How many characters can a String have?

Karol Dowbecki · December 2, 2022

Since String.length() method returns an int we could guess that the maximum length would be Integer.MAX_VALUE characters. That’s not correct. Let’s forget about Unicode for now, and try to create the longest possible string by repeating a lowercase letter a.

String text = "a".repeat(Integer.MAX_VALUE);
java.lang.OutOfMemoryError: Requested array size exceeds VM limit
  at java.base/java.lang.String.repeat(

Let’s try to understand what happened and why the code complied but threw an error in runtime.

How is the String implemented

Since Java 9 and JEP 254: Compact Strings the String class is internally storing the characters in a byte[] array. The stack trace from OutOfMemoryError points to line, which in Java 17 source code is an array creation expression:

final byte[] single = new byte[count];

As per Java Language Specification, Java SE 17 Edition, Chapter 10

The variables contained in an array have no names; instead they are referenced by array access expressions that use non-negative integer index values.

The language specification doesn’t prohibit the Integer.MAX_VALUE array index so the compiler doesn’t complain if we try to allocate new byte[Integer.MAX_VALUE]. However, we will get the familiar OutOfMemoryError: Requested array size exceeds VM limit error in runtime.

This is not a new behaviour. The ancient java.util.Hastable class present since Java 1.0 mentions it.

 * The maximum size of array to allocate.
 * Some VMs reserve some header words in an array.
 * Attempts to allocate larger arrays may result in
 * OutOfMemoryError: Requested array size exceeds VM limit
private static final int MAX_ARRAY_SIZE = Integer.MAX_VALUE - 8;

How is the array creation implemented

The maximum array length limitation is coming from the JVM implementation. In OpenJDK 17 source code it will be arrayOop.hpp max_array_length(BasicType type) method which performs the below calculation.

const size_t max_element_words_per_size_t =
  align_down((SIZE_MAX/HeapWordSize - header_size(type)), MinObjAlignment);
const size_t max_elements_per_size_t =
  HeapWordSize * max_element_words_per_size_t / type2aelembytes(type);
if ((size_t)max_jint < max_elements_per_size_t) {
  // It should be ok to return max_jint here, but parts of the code
  // (CollectedHeap, Klass::oop_oop_iterate(), and more) uses an int for
  // passing around the size (in words) of an object. So, we need to avoid
  // overflowing an int when we add the header. See CRs 4718400 and 7110613.
  return align_down(max_jint - header_size(type), MinObjAlignment);
return (int32_t)max_elements_per_size_t;

After going through OpenJDK code it looks like MinObjAlignment and other values here will depend on the CPU architecture. If so, there won’t be just a single answer.

On Linux x86_64 debugging with gdb shows that the expression align_down(max_jint - header_size(type), MinObjAlignment) is executed and the method returns 2147483645. This value is equal to Integer.MAX_VALUE - 2. Let’s try to create the longest possible string again:

String text = "a".repeat(Integer.MAX_VALUE - 2);

This time the code doesn’t throw any errors, confirming that for Java 17 running on Linux x86_64 a string can have up to Integer.MAX_VALUE - 2 characters.

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