A List helper class that attempts to avoid unneccessary List creation.
//
// Copyright 2004-2005 Mort Bay Consulting Pty. Ltd.
// ------------------------------------------------------------------------
// Licensed under the Apache License, Version 2.0 (the "License");
// you may not use this file except in compliance with the License.
// You may obtain a copy of the License at
// http://www.apache.org/licenses/LICENSE-2.0
// Unless required by applicable law or agreed to in writing, software
// distributed under the License is distributed on an "AS IS" BASIS,
// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
// See the License for the specific language governing permissions and
// limitations under the License.
//
import java.io.Serializable;
import java.lang.reflect.Array;
import java.util.ArrayList;
import java.util.Arrays;
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 unneccessary 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 Collections.EMPTY_LIST or
* Collections.singletonList where possible.
*
* <p><h4>Usage</h4>
* <pre>
* Object lazylist =null;
* while(loopCondition)
* {
* Object item = getItem();
* if (item.isToBeAdded())
* lazylist = LazyList.add(lazylist,item);
* }
* return LazyList.getList(lazylist);
* </pre>
*
* An ArrayList of default size is used as the initial LazyList.
*
* @see java.util.List
* @author Greg Wilkins (gregw)
*/
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.
*/
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.
*/
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 collection The Collection 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 capcity of the underlying list.
*
*/
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.
*/
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.
*/
public static List getList(Object list, boolean nullForEmpty)
{
if (list==null)
return nullForEmpty?null:Collections.EMPTY_LIST;
if (list instanceof List)
return (List)list;
List l = new ArrayList(1);
l.add(list);
return l;
}
/* ------------------------------------------------------------ */
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()};
}
/* ------------------------------------------------------------ */
public static Object toArray(Object list,Class aClass)
{
if (list==null)
return (Object[])Array.newInstance(aClass,0);
if (list instanceof List)
{
List l = (List)list;
if (aClass.isPrimitive())
{
Object a = Array.newInstance(aClass,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(aClass,l.size()));
}
Object a = Array.newInstance(aClass,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.
*/
public static Object get(Object list, int i)
{
if (list==null)
throw new IndexOutOfBoundsException();
if (list instanceof List)
return ((List)list).get(i);
if (i==0)
return 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)list).toString();
return "["+list+"]";
}
/* ------------------------------------------------------------ */
public static Iterator iterator(Object list)
{
if (list==null)
return Collections.EMPTY_LIST.iterator();
if (list instanceof List)
return ((List)list).iterator();
return getList(list).iterator();
}
/* ------------------------------------------------------------ */
public static ListIterator listIterator(Object list)
{
if (list==null)
return Collections.EMPTY_LIST.listIterator();
if (list instanceof List)
return ((List)list).listIterator();
return getList(list).listIterator();
}
/* ------------------------------------------------------------ */
/**
* @param array Any array of object
* @return A new <i>modifiable</i> list initialised with the elements from <code>array</code>.
*/
public static List array2List(Object[] array)
{
if (array==null || array.length==0)
return new ArrayList();
return new ArrayList(Arrays.asList(array));
}
/* ------------------------------------------------------------ */
/** Add element to an array
* @param array The array to add to (or null)
* @param item The item to add
* @param type The type of the array (in case of null array)
* @return new array with contents of array plus item
*/
public static Object[] addToArray(Object[] array, Object item, Class type)
{
if (array==null)
{
if (type==null && item!=null)
type= item.getClass();
Object[] na = (Object[])Array.newInstance(type, 1);
na[0]=item;
return na;
}
else
{
Class c = array.getClass().getComponentType();
Object[] na = (Object[])Array.newInstance(c, Array.getLength(array)+1);
System.arraycopy(array, 0, na, 0, array.length);
na[array.length]=item;
return na;
}
}
/* ------------------------------------------------------------ */
public static Object[] removeFromArray(Object[] array, Object item)
{
if (item==null || array==null)
return array;
for (int i=array.length;i-->0;)
{
if (item.equals(array[i]))
{
Class c = array==null?item.getClass():array.getClass().getComponentType();
Object[] na = (Object[])Array.newInstance(c, Array.getLength(array)-1);
if (i>0)
System.arraycopy(array, 0, na, 0, i);
if (i+1<array.length)
System.arraycopy(array, i+1, na, i, array.length-(i+1));
return na;
}
}
return array;
}
}
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