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com.firefly.utils.collection.LazyList Maven / Gradle / Ivy
package com.firefly.utils.collection;
import java.io.Serializable;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Collection;
import java.util.Collections;
import java.util.Iterator;
import java.util.List;
import java.util.ListIterator;
/**
* Lazy List creation.
*
* A List helper class that attempts to avoid unnecessary List creation. If a
* method needs to create a List to return, but it is expected that this will
* either be empty or frequently contain a single item, then using LazyList will
* avoid additional object creations by using {@link Collections#EMPTY_LIST} or
* {@link Collections#singletonList(Object)} where possible.
*
*
* LazyList works by passing an opaque representation of the list in and out of
* all the LazyList methods. This opaque object is either null for an empty
* list, an Object for a list with a single entry or an {@link ArrayList} for a
* list of items.
*
* Usage
*
*
* Object lazylist = null;
* while (loopCondition) {
* Object item = getItem();
* if (item.isToBeAdded())
* lazylist = LazyList.add(lazylist, item);
* }
* return LazyList.getList(lazylist);
*
*
* An ArrayList of default size is used as the initial LazyList.
*
* @see java.util.List
*/
@SuppressWarnings("serial")
public class LazyList implements Cloneable, Serializable {
private static final String[] __EMTPY_STRING_ARRAY = new String[0];
private LazyList() {
}
/**
* Add an item to a LazyList
*
* @param list
* The list to add to or null if none yet created.
* @param item
* The item to add.
* @return The lazylist created or added to.
*/
@SuppressWarnings("unchecked")
public static Object add(Object list, Object item) {
if (list == null) {
if (item instanceof List || item == null) {
List l = new ArrayList();
l.add(item);
return l;
}
return item;
}
if (list instanceof List) {
((List) list).add(item);
return list;
}
List l = new ArrayList();
l.add(list);
l.add(item);
return l;
}
/**
* Add an item to a LazyList
*
* @param list
* The list to add to or null if none yet created.
* @param index
* The index to add the item at.
* @param item
* The item to add.
* @return The lazylist created or added to.
*/
@SuppressWarnings("unchecked")
public static Object add(Object list, int index, Object item) {
if (list == null) {
if (index > 0 || item instanceof List || item == null) {
List l = new ArrayList();
l.add(index, item);
return l;
}
return item;
}
if (list instanceof List) {
((List) list).add(index, item);
return list;
}
List l = new ArrayList();
l.add(list);
l.add(index, item);
return l;
}
/**
* Add the contents of a Collection to a LazyList
*
* @param list
* The list to add to or null if none yet created.
* @param collection
* The Collection whose contents should be added.
* @return The lazylist created or added to.
*/
public static Object addCollection(Object list, Collection collection) {
Iterator i = collection.iterator();
while (i.hasNext())
list = LazyList.add(list, i.next());
return list;
}
/**
* Add the contents of an array to a LazyList
*
* @param list
* The list to add to or null if none yet created.
* @param array
* The array whose contents should be added.
* @return The lazylist created or added to.
*/
public static Object addArray(Object list, Object[] array) {
for (int i = 0; array != null && i < array.length; i++)
list = LazyList.add(list, array[i]);
return list;
}
/**
* Ensure the capacity of the underlying list.
*
* @param list
* the list to grow
* @param initialSize
* the size to grow to
* @return the new List with new size
*/
public static Object ensureSize(Object list, int initialSize) {
if (list == null)
return new ArrayList(initialSize);
if (list instanceof ArrayList) {
ArrayList ol = (ArrayList) list;
if (ol.size() > initialSize)
return ol;
ArrayList nl = new ArrayList(initialSize);
nl.addAll(ol);
return nl;
}
List l = new ArrayList(initialSize);
l.add(list);
return l;
}
public static Object remove(Object list, Object o) {
if (list == null)
return null;
if (list instanceof List) {
List l = (List) list;
l.remove(o);
if (l.size() == 0)
return null;
return list;
}
if (list.equals(o))
return null;
return list;
}
public static Object remove(Object list, int i) {
if (list == null)
return null;
if (list instanceof List) {
List l = (List) list;
l.remove(i);
if (l.size() == 0)
return null;
return list;
}
if (i == 0)
return null;
return list;
}
/**
* Get the real List from a LazyList.
*
* @param list
* A LazyList returned from LazyList.add(Object)
* @return The List of added items, which may be an EMPTY_LIST or a
* SingletonList.
* @param
* the list entry type
*/
public static List getList(Object list) {
return getList(list, false);
}
/**
* Get the real List from a LazyList.
*
* @param list
* A LazyList returned from LazyList.add(Object) or null
* @param nullForEmpty
* If true, null is returned instead of an empty list.
* @return The List of added items, which may be null, an EMPTY_LIST or a
* SingletonList.
* @param
* the list entry type
*/
@SuppressWarnings("unchecked")
public static List getList(Object list, boolean nullForEmpty) {
if (list == null) {
if (nullForEmpty)
return null;
return Collections.emptyList();
}
if (list instanceof List)
return (List) list;
return (List) Collections.singletonList(list);
}
/**
* Simple utility method to test if List has at least 1 entry.
*
* @param list
* a LazyList, {@link List} or {@link Object}
* @return true if not-null and is not empty
*/
public static boolean hasEntry(Object list) {
if (list == null)
return false;
if (list instanceof List)
return !((List) list).isEmpty();
return true;
}
/**
* Simple utility method to test if List is empty
*
* @param list
* a LazyList, {@link List} or {@link Object}
* @return true if null or is empty
*/
public static boolean isEmpty(Object list) {
if (list == null)
return true;
if (list instanceof List)
return ((List) list).isEmpty();
return false;
}
public static String[] toStringArray(Object list) {
if (list == null)
return __EMTPY_STRING_ARRAY;
if (list instanceof List) {
List l = (List) list;
String[] a = new String[l.size()];
for (int i = l.size(); i-- > 0;) {
Object o = l.get(i);
if (o != null)
a[i] = o.toString();
}
return a;
}
return new String[] { list.toString() };
}
/**
* Convert a lazylist to an array
*
* @param list
* The list to convert
* @param clazz
* The class of the array, which may be a primitive type
* @return array of the lazylist entries passed in
*/
public static Object toArray(Object list, Class clazz) {
if (list == null)
return Array.newInstance(clazz, 0);
if (list instanceof List) {
List l = (List) list;
if (clazz.isPrimitive()) {
Object a = Array.newInstance(clazz, l.size());
for (int i = 0; i < l.size(); i++)
Array.set(a, i, l.get(i));
return a;
}
return l.toArray((Object[]) Array.newInstance(clazz, l.size()));
}
Object a = Array.newInstance(clazz, 1);
Array.set(a, 0, list);
return a;
}
/**
* The size of a lazy List
*
* @param list
* A LazyList returned from LazyList.add(Object) or null
* @return the size of the list.
*/
public static int size(Object list) {
if (list == null)
return 0;
if (list instanceof List)
return ((List) list).size();
return 1;
}
/**
* Get item from the list
*
* @param list
* A LazyList returned from LazyList.add(Object) or null
* @param i
* int index
* @return the item from the list.
* @param
* the list entry type
*/
@SuppressWarnings("unchecked")
public static E get(Object list, int i) {
if (list == null)
throw new IndexOutOfBoundsException();
if (list instanceof List)
return (E) ((List) list).get(i);
if (i == 0)
return (E) list;
throw new IndexOutOfBoundsException();
}
public static boolean contains(Object list, Object item) {
if (list == null)
return false;
if (list instanceof List)
return ((List) list).contains(item);
return list.equals(item);
}
public static Object clone(Object list) {
if (list == null)
return null;
if (list instanceof List)
return new ArrayList((List) list);
return list;
}
public static String toString(Object list) {
if (list == null)
return "[]";
if (list instanceof List)
return list.toString();
return "[" + list + "]";
}
@SuppressWarnings("unchecked")
public static Iterator iterator(Object list) {
if (list == null) {
List empty = Collections.emptyList();
return empty.iterator();
}
if (list instanceof List) {
return ((List) list).iterator();
}
List l = getList(list);
return l.iterator();
}
@SuppressWarnings("unchecked")
public static ListIterator listIterator(Object list) {
if (list == null) {
List empty = Collections.emptyList();
return empty.listIterator();
}
if (list instanceof List)
return ((List) list).listIterator();
List l = getList(list);
return l.listIterator();
}
}
