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

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 [];

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 < 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 < 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.

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 > 65535) {   
   // If no. of characters> 65535, than however UTF-8 encoded length, wont fit in 2 bytes.
      throw new IllegalArgumentException("UTF8 string too large");
    }
    for (int i = 0; i < charLength; ++i) {
      char charValue = stringValue.charAt(i);
      if (charValue >= '\u0001' && charValue <= '\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 > 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 < charLength; ++i) {
      char charValue = stringValue.charAt(i);
      if (charValue >= 0x0001 && charValue <= 0x007F) {
        byteLength++;
      } else if (charValue <= 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.

apparently it's bound to an int, which is 0x7FFFFFFF (2147483647).

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

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.

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).