How to Efficiently Concatenate Two Strings in Java?

What is the most efficient method to concatenate two strings in Java, especially in performance-critical scenarios?

String ccyPair = ccy1 + ccy2;

In Java, string concatenation can impact performance significantly, especially in tight loops or when processing large volumes of data. Below, we explore various methods to concatenate strings effectively, with a focus on performance optimization using StringBuilder.

StringBuilder sb = new StringBuilder(ccy1.length() + ccy2.length());
sb.append(ccy1);
sb.append(ccy2);
String ccyPair = sb.toString();

Causes

• Using the + operator results in the creation of multiple temporary String objects due to immutability of Strings.
• In tight loops, repeated concatenation can severely degrade performance because of repetitive memory allocation and garbage collection.

Solutions

• Use StringBuilder for concatenation within loops to improve performance by appending to a mutable sequence of characters.
• Consider using String.concat() or String.join() for specific use cases, although they may not outperform StringBuilder in high-frequency scenarios.
• If concatenation is done a fixed number of times, consider pre-sizing the StringBuilder to avoid resizing overhead.

Mistake: Using the + operator in performance-sensitive areas without considering the overhead.

Solution: Switch to StringBuilder when performing multiple concatenations or operations in a loop.

Mistake: Not initializing StringBuilder with initial capacity based on expected string sizes.

Solution: Determine an approximate size for the resulting string and initialize StringBuilder accordingly.