String defaultField = ...;
Analyzer analyzer = ...;
QueryParser queryParser = new QueryParser(defaultField, analyzer);

queryParser.setDefaultOperator(QueryParser.Operator.AND);

Query query = queryParser.parse("Searching is fun");
	
if (required && !prohibited)
  clauses.addElement(new BooleanClause(q, BooleanClause.Occur.MUST));
else if (!required && !prohibited)
  clauses.addElement(new BooleanClause(q, BooleanClause.Occur.SHOULD));
else if (!required && prohibited)
  clauses.addElement(new BooleanClause(q, BooleanClause.Occur.MUST_NOT));
else
  throw new RuntimeException("Clause cannot be both required and prohibited");