ssssssss-team/spider-flow · error · IllegalArgumentException

Start must be <= end.

Error message

Start must be <= end.

What it means

Span represents a character range within an expression source, used for error positioning. Its start/end constructor validates bounds and throws IllegalArgumentException("Start must be <= end.") when start > end — an inverted range.

Solutions

1. Verify the start and end values passed to the Span constructor
2. Ensure end is set to a position found after start; if a terminator wasn't found, bail out before creating the Span
3. Clamp or swap: end = Math.max(start, end)
4. Prefer the single-argument Span(source) constructor for whole-source spans

Example fix

// before
Span span = new Span(source, start, endIdx); // endIdx may be -1
// after
if (endIdx < start) {
    throw new IllegalArgumentException("Terminator not found after position " + start);
}
Span span = new Span(source, start, endIdx);

if (start > end) throw new IllegalArgumentException("Span start (" + start + ") must be <= end (" + end + ")");

try {
    Span span = new Span(source, start, end);
} catch (IllegalArgumentException e) {
    logger.error("Invalid span range: {}..{}", start, end);
}

When it happens

Trigger: new Span(source, start, end) with start > end, typically from failed matches producing end markers before start markers or from swapped variables.

Common situations: Custom expression parsers building error spans from scan results where the terminator was not found (end left at -1 or below start); refactors swapping start/end.

Understand the failure class

Background: "Must be a positive integer", "Invalid value", "Unsupported": the invalid-argument-value error family, when a library rejects the value you pass — this error's family across 35 libraries.

Related errors

• Start must be <= end.
• Start must be >= 0.
• Start outside of string.
• End outside of string.
• Start must be >= 0.

AI-assisted analysis of ssssssss-team/spider-flow@c799cca99c (2026-09-08). Data as JSON: /api/errors/a4b8d5c450a2d244. Report an issue: GitHub.

Appendix: source

Thrown at spider-flow-core/src/main/java/org/spiderflow/core/expression/parsing/Span.java:23

public class Span {
	/** the source string this span refers to **/
	private final String source;

	/** start index in source string, starting at 0 **/
	private int start;

	/** end index in source string, exclusive, starting at 0 **/
	private int end;

	/** Cached String instance to reduce pressure on GC **/
	private final String cachedText;

	public Span (String source) {
		this(source, 0, source.length());
	}

	public Span (String source, int start, int end) {
		if (start > end) throw new IllegalArgumentException("Start must be <= end.");
		if (start < 0) throw new IndexOutOfBoundsException("Start must be >= 0.");
		if (start > source.length() - 1) 
			throw new IndexOutOfBoundsException("Start outside of string.");
		if (end >source.length()) throw new IndexOutOfBoundsException("End outside of string.");

		this.source = source;
		this.start = start;
		this.end = end;
		this.cachedText = source.substring(start, end);
	}

	public Span (Span start, Span end) {
		if (!start.source.equals(end.source)) throw new IllegalArgumentException("The two spans do not reference the same source.");
		if (start.start > end.end) throw new IllegalArgumentException("Start must be <= end.");
		if (start.start < 0) throw new IndexOutOfBoundsException("Start must be >= 0.");
		if (start.start > start.source.length() - 1) throw new IndexOutOfBoundsException("Start outside of string.");
		if (end.end > start.source.length()) throw new IndexOutOfBoundsException("End outside of string.");

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