PropertyComparator.java

package edu.revtek.util;

import java.lang.reflect.*;
import java.util.*;

/**
 * @author Caleb Whiting
 *
 * A convenient class for comparing values within objects
 */
public class PropertyComparator<T> implements Comparator<T> {

	private final Class<? extends Member> memberType;
	private final String name;

	public PropertyComparator(Class<? extends Member> memberType, String name) {
		this.memberType = memberType;
		this.name = name;
	}

	private Member getMember(Object o) {
		Class c = o.getClass();
		if (memberType == Field.class) {
			for (Field f : c.getDeclaredFields()) {
				if (f.getName().equals(name)) {
					return f;
				}
			}
		} else if (memberType == Method.class) {
			for (Method method : c.getDeclaredMethods()) {
				if (method.getName().equals(name)) {
					return method;
				}
			}
		} else {
			throw new RuntimeException("Unsupported member type: " + c);
		}
		throw new RuntimeException(String.format("Couldn't find member for %s in class %s", name, c));
	}

	private Object getValue(T t) throws ReflectiveOperationException {
		Member member = getMember(t);
		if (member instanceof Method) {
			return ((Method) member).invoke(t);
		} else {
			// no need to check, getMember would throw an exception
			// if the type wasn't field or method
			return ((Field) member).get(t);
		}
	}

	@Override
	public int compare(T t, T t1) {
		try {
			Object o = getValue(t);
			Object o1 = getValue(t1);
			try {
				//noinspection unchecked
				return ((Comparable) o).compareTo(o1);
			} catch (ClassCastException e) {
				throw new RuntimeException("Compared values must be instances of java.lang.Comparable", e);
			}
		} catch (ReflectiveOperationException e) {
			throw new RuntimeException(e);
		}
	}

}