EXP00-J. Do not ignore values returned by methods

Methods can return values to communicate failure or success or to update local objects or fields. Security risks can arise when method return values are ignored or when the invoking method fails to take suitable action. Consequently, programs must not ignore method return values.

When getter methods are named after an action, a programmer could fail to realize that a return value is expected. For example, the only purpose of the ProcessBuilder.redirectErrorStream() method is to report via return value whether the process builder successfully merges standard error and standard output. The method that actually performs redirection of the error stream is the overloaded single-argument version.

Noncompliant Code Example (File Deletion)

This noncompliant code example attempts to delete a file but fails to check whether the operation has succeeded.

public void deleteFile(){

  File someFile = new File("someFileName.txt");
  // do something with someFile
  someFile.delete();

}

Compliant Solution

This compliant solution checks the (boolean) value returned by the delete() method and handles any resulting errors.

public void deleteFile(){

  File someFile = new File("someFileName.txt");
  // do something with someFile
  if (!someFile.delete()) {
    // handle failure to delete the file
  }

}

Noncompliant Code Example (String Replacement)

This noncompliant code example ignores the return value of the String.replace() method, failing to update the original string. The String.replace() method cannot modify the state of the String (because String objects are immutable); rather, it returns a reference to a new String object containing the desired result.

public class Ignore {
  public static void main(String[] args) {
    String original = "insecure";
    original.replace( 'i', '9' );
    System.out.println(original);
  }
}

It is especially important to process the return values of immutable object methods. While many methods of mutable objects operate by changing some internal state of the object, methods of immutable objects can't change the object and often will return a new object with some mutation, leaving the original object unchanged.

Compliant Solution

This compliant solution correctly updates the String reference original with the return value from the String.replace() method.

public class DoNotIgnore {
  public static void main(String[] args) {
    String original = "insecure";
    original = original.replace( 'i', '9' );
    System.out.println(original);
  }
}

Risk Assessment

Ignoring method return values can lead to unexpected program behavior.

Automated Detection

The Coverity Prevent Version 5.0 CHECKED_RETURN checker can detect the instance where the value returned from a function is not checked for errors before being used.

Related Guidelines

Bibliography

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