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 [];

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.

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).

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.

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.