How many characters can a Java String have?

Java Problem Overview

I'm trying The Next Palindrome problem from Sphere Online Judge (SPOJ) where I need to find a palindrome for a integer of up to a million digits. I thought about using Java's functions for reversing Strings, but would they allow for a String to be this long?

Java Solutions

Solution 1 - Java

You should be able to get a String of length

Integer.MAX_VALUE always 2,147,483,647 (2³¹ - 1)
(Defined by the Java specification, the maximum size of an array, which the String class uses for internal storage)
OR
Half your maximum heap size (since each character is two bytes) whichever is smaller.

Solution 2 - Java

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.

Solution 3 - Java

While you can in theory Integer.MAX_VALUE characters, the JVM is limited in the size of the array it can use.

public static void main(String... args) {
    for (int i = 0; i < 4; i++) {
        int len = Integer.MAX_VALUE - i;
        try {
            char[] ch = new char[len];
            System.out.println("len: " + len + " OK");
        } catch (Error e) {
            System.out.println("len: " + len + " " + e);
        }
    }
}

on Oracle Java 8 update 92 prints

len: 2147483647 java.lang.OutOfMemoryError: Requested array size exceeds VM limit
len: 2147483646 java.lang.OutOfMemoryError: Requested array size exceeds VM limit
len: 2147483645 OK
len: 2147483644 OK

Note: in Java 9, Strings will use byte[] which will mean that multi-byte characters will use more than one byte and reduce the maximum further. If you have all four byte code-points e.g. emojis, you will only get around 500 million characters

Solution 4 - Java

Have you considered using BigDecimal instead of String to hold your numbers?

Solution 5 - Java

Integer.MAX_VALUE is max size of string + depends of your memory size but the Problem on sphere's online judge you don't have to use those functions

Solution 6 - Java

Java9 uses byte[] to store String.value, so you can only get about 1GB Strings in Java9. Java8 on the other hand can have 2GB Strings.

By character I mean "char"s, some character is not representable in BMP(like some of the emojis), so it will take more(currently 2) chars.

Solution 7 - Java

The heap part gets worse, my friends. UTF-16 isn't guaranteed to be limited to 16 bits and can expand to 32

Content Type	Original Author	Original Content on Stackoverflow
Question	andandandand	View Question on Stackoverflow
Solution 1 - Java	Bill the Lizard	View Answer on Stackoverflow
Solution 2 - Java	aperkins	View Answer on Stackoverflow
Solution 3 - Java	Peter Lawrey	View Answer on Stackoverflow
Solution 4 - Java	Thorbjørn Ravn Andersen	View Answer on Stackoverflow
Solution 5 - Java	Mite Mitreski	View Answer on Stackoverflow
Solution 6 - Java	Revin	View Answer on Stackoverflow
Solution 7 - Java	Joe Plante	View Answer on Stackoverflow