Discarded Read Result

ID

java.read_readline_return_values

Severity

high

Remediation Complexity

trivial

Remediation Risk

low

Remediation Effort

low

Resource

Reliability

Language

Java

Tags

reliability

Description

Reports a while (reader.readLine() != null) loop — one driven by BufferedReader.readLine() whose return value is never captured, so every line the loop reads is discarded.

Reader.read()/InputStream.read() compared against -1 is deliberately out of scope: while (in.read() != -1) {} is the ordinary drain idiom, and consuming a byte to make progress is the point of the call rather than a lost result.

Rationale

read() and readLine() return the data they just consumed from the stream — a character/byte, or a line of text — alongside the end-of-stream signal. Comparing the call directly against the sentinel, as in while (reader.readLine() != null), uses the call only to detect end-of-stream: the line or byte it returned is never stored anywhere and is gone the moment the comparison finishes.

// Bad -- every line read is immediately discarded
while (reader.readLine() != null) {
    // nothing here has access to what was just read
}

Remediation

Assign the call’s result to a variable as part of the comparison, and use that variable inside the loop or branch.

// Good -- the line is captured before the comparison consumes it
String line;
while ((line = reader.readLine()) != null) {
    System.out.println(line);
}

References

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