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org.eclipse.jetty.util.LazyList Maven / Gradle / Ivy
//
// ========================================================================
// Copyright (c) 1995 Mort Bay Consulting Pty Ltd and others.
//
// This program and the accompanying materials are made available under the
// terms of the Eclipse Public License v. 2.0 which is available at
// https://www.eclipse.org/legal/epl-2.0, or the Apache License, Version 2.0
// which is available at https://www.apache.org/licenses/LICENSE-2.0.
//
// SPDX-License-Identifier: EPL-2.0 OR Apache-2.0
// ========================================================================
//
package org.eclipse.jetty.util;
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[] __EMPTY_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)
* @param the list entry type
* @return The List of added items, which may be an EMPTY_LIST
* or a SingletonList.
*/
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.
* @param the list entry type
* @return The List of added items, which may be null, an EMPTY_LIST
* or a SingletonList.
*/
@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 __EMPTY_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
* @param the list entry type
* @return the item from the list.
*/
@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();
}
}
