Mastering Java String Concatenation: Techniques, Internals, and Performance

1. String Concatenation Overview

This article supplements Java string knowledge by focusing on string concatenation, based on JDK 1.8.0_181. It explains why concatenation creates new objects because String is immutable.

2. Using the + Operator

The simplest way to concatenate strings in Java is the + operator, which is syntactic sugar, not operator overloading. Internally the compiler rewrites + expressions to use StringBuilder.append.

Decompiled bytecode shows a new StringBuilder is created, the original String is converted, and append is called.

3. concat Method

The String.concat method creates a new character array whose length equals the sum of the two strings, copies both strings into it, and constructs a new String object.

4. StringBuilder and StringBuffer

Both classes encapsulate a mutable character array. StringBuilder is not synchronized, while StringBuffer adds synchronized methods for thread safety.

The internal append method copies characters directly into the array, expanding it when necessary.

For StringBuffer, append is declared synchronized, making it slower than StringBuilder.

5. StringUtils.join

The Apache StringUtils.join method also uses a StringBuilder internally to concatenate arrays or collections.

6. Performance Comparison

A benchmark comparing five concatenation techniques shows the order from fastest to slowest: StringBuilder, +, StringBuffer, concat, StringUtils.join. The + operator is much slower in loops because the compiler creates a new StringBuilder on each iteration.

7. Recommendations

• Use StringBuilder (or StringBuffer in concurrent scenarios) for concatenation inside loops. • Use the + operator for simple, non‑loop concatenations. • Prefer StringUtils.join when joining collections or arrays.

8. Conclusion

The article clarifies that although String is immutable, concatenation is achieved by creating new objects. Among the common methods, StringBuilder offers the best performance, especially in loops, while StringBuffer provides thread safety when needed.