Understanding Java String Length Limits: JVM Specification and Practical Constraints

Java strings are internally stored as a char[] array, and the length() method returns an int , so the theoretical maximum size is limited by the maximum value of an integer (2^31‑1, about 4 GB).

However, when a string literal is compiled, the JVM places it in the class file constant pool, where the CONSTANT_String entry uses a u2 index. A u2 can represent values up to 2^16‑1 (65535), and one byte is needed for the terminating instruction, leaving an effective compile‑time limit of 65534 characters.

The article demonstrates this limit with a small experiment: constructing a 65534‑character literal compiles successfully, while a longer literal (e.g., 100 000 characters) triggers a compilation error.

At runtime, strings can be built dynamically (e.g., via concatenation or reading data) and can grow up to the int maximum, bypassing the constant‑pool restriction.

Key points include:

The array backing a String is subject to the integer range, giving a theoretical 4 GB limit.

The JVM class‑file format restricts literal strings to 65534 characters because of the u2 index size.

Runtime‑generated strings are only limited by the int range.

int[] arr1 = new int[10]; // define an array of length 10
int[] arr2 = {1,2,3,4,5}; // array length is 5

Understanding these constraints helps answer interview questions about String length limits and explains why a seemingly small literal can cause compilation failures.