Java List Compare compareByList(List list, T a, T b)

Description

Compare two items, with the ordering determined by a list of those items.

License

Apache License

Parameter

Parameter	Description
T	the list type
list	the list, not null
a	the first object, may be null
b	the second object, may be null

Return

0, if equal, -1 if a < b, +1 if a > b

Declaration

public static <T> int compareByList(List<T> list, T a, T b) 

Method Source Code

//package com.java2s;
//License from project: Apache License 

import java.util.List;

public class Main {
    /**/*from  ww w. j  a v  a2s.c o m*/
     * Compare two items, with the ordering determined by a list of those items.
     * <p>
     * Nulls are permitted and sort low, and if a or b are not in the list, then
     * the result of comparing the toString() output is used instead.
     * 
     * @param <T> the list type
     * @param list  the list, not null
     * @param a  the first object, may be null
     * @param b  the second object, may be null
     * @return 0, if equal, -1 if a < b, +1 if a > b
     */
    public static <T> int compareByList(List<T> list, T a, T b) {
        if (a == null) {
            if (b == null) {
                return 0;
            } else {
                return -1;
            }
        } else {
            if (b == null) {
                return 1;
            } else {
                if (list.contains(a) && list.contains(b)) {
                    return list.indexOf(a) - list.indexOf(b);
                } else {
                    return compareWithNullLow(a.toString(), b.toString());
                }
            }
        }
    }

    /**
     * Compares two objects, either of which might be null, sorting nulls low.
     * 
     * @param <E>  the type of object being compared
     * @param a  item that compareTo is called on
     * @param b  item that is being compared
     * @return negative when a less than b, zero when equal, positive when greater
     */
    public static <E> int compareWithNullLow(Comparable<E> a, E b) {
        if (a == null) {
            return b == null ? 0 : -1;
        } else if (b == null) {
            return 1; // a not null
        } else {
            return a.compareTo(b);
        }
    }
}

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