When Does Java Use StringBuilder for String Concatenation? A Deep Dive

Introduction

Recently I wondered about Java string concatenation behavior and created a demo to see when the compiler uses StringBuilder and when it optimizes the code.

1. Problem

Case 1

Comparing str == str2 yields false .

Case 2

Here the comparison returns true .

2. Investigation

Why do the results differ? By running javap -c TestDemo.class to decompile the bytecode, the underlying cause becomes clear.

In Case 1 the bytecode shows that Java creates a StringBuilder, appends the strings, and then calls toString(). The resulting object resides at a different memory address from the constant‑pool string "111", so == is false.

In Case 2 the compiler folds the expression "111" + "" into the constant "111" at compile time. Both variables therefore reference the same interned string, making == true.

3. Summary

1. When a variable participates in concatenation, the compiler cannot optimize; it must use StringBuilder, producing a new object whose reference differs from the constant‑pool string.

2. When only constant strings are concatenated, the compiler performs constant‑folding, and the resulting string is taken from the constant pool, so reference equality holds.