stanfordnlp/CoreNLP · error · IllegalArgumentException

Need an even number of arguments but there were " +…

Error message

Need an even number of arguments but there were " + args.length

What it means

PropertiesUtils.asProperties(String... args) builds a Properties object from alternating key/value strings, so it requires an even number of arguments. If an odd count is passed it throws IllegalArgumentException immediately, since the last key would have no value. This is a fail-fast guard on command-line style property arguments.

Solutions

1. Count and pair your arguments; add the missing value for the last key
2. Quote argument values containing spaces so they stay a single token
3. Validate args.length % 2 == 0 in your main before calling asProperties
4. Print usage/help when the argument count is wrong

Example fix

// before
Properties p = PropertiesUtils.asProperties("-filelist"); // odd
// after
Properties p = PropertiesUtils.asProperties("-filelist", "input.txt");

if (args.length % 2 != 0) {
  throw new IllegalArgumentException("Expected key/value pairs; got " + args.length
      + " args (dangling key: " + args[args.length - 1] + ")");
}

try {
  Properties p = PropertiesUtils.asProperties(args);
} catch (IllegalArgumentException e) {
  if (e.getMessage().startsWith("Need an even number")) {
    printUsage("Each key needs a value; missing value for last argument.");
    System.exit(2);
  }
  throw e;
}

When it happens

Trigger: Calling PropertiesUtils.asProperties("key1", "val1", "key2") — any invocation where args.length is odd, typically from a main() passing through raw command-line arguments with a missing value.

Common situations: User forgets a value on the command line (e.g. '-output' with no filename); a value containing spaces was not quoted so it shifted the argument positions; shell stripping an empty quoted argument.

Understand the failure class

Background: "missing required argument" and "the following required arguments were not provided": what required-argument errors mean and how to fix them — this error's family across 20 libraries.

Related errors

• Unknown properties: " + names
• Unknown property: " + names.iterator().next()
• args: treebankPath trainNums testNums
• argsToProperties could not read properties file: " + file
• Array lengths don't match

AI-assisted analysis of stanfordnlp/CoreNLP@1b7edd19c4 (2026-09-10). Data as JSON: /api/errors/4415b3fb042432fb. Report an issue: GitHub.

Appendix: source

Thrown at src/edu/stanford/nlp/util/PropertiesUtils.java:54

    return ! (value.equals("false") || value.equals("no") || value.equals("off"));
  }

  public static boolean hasPropertyPrefix(Properties props, String prefix) {
    for (Object o : props.keySet()) {
      if (o instanceof String && ((String) o).startsWith(prefix)) return true;
    }
    return false;
  }

  /** Create a Properties object from the passed in String arguments.
   *  The odd numbered arguments are the names of keys, and the even
   *  numbered arguments are the value of the preceding key
   *
   *  @param args An even-length list of alternately key and value
   */
  public static Properties asProperties(String... args) {
    if (args.length % 2 != 0) {
      throw new IllegalArgumentException("Need an even number of arguments but there were " + args.length);
    }
    Properties properties = new Properties();
    for (int i = 0; i < args.length; i += 2) {
      properties.setProperty(args[i], args[i + 1]);
    }
    return properties;
  }

  /** Convert from Properties to String. */
  public static String asString(Properties props) {
    try {
      StringWriter sw = new StringWriter();
      props.store(sw, null);
      return sw.toString();
    } catch (IOException ex) {
      throw new RuntimeException(ex);
    }
  }

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