String Concatenation in Loop

ID

java.string_concat_in_loop

Severity

low

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Efficiency

Language

Java

Tags

code-style

Description

Reports String concatenation using the + or += operator inside loop bodies. Each concatenation creates a new String object, leading to O(n^2) memory allocations.

Rationale

Java strings are immutable. Every + concatenation allocates a new String and copies the accumulated characters. Inside a loop, this turns a linear operation into a quadratic one.

// Bad — quadratic allocation
String result = "";
for (String item : items) {
    result += item;
}

Remediation

Use StringBuilder (or StringJoiner / String.join() when joining with a delimiter):

// Good — linear allocation
StringBuilder sb = new StringBuilder();
for (String item : items) {
    sb.append(item);
}
String result = sb.toString();

References