XRefService with LookupResult

LookupResult.java

package au.com.sixtree.common.services.xref;

import java.util.ArrayList;
import java.util.Collection;

import au.com.sixtree.common.services.xref.exceptions.UnexpectedResultsException;

public class LookupResult {
    private ArrayList<String> _result;

    public LookupResult() {
        _result = new ArrayList<String>();
    }

    public LookupResult(Collection<String> from) {
        _result = new ArrayList<String>(from);
    }

    public Collection<String> all() {
        return _result;
    }

    public String firstOrDefault(String defaultValue) {
        if (_result.size() > 0) {
            return _result.get(0);
        } else {
            return defaultValue;
        }
    }

    public String first() throws UnexpectedResultsException {
        if (_result.size() == 0) throw new UnexpectedResultsException(UnexpectedResultsException.AT_LEAST_ONE, _result.size());

        return _result.get(0);
    }

    public String single() throws UnexpectedResultsException {
        if (_result.size() > 1) throw new UnexpectedResultsException(UnexpectedResultsException.EXACTLY_ONE, _result.size());

        return first();
    }

    public String singleOrDefault(String defaultValue) throws UnexpectedResultsException {
        if (_result.size() > 1) throw new UnexpectedResultsException(UnexpectedResultsException.AT_MOST_ONE, _result.size());

        return firstOrDefault(defaultValue);
    }
    
    public Collection<String> many() throws UnexpectedResultsException {
        if (_result.size() == 0) throw new UnexpectedResultsException(UnexpectedResultsException.AT_LEAST_ONE, _result.size());
        
        return _result;
    }
    
    public Collection<String> manyOrDefault(String defaultValue) {
        if (_result.size() == 0) {
            _result = new ArrayList<String>();
            _result.add(defaultValue);
        }

        return _result;
    }
}

UnexpectedResultsException.java

package au.com.sixtree.common.services.xref.exceptions;

public class UnexpectedResultsException extends XRefException {
    public static final String AT_LEAST_ONE = "at least one";
    public static final String EXACTLY_ONE = "exactly one";
    public static final String AT_MOST_ONE = "at most one";

    public UnexpectedResultsException(String message) {
        super(message);
    }

    public UnexpectedResultsException(String expected, int actual) {
        this(String.format("Expected %s result returned, got %i", expected, actual));
    }
}

XRefException.java

package au.com.sixtree.common.services.xref.exceptions;

public class XRefException extends Exception {
    public XRefException(String message) {
        super(message);
    }
}

XRefService.java

package au.com.sixtree.common.services.xref;

public interface XRefService {
    public LookupResult lookup(String entity, String sourceValue,
                                     String sourceAttribute, String sourceParticipant,
                                     String targetAttribute, String targetParticipant);
}