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Utility Class for lists.
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;
}
}
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