Utility Class for lists.

ListUtils.java

package name.mymiller.utils;

import java.util.ArrayList;
import java.util.List;
import java.util.stream.Collectors;

/**
 * @author jmiller Provide a set of utilities to use on lists.
 */
public class ListUtils {

    /**
     * Find the common elements between two Lists
     *
     * @param <E> Type used in lists, which must have overridden equals();
     * @param a   List A
     * @param b   List B
     * @return List<E> that contains the common elements between the Lists.
     */
    public static <E> List<E> intersection(List<E> a, List<E> b) {
        return a.stream().distinct().filter(b::contains).collect(Collectors.toList());
    }

    /**
     * Join two lists together to form a new List
     *
     * @param <E> Type used in lists, which must have overridden equals();
     * @param a   List A
     * @param b   List B
     * @return List<E> that contains the contents of both Lists.
     */
    public static <E> List<E> union(List<E> a, List<E> b) {
        final List<E> unionList = new ArrayList<>(a);
        unionList.addAll(b);
        return unionList.stream().distinct().collect(Collectors.toList());
    }

    /**
     * Create a list of the unique elements in both Lists.
     *
     * @param <E> Type used in lists, which must have overridden equals();
     * @param a   List A
     * @param b   List B
     * @return List<E> that contains the unique elements not shared between lists.
     */
    public static <E> List<E> unique(List<E> a, List<E> b) {
        final List<E> unionList = ListUtils.union(a, b);
        unionList.removeAll(ListUtils.intersection(a, b));
        return unionList.stream().distinct().collect(Collectors.toList());
    }

    /**
     * Returns boolean indicating if the List is empty
     * @param list List to check
     * @param <E> List type
     * @return boolean indicating if LIst is empty
     */
    public static <E> boolean isEmpty(List<E> list) {
        if (list != null) {
            if (list.size() > 0) {
                return false;
            }
        }

        return true;
    }

    /**
     * Returns boolean indicating if the List is not empty
     * @param list List to check
     * @param <E> List type
     * @return boolean indicating if List is not empty
     */
    public static <E> boolean notEmpty(List<E> list) {
        return !ListUtils.isEmpty(list);
    }

    /**
     * Returns the number of elements in the list.  Checks for Null.
     * @param list List to check
     * @param <E> List Type
     * @return int indicating the number of elements in the list or 0 if null.
     */
    public static <E> int size( List<E> list) {
        if(list == null) {
            return 0;
        }

        return list.size();
    }
    
    /**
     *  Gurantees a list will be safe to use, even if null.
     * @param list List to check if null
     * @param <E> List Type
     * @return List, or an empty list of E.
     */
    public static <E> List<E> safe(List<E> list) {
        if(list != null) {
            return list;
        }
        
        return Collections.emptyList();
    }
    
     /**
     *  Performs pagination on the List,  returning the indicated page of elements, or an empty ist.
     * @param list List to paginate
     * @param pageSize Number of elements per page
     * @param page The Page to return
     * @param <E> Type of Elements
     * @return Empty list if no matching page, or List containing the page's elements.
     */
    public static <E> List<E> page( List<E> list, int pageSize, int page) {
        List<E> safe = ListUtils.safe(list);

        if(pageSize > 0 && page >= 0 && safe.size() > 0) {
            int startIndex = pageSize * (page - 1);
            int endIndex = startIndex + pageSize;
            if(startIndex > safe.size()) {
                return Collections.EMPTY_LIST;
            }

            if(endIndex > safe.size()) {
                endIndex = safe.size() - 1;
            }

            return safe.subList(startIndex,endIndex);
        }

        return Collections.EMPTY_LIST;
    }
}