CodeQL documentation

Character passed to StringBuffer or StringBuilder constructor

ID: java/string-buffer-char-init
Kind: problem
Severity: error
Precision: very-high
   - reliability
   - maintainability
Query suites:
   - java-security-and-quality.qls

Click to see the query in the CodeQL repository

Passing a character to the constructor of StringBuffer or StringBuilder is probably intended to insert the character into the newly created buffer. In fact, however, the character value is converted to an integer and interpreted as the buffer’s initial capacity, which may yield unexpected results.


The following example shows a class representing points in two-dimensional Cartesian coordinates. The toString method uses a StringBuffer to construct a human-readable representation of the form (x, y), where x and y are the point’s coordinates.

However, the opening parenthesis is passed to the StringBuffer constructor as character literal. Instead of being used to initialise the buffer’s contents, the character is converted to the integer value 40 and interpreted as the buffer’s initial capacity. Thus, the string representation returned by toString will be missing the opening parenthesis. (Note that passing a character to append, on the other hand, is unproblematic.)

class Point {
	private double x, y;
	public Point(double x, double y) {
		this.x = x;
		this.y = y;
	public String toString() {
		StringBuffer res = new StringBuffer('(');
		res.append(", ");
		return res.toString();


If the character used to initialize the buffer is a character literal, simply replace it with the corresponding string literal. So, in our example, replace new StringBuffer('(') with new StringBuffer("("). If the character is not a literal value, use method String.valueOf to convert it to a string.