Why Does Java’s String Have a 65,534‑Character Limit? Deep Dive into JVM Specs

How String Is Stored

In Java a String is backed by a char[] array, so the characters are stored in a contiguous array of 16‑bit Unicode code units.

Theoretical Array Limit

The length of the backing array is an int, and Java’s int type ranges from -2^31 to 2^31‑1. Because array indices start at 0, the maximum number of elements a char[] can hold is 2^31‑1 (≈2 147 483 647), which corresponds to roughly 4 GB of character data.

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

JVM Constant‑Pool Limit

When a string literal is compiled, the JVM stores it in the class file’s constant pool. Each constant‑pool entry is identified by a u2 index, i.e., an unsigned 16‑bit value whose maximum is 2^16‑1 = 65535. The JVM also needs one extra byte for the terminating instruction, so the effective compile‑time limit for a literal string is 65534 characters.

Practical Experiment

To verify the limit, a loop was used to build a string of exactly 65 534 characters, its length was confirmed with a character‑count tool, and the literal was placed directly in source code. The code compiled and ran successfully, demonstrating that the compile‑time limit is indeed 65 534 characters.

Conclusion

In summary, a Java String can hold up to roughly 2 billion characters at runtime because of the int ‑based array limit, but when the string is expressed as a literal in source code the JVM imposes a stricter compile‑time ceiling of 65 534 characters due to the constant‑pool’s u2 index size.