package edu.berkeley.nlp.io;

/**
 * Something that implements the <code>Label</code> interface can act as a
 * constituent, node, or word label with linguistic attributes. A
 * <code>Label</code> is required to have a "primary" <code>String</code>
 * <code>value()</code> (although this may be null). This is referred to as its
 * <code>value</code>.
 * <p>
 * Implementations of Label split into two groups with respect to equality.
 * Classes that extend ValueLabel define equality solely in terms of String
 * equality of its value (secondary facets may be present but are ignored for
 * purposes of equality), and have equals and compareTo defined across all
 * subclasses of ValueLabel. This behavior should not be changed. Other classes
 * that implement Label define equality only with their own type and require all
 * fields of the type to be equal.
 * <p>
 * A subclass that extends another Label class <i>should</i> override the
 * definition of <code>labelFactory()</code>, since the contract for this method
 * is that it should return a factory for labels of the exact same object type.
 * 
 * @author Christopher Manning
 */
public interface Label {

	/**
	 * Return a String representation of just the "main" value of this label.
	 * 
	 * @return the "value" of the label
	 */
	public String value();

	/**
	 * Set the value for the label (if one is stored).
	 * 
	 * @param value
	 *            - the value for the label
	 */
	public void setValue(String value);

	/**
	 * Return a String representation of the label. For a multipart label, this
	 * will return all parts. The <code>toString()</code> method causes a label
	 * to spill its guts. It should always return an empty string rather than
	 * <code>null</code> if there is no value.
	 * 
	 * @return a text representation of the full label contents
	 */
	public String toString();

	/**
	 * Set the contents of this label to this <code>String</code> representing
	 * the complete contents of the label. A class implementing label may throw
	 * an <code>UnsupportedOperationException</code> for this method (only).
	 * Typically, this method would do some appropriate decoding of the string
	 * in a way that sets multiple fields in an inverse of the
	 * <code>toString()</code> method.
	 * 
	 * @param labelStr
	 *            the String that translates into the content of the label
	 */
	public void setFromString(String labelStr);

	/**
	 * Returns a factory that makes labels of the exact same type as this one.
	 * May return <code>null</code> if no appropriate factory is known.
	 * 
	 * @return the LabelFactory for this kind of label
	 */
	public LabelFactory labelFactory();

}
