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AIMA-Java Core Algorithms from the book Artificial Intelligence a Modern Approach 3rd Ed.

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package aima.core.logic.fol.kb.data;

import aima.core.logic.fol.parsing.ast.AtomicSentence;
import aima.core.logic.fol.parsing.ast.Term;

/**
 * Artificial Intelligence A Modern Approach (3rd Edition): page 244.

 * 

 * A literal is either an atomic sentence (a positive literal) or a negated
 * atomic sentence (a negative literal).
 * 
 * @author Ciaran O'Reilly
 * 
 */
public class Literal {
	private AtomicSentence atom = null;
	private boolean negativeLiteral = false;
	private String strRep = null;
	private int hashCode = 0;

	public Literal(AtomicSentence atom) {
		this.atom = atom;
	}

	public Literal(AtomicSentence atom, boolean negated) {
		this.atom = atom;
		this.negativeLiteral = negated;
	}

	public Literal newInstance(AtomicSentence atom) {
		return new Literal(atom, negativeLiteral);
	}

	public boolean isPositiveLiteral() {
		return !negativeLiteral;
	}

	public boolean isNegativeLiteral() {
		return negativeLiteral;
	}

	public AtomicSentence getAtomicSentence() {
		return atom;
	}

	@Override
	public String toString() {
		if (null == strRep) {
			StringBuilder sb = new StringBuilder();
			if (isNegativeLiteral()) {
				sb.append("~");
			}
			sb.append(getAtomicSentence().toString());
			strRep = sb.toString();
		}

		return strRep;
	}

	@Override
	public boolean equals(Object o) {

		if (this == o) {
			return true;
		}
		if (o.getClass() != getClass()) {
			// This prevents ReducedLiterals
			// being treated as equivalent to
			// normal Literals.
			return false;
		}
		if (!(o instanceof Literal)) {
			return false;
		}
		Literal l = (Literal) o;
		return l.isPositiveLiteral() == isPositiveLiteral()
				&& l.getAtomicSentence().getSymbolicName()
						.equals(atom.getSymbolicName())
				&& l.getAtomicSentence().getArgs().equals(atom.getArgs());
	}

	@Override
	public int hashCode() {
		if (0 == hashCode) {
			hashCode = 17;
			hashCode = 37 * hashCode + (getClass().getSimpleName().hashCode())
					+ (isPositiveLiteral() ? "+".hashCode() : "-".hashCode())
					+ atom.getSymbolicName().hashCode();
			for (Term t : atom.getArgs()) {
				hashCode = 37 * hashCode + t.hashCode();
			}
		}
		return hashCode;
	}
}