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).

I believe they can be up to 2^31-1 characters, as they are held by an internal array, and arrays are indexed by integers in Java.

It's not that 509 characters is the limit for a string, it's the minimum required for ANSI compatibility, as explained here.

I think that the makers of the standard pulled the number 509 out of their ass, but unless we get some official documentation from this, there is no way for us to know.

As far as how many characters can actually be in a string literal, that is compiler-dependent.

Here are some examples:

• MSVC: 2048
• GCC: No Limit (up to 100,000 characters), but gives warning after 510 characters:

  String literal of length 100000 exceeds maximum length 509 that C90 compilers are required to support

Based on my highly scientific and accurate experiment</sarcasm>, it tops out on my machine well before 1,000,000,000 characters.

After a few hours, I've given up. Final results: It can go a lot bigger than 100,000,000 characters, instantly given System.OutOfMemoryException at 1,000,000,000 characters.

using System;
using System.Collections.Generic;

public class MyClass
{
    public static void Main()
    {
        int i = 100000000;
        try
        {
            for (i = i; i <= int.MaxValue; i += 5000)
            {
                string value = new string('x', i);
                //WL(i);
            }
        }
        catch (Exception exc)
        {
            WL(i);
            WL(exc);
        }
        WL(i);
        RL();
    }

    #region Helper methods

    private static void WL(object text, params object[] args)
    {
        Console.WriteLine(text.ToString(), args);
    }

    private static void RL()
    {
        Console.ReadLine();
    }

    private static void Break()
    {
        System.Diagnostics.Debugger.Break();
    }

    #endregion
}

I ran this code on an x2iedn.16xlarge EC2 instance, which has 2048 GiB (2.2 TB) of RAM

>>> one_gigabyte = 1_000_000_000
>>> my_str = 'A' * (2000 * one_gigabyte)

It took a couple minutes but I was able to allocate a 2TB string on Python 3.10 running on Ubuntu 22.04.

>>> import sys
>>> sys.getsizeof(my_str)
2000000000049
>>> my_str
'AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA...

The last line actually hangs, but it would print 2 trillion As.