtual machine specification defines the structure as follows   CONSTANT Utf8 info       u1 tag      u2 length      u1 bytes length       You can find that the size of  length  is two bytes   That the return type of a certain method  e g  String length    is int does not always mean that its allowed maximum value is Integer MAX VALUE  Instead  in most cases  int is chosen just for performance reasons  The Java language specification says that integers whose size is smaller than that of int are converted to int before calculation  if my memory serves me correctly  and it is one reason to choose int when there is no special reason   The maximum length at compilation time is at most 65536  Note again that the length is the number of bytes of the modified UTF-8 representation  not the number of characters in a String object   String objects may be able to have much more characters at runtime  However  if you want to use String objects with DataInput and DataOutput interfaces  it is better to avoid using too long String objects  I found this limitation when I implemented Objective-C equivalents of DataInput readUTF   and DataOutput writeUTF String

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The Return type of the length   method of the String class is int      public int length     Refer http   docs oracle com javase 7 docs api java lang String html length    So the maximum value of int is 2147483647   String is considered as char array internally So indexing is done within the maximum range  This means we cannot index the 2147483648th member So the maximum length of String in java is 2147483647   Primitive data type int is 4 bytes 32 bits  in java As 1 bit  MSB  is used as a sign bit The range is constrained within -2 31 to 2 31-1  -2147483648 to 2147483647   We cannot use negative values for indexing So obviously the range we can use is from 0 to 2147483647

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Since arrays must be indexed with integers  the maximum length of an array is Integer MAX INT  231-1  or 2 nbsp 147 nbsp 483 nbsp 647   This is assuming you have enough memory to hold an array of that size  of course

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Considering the String class  length method returns an int  the maximum length that would be returned by the method would be Integer MAX VALUE  which is 2 31 - 1  or approximately 2 billion    In terms of lengths and indexing of arrays   such as char    which is probably the way the internal data representation is implemented for Strings   Chapter 10  Arrays of The Java Language Specification  Java SE 7 Edition says the following      The variables contained in an array   have no names  instead they are   referenced by array access expressions   that use nonnegative integer index   values  These variables are called the   components of the array  If an array   has n components  we say n is the   length of the array  the components of   the array are referenced using integer   indices from 0 to n - 1  inclusive    Furthermore  the indexing must be by int values  as mentioned in Section 10 4      Arrays must be indexed by int values    Therefore  it appears that the limit is indeed 2 31 - 1  as that is the maximum value for a nonnegative int value   However  there probably are going to be other limitations  such as the maximum allocatable size for an array

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apparently it s bound to an int  which is 0x7FFFFFFF  2147483647

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I have a 2010 iMac with 8GB of RAM  running Eclipse Neon 2 Release  4 6 2  with Java 1 8 0 25  With the VM argument -Xmx6g  I ran the following code   StringBuilder sb   new StringBuilder    for  int i   0  i  lt  Integer MAX VALUE  i          try           sb append  a          catch  Throwable e            System out println i           break          System out println sb toString   length       This prints   Requested array size exceeds VM limit 1207959550   So  it seems that the max array size is  1 207 959 549  Then I realized that we don t actually care if Java runs out of memory  we re just looking for the maximum array size  which seems to be a constant defined somewhere   So   for  int i   0  i  lt  1 000  i          try           char   array   new char Integer MAX VALUE - i           Arrays fill array   a            String string   new String array           System out println string length           catch  Throwable e            System out println e getMessage             System out println  Last       Integer MAX VALUE - i            System out println  Last      i             Which prints   Requested array size exceeds VM limit Last  2147483647 Last  0 Requested array size exceeds VM limit Last  2147483646 Last  1 Java heap space Last  2147483645 Last  2   So  it seems the max is Integer MAX VALUE - 2  or  2 31  - 3  P S  I m not sure why my StringBuilder maxed out at 1207959550 while my char   maxed out at  2 31 -3  It seems that AbstractStringBuilder doubles the size of its internal char   to grow it  so that probably causes the issue

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As mentioned in Takahiko Kawasaki s answer  java represents Unicode strings in the form of modified UTF-8 and in JVM-Spec CONSTANT UTF8 info Structure  2 bytes are allocated to length  and not the no  of characters of String   To extend the answer  the ASM jvm bytecode library s putUTF8 method  contains this  public ByteVector putUTF8 final String stringValue        int charLength   stringValue length        if  charLength  gt  65535             If no  of characters gt  65535  than however UTF-8 encoded length  wont fit in 2 bytes        throw new IllegalArgumentException  quot UTF8 string too large quot              for  int i   0  i  lt  charLength    i          char charValue   stringValue charAt i         if  charValue  gt     u0001   amp  amp  charValue  lt     u007F                Unicode code-point encoding in utf-8 fits in 1 byte          currentData currentLength       byte  charValue          else              doesnt fit in 1 byte          length   currentLength          return encodeUtf8 stringValue  i  65535                            But when code-point mapping  gt  1byte  it calls encodeUTF8 method  final ByteVector encodeUtf8 final String stringValue  final int offset  final int maxByteLength     65535           int charLength   stringValue length        int byteLength   offset      for  int i   offset  i  lt  charLength    i          char charValue   stringValue charAt i         if  charValue  gt   0x0001  amp  amp  charValue  lt   0x007F            byteLength            else if  charValue  lt   0x07FF            byteLength    2          else           byteLength    3                          In this sense  the max string length is 65535 bytes  i e the utf-8 encoding length  and not char count  You can find the modified-Unicode code-point range of JVM  from the above utf8 struct link

[java] String's Maximum length in Java - calling length() method

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