Sorting in array list
Hai
I have an ArrayList object that contains 10 Book objects each book object contains two properties named name and price i want to sort the book objects as per name property or as per price propery.
how can i do it
Thanks in advance
malcolmmc wrote:
cotton.m wrote:
malcolmmc wrote:
.. or make your Book class implement the Comparable<Book> interface if there's only likely to be one sort sequence.
i want to sort the book objects as per name property or as per price propery.It's just a matter of writing a compareTo method accordingly when you implement Comparable. Something like:
public class Book implements Comparable<Book> {
public int compareTo(Book other) {
int nameComp = title.compareTo(other.title);
if(nameComp != 0)
return nameComp;
return Double.compare(price, other.price);
That, to me, is an AND sort not an OR. It is sorted by name AND then price. But it doesn't work if you want to sort by price only.
Similar Messages
-
SAY I HAVE TO STORE COINS IN A LIST....HOW CAN I SORT THEM IN INCREASING ORDER IN THE LIST?
THANKSYou can also, implement comparable and use a sorted collection, if you are shore it's type will not change.
-
Sort array list and using comparable
With the following code I would like to setup a score object for the current player.
Change it to a string(Is it correct to say parse it to a string type)
Then add it to the arraylist.
So I can sort the array list according to the string size.
That's why I have the variables in that order.
So if 3 players have the same amount of guesses, they can be positioned on the high score list according to their gameTime.
//create score object
Score currentScore = new Score(guessCount, gameTime, name);
String currentScoreToString = currentScore.toString();
//add score to arrayList
scores.add(currentScoreToString);So the error message says " The method add(Score) in the type arrayList <Score> is not applicable for the arguments(string)"
Now, I understand that, I would like to know if there is another way to achieve what I am trying to do.
Is the string idea I am trying here possible? is it practical or should I use comparable?
I have looked at comparable, but I don't get it.
Will my Score class implement comparable. I am looking at an example with the following code.
Employee.java
public class Employee implements Comparable {
int EmpID;
String Ename;
double Sal;
static int i;
public Employee() {
EmpID = i++;
Ename = "dont know";
Sal = 0.0;
public Employee(String ename, double sal) {
EmpID = i++;
Ename = ename;
Sal = sal;
public String toString() {
return "EmpID " + EmpID + "\n" + "Ename " + Ename + "\n" + "Sal" + Sal;
public int compareTo(Object o1) {
if (this.Sal == ((Employee) o1).Sal)
return 0;
else if ((this.Sal) > ((Employee) o1).Sal)
return 1;
else
return -1;
ComparableDemo.java
import java.util.*;
public class ComparableDemo{
public static void main(String[] args) {
List ts1 = new ArrayList();
ts1.add(new Employee ("Tom",40000.00));
ts1.add(new Employee ("Harry",20000.00));
ts1.add(new Employee ("Maggie",50000.00));
ts1.add(new Employee ("Chris",70000.00));
Collections.sort(ts1);
Iterator itr = ts1.iterator();
while(itr.hasNext()){
Object element = itr.next();
System.out.println(element + "\n");
}The thing I don't understand is why it returns 0, 1 or -1(does it have to do with the positioning according to the object is being compared with?)
What if I only use currentScore in a loop which loops every time the player restarts?
//create score object
Score currentScore = new Score(guessCount, gameTime, name);
String currentScoreToString = currentScore.toString();
//add score to arrayList
scores.add(currentScoreToString);Also why there is a method compareTo, and where is it used?
Thanks in advance.
Edited by: Implode on Oct 7, 2009 9:27 AM
Edited by: Implode on Oct 7, 2009 9:28 AMjverd wrote:
Implode wrote:
I have to hand in an assignment by Friday, and all I have to do still is write a method to sort the array list. Okay, if you have to write your own sort method, then the links I provided may not be that useful. They show you how to use the sort methods provided by the core API. You COULD still implement Comparable or Comparator. It would just be your sort routine calling it rather than the built-in ones.
You have two main tasks: 1) Write a method that determines which of a pair of items is "less than" the other, and 2) Figure out a procedure for sorting a list of items.
The basic idea is this: When you sort, you compare pairs of items, and swap them if they're out of order. The two main parts of sorting are: 1) The rules for determining which item is "less than" another and 2) Determining which pairs of items to compare. When you implement Comparable or create a Comparator, you're doing #1--defining the rules for what makes one object of your class "less than" another. Collections.sort() and other methods in the core API will call your compare() or compareTo() method on pairs of objects to produce a sorted list.
For instance, if you have a PersonName class that consists of firstName and lastName, then your rules might be, "Compare last names. If they're different, then whichever lastName is less indicates which PersonName object is less. If they're the same, then compare firstNames." This is exactly what we do in many real-life situations. And of course the "compare lastName" and "compare firstName" steps have their own rules, which are implemented by String's compareTo method, and which basically say, "compare char by char until there's a difference or one string runs out of chars."
Completely independent of the rules for comparing two items is the algorithm for which items get compared and possibly swapped. So, if you have 10 Whatsits (W1 through W10) in a row, and you're asked to sort them, you might do something like this:
Compare the current W1 to each of W2 through W10 (call the current one being compared Wn). If any of them are less than W1, swap that Wn with W1 and continue on, comparing the new W1 to W(n+1) (that is, swap them, and then compare the new first item to the next one after where you just swapped.)
Once we reach the end of the list, the item in position 1 is the "smallest".
Now repeat the process, comparing W2 to W3 through W10, then W3 to W4 through W10, etc. After N steps, the first N positions have the correct Whatsit.
Do you see how the comparison rules are totally independent of the algorithm we use to determine which items to compare? You can define one set of comparison rules ("which item is less?") for Whatsits, another for Strings, another for Integers, and use the same sorting algorithm on any of those types of items, as long as you use the appropriate comparison rules.Thanks ;)
massive help
I understand that now, but I am clueless on how to implement it.
Edited by: Implode on Oct 7, 2009 10:56 AM -
Sort array(list) of files on filename
Hey all,
Is there a way to sort an array(list) of files on the names of those files. (the files are not in same directory)
ThanksGogoe wrote:
Hey all,
Is there a way to sort an array(list) of files on the names of those files. (the files are not in same directory)
Thanks
Why do you need to sort the actual Files? Can't you just sort the list of name using Arrays.sort()? Once the names are in order you call up the Files in the order you read the names. Or is there more to the request yet? -
Hi,
I have an array List which have values like empid, amount, corpcode eg.
223, 345.95, SDB
791, 567.75, XYZ
115, 345.95, SDB
I need to sort this array like this
115, 345.95, SDB
223, 345.95, SDB
791, 567.75, XYZ
How can I do this?
ThanksYou have to implement a natural order for your object, so that it can be sorted. There are a lot of tutorials out there for this, but I will do it anyway for you:
public YourClass implements Comparable<YourClass>{
int firstNumber;
int secondNumber;
int thirdNumber;
String someIdentifier;
public int compareTo(YourClass otherObject){
int difference = this.firstNumber - otherObject.firstNumber;
if(difference != 0) return difference;
difference = this.secondNumber - otherObject.secondNumber;
if(difference != 0) return difference;
return this.someIdentifier.compareTo(otherObject.someIdentifier);
}As you can see, the basic idea is, that you return if an object is greater, equal or less than another. You begin with the most important criteria. If this one is equal, you check the next one.
Some objects already include the comparable interface, like String. If this is the case, you can take advantage of this, and use their compareTo Method.
After this, you can sort your objects with
Collections.sort(someList); -
can u please give me the source code for:
sorting array list for ascending and descending orderYou already have the source code.
-
I need to sort an array of strings based on the number in each string.
Basically, I have a directory of files that all have the same name but each with a different number on the end.
example: image 1.jpg, image 2.jpg, etc.
When I use the List Directory function that returns an array of strings containing the file names in the directory, they don't come out in a 1, 2, 3, order like they appear in the directory. It sorts them character by character so that they come out like: 1, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 2, 20, 21, 22 etc.
Is there a simple way of sorting this array of strings with the file names (as above) so that they are in numerical order?It's a while since this thread was started, but I am sure others will have use for this so here goes:
The in-built array sort function sorts the strings the same way DOS and Windows do. Microsoft has fixed this in the Explorer that comes with XP, however the rest of the OS(s) still sorts the old way.
The attached "AlphaLogical String Array Sort" VIs will sort strings arrays the same way as the new XP Explorer. There are three different implementations of the sorting, one based on the Insertion sort algorithm, a Quick Sort based on recursive calls (the most elegant way, but unfortunately LabVIEW has too much overhead when doing recursive calls so this is actually a very slow alternative) and finally the fastest; a stack based Quick Sort. There is also a test VI that will show you how the different implementations perform.
I had not used recursive calls in LV much until I made the attached quick sort and was a bit disappointed by the fact that it is so slow, but it's a good learning example...The ability to do recursive calls this way was introduced in LV7 I believe...There is an example here on the zone that shows how you can calulate a factorial by using recursive calls, however - unlike for the quick sort (normally) - recursive calls are actually not the optimal solution for that calculation.
Message Edited by Mads on 09-13-2005 02:30 AM
MTO
Attachments:
AlphaLogical Sorting.zip 142 KB -
Sorting an array of filenames by filetype?
I have an array of filenames read from a directory list. I
can parse the
file type from each filename using a function. How would I
then sort the
array by their respective file type, so that all PDF files
sort together in
the array, for example? Or would it be easier to just loop
through the
array by filetype, printing each matching array value?
Murray --- ICQ 71997575
Adobe Community Expert
(If you *MUST* email me, don't LAUGH when you do so!)
==================
http://www.projectseven.com/go
- DW FAQs, Tutorials & Resources
http://www.dwfaq.com - DW FAQs,
Tutorials & Resources
==================That's exactly the code I am using except without the
multisort. It should
be an easy retrofit!
Thanks, Joe!
Murray --- ICQ 71997575
Adobe Community Expert
(If you *MUST* email me, don't LAUGH when you do so!)
==================
http://www.projectseven.com/go
- DW FAQs, Tutorials & Resources
http://www.dwfaq.com - DW FAQs,
Tutorials & Resources
==================
"Joe Makowiec" <[email protected]> wrote in
message
news:[email protected]...
> On 14 Apr 2008 in macromedia.dreamweaver.appdev, Joe
Makowiec wrote:
>
>> On 14 Apr 2008 in macromedia.dreamweaver.appdev,
Murray *ACE* wrote:
>>
>>> I have an array of filenames read from a
directory list. I can
>>> parse the file type from each filename using a
function. How would
>>> I then sort the array by their respective file
type, so that all
>>> PDF files sort together in the array, for
example? Or would it be
>>> easier to just loop through the array by
filetype, printing each
>>> matching array value?
>>
>>
http://www.php.net/manual/en/ref.array.php
>>
>> There are a bunch of sorting functions there.
array_multisort and
>> uasort both look interesting. Got code?
>
> Dropin for a directory listing in a <ul>:
>
> <pre>
> <?php
> $files2 = array();
> if ($handle = opendir('.')) {
> echo "Input: .\n";
> echo "Directory handle: $handle\n";
> /* This is the correct way to loop over the directory.
> while (false !== ($file = readdir($handle))) {
> $files[] = $file;
> if (substr($file, 0, 1) != '.') {
> $separator = split('\.', $file);
> $files2[] = array('filename' => $separator[0], 'ext'
=>
> $separator[1]);
> }
> echo "$file\n";
> }
> closedir($handle);
>
> // print_r($files);
> // print_r($files2);
>
> foreach ($files2 as $key => $row) {
> $filename[$key] = $row['filename'];
> $ext[$key] = $row['ext'];
> }
>
> array_multisort($ext, SORT_ASC, $filename, SORT_DESC,
$files2);
>
> // print_r($files2);
>
> reset($files2);
>
> }
> ?>
> </pre>
> <ul>
> <?php
> foreach ($files2 as $myfile) {
> echo "\t<li>";
> echo $myfile['filename'];
> echo (empty($myfile['ext'])) ? '' : '.' .
$myfile['ext'];
> echo "</li>\n";
> }
> ?>
> </ul>
>
> Code's probably uglier than it needs to be, but it
works.
>
> --
> Joe Makowiec
>
http://makowiec.net/
> Email:
http://makowiec.net/contact.php -
Sorting singly linked list with minimum time complexity
Hi ...
anyone could tell me how can i sort singly linked list with minimum time complexity .... ????
Regards...By MergeSort or QuickSort O(n log n). But then you
have to first extract the objects in the list,sort
them, then rebuild the list. But it will still bealot
faster than by keeping the list linked.Technically, I believe insertion sort is marginally
faster for small n ( <20 or so).Woohoo! So for 20 out of the possible 2147483648 array
sizes Insetion is faster!
Unfortunately, checking for that case probably wastes
all the time you get from using the faster sort...
That would depend on the actual distribution off array sizes. So it's an engineering decision.
Sylvia. -
Previous element in array list
hello ppl..
i hope u guys can help me here.. i am developing a program that sort an amount of strings and output to a file..and i place them inside an array list..i use Collections.sort to sort it..
now the problem is..i want to handle duplicate strings meaning deleting the duplicate strings..however my code doesnt seem to work..here it is
Iterator check = c.iterator();
while (check.hasNext() == true)
if (check.previous() == check.next())
check.remove();
br.write(check.next() + "\r\n");
c is my arraylist name. JDK's error is they cannot resolve symbol previous () method but i found it inside util and i imported the whole util..by import java.util.*;
Please help..thank youGood catch. It's very rare that you use == to compare objects. You only do that when you want to see if the two references are both aimed at the same object. Normally what you want to do (and what should be done here) is to use equals to see if the two objects' "contents" or "values" are the same.
>
if (check.previous() == check.next())Did you use this piece in the program?
If so could this be the problem?
I think that when you use the "==" operator it checks
to see if the two objects share the same memory
location.
Like when you are comparing Strings you must use the
String.equals(String s) method they they will never
match with "==".
Maybe this is not the problem but I like to
ramble...
-- Ian -
A suggestion : use "loop array list" instead of ArrayList / LinkedList
ArrayList is good at:
get / set by index : O(1)
appending : O(log(n))
remove last : O(1)
and very bad at:
add middle : O(n)
remove from middle : O(n)
LinkedList is good at:
fast remove from middle, if your iteraror already goes there : O(1)
convenient methods : addFirst, addLast, removeFirst, removeLast : O(1)
and very bad at :
get / set by index : O(n)
here I want to make a suggestion : use "loop array list" instead of the ArrayList and LinkedList.
a "loop array list" is based on array, just like ArrayList. But it has 2 member-variables : the start position, and the real size. the start position can be anywhere within array.length. an element of index "i" is stored in array[ start + i ], and when (start + i > array.length), loop to the beginning of the array; when (start + i < 0), loop to the end of the array.
this "loop array list" has all the good sides:
get / set by index : O(1)
add first / last : O(log(n))
remove first / last : O(log(n))
add / remove from middle : O(n)
(note : because we shrink the backup-array when the real size is too small, add / remove operation take O(log(n)) now.)
the only problem is : add / remove from middle. let's take a look at it.
1. the LinkedList does NOT really add/remove from middle in O(1), you has to locate the element, which is O(n).
2. O(n) is acceptable, O(n^2) is not acceptable. try this : keep removing first element from an very big ArrayList ( size>10^6 ) until it's empty.
the fact is, any list can perform batch-remove / batch-add operation in O(n) instead of O(n^2). it's easy : allocate a new list, iterate the original list, copy the element into the new list if condition is satisfied.
so, instead of "remove from middle", what we need is a new methods : removeAllByCondition( Condition condition ), and now the batch-remove operation can be done in O(n)
here is an implementation of mine. I've tested it on my computer( 512mem + 2G cpu, win2k + jdk1.5 ), it's amazing, just a little slower then ArrayList when add last, and a liitle slower then LinkedList when batch-remove. in all other cases, it's far more better then those 2 kinds of List.
// source code : List2
import java.util.AbstractList;
import java.util.ArrayList;
import java.util.Arrays;
import java.util.Collection;
import java.util.Collections;
import java.util.Comparator;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.NoSuchElementException;
import java.util.Set;
public final class List2<T> extends AbstractList<T> {
private static int initialArrayLength = 4;
private T[] array;
private int start;
private int size;
private void init( T[] newArray, int start_a, int size_a ) {
array = newArray;
start = start_a;
size = size_a;
@SuppressWarnings("unchecked")
private void init() {
init( (T[]) new Object[ initialArrayLength ], 0, 0 );
public List2() {
init();
@SuppressWarnings("unchecked")
public List2( Collection<? extends T> collection ) {
init(
collection.toArray( (T[]) new Object[ collection.size() * 11 / 10 + 1 ] ),
0,
collection.size()
private List2( T[] array_a, int start_a, int size_a ) {
init( array_a, start_a, size_a );
@SuppressWarnings("unchecked")
public static <TT> List2<TT> createV( TT... elements ) {
TT[] array = (TT[]) new Object[ elements.length * 11 / 10 + 1 ];
System.arraycopy( elements, 0, array, 0, elements.length );
return new List2<TT>( array, 0, elements.length );
public static List2<Double> create( double... elements ) {
Double[] array = new Double[ elements.length * 11 / 10 + 1 ];
for( int i=0; i < elements.length; i++ )
array[i] = elements;
return new List2<Double>( array, 0, elements.length );
public static List2<Integer> create( int... elements ) {
Integer[] array2 = new Integer[ elements.length * 11 / 10 + 1 ];
for( int i=0; i < elements.length; i++ )
array2[i] = elements[i];
return new List2<Integer>( array2, 0, elements.length );
public static List2<Character> create( char... elements ) {
Character[] array2 = new Character[ elements.length * 11 / 10 + 1 ];
for( int i=0; i < elements.length; i++ )
array2[i] = elements[i];
return new List2<Character>( array2, 0, elements.length );
public static List2<Character> create( String s ) {
return create( s.toCharArray() );
public List2<T> clone() {
return new List2<T>( this );
// basic
public int size() {
return size;
private int index( int index ) {
int i = start + index;
if( i >= array.length )
i -= array.length;
return i;
public T get( int d ) {
if( d < 0 || d >= size )
throw new IndexOutOfBoundsException();
if( size == 0 )
throw new NoSuchElementException();
return array[ index( d ) ];
public T set( int index, T element ) {
if( index < 0 || index >= size )
throw new IndexOutOfBoundsException();
int i = index( index );
T oldElement = array[i];
array[i] = element;
return oldElement;
@SuppressWarnings("unchecked")
private void copyAndSetToNewArray( int newArrayLength ) {
T[] newArray = (T[]) new Object[ newArrayLength ];
for( int i=0; i<size; i++ )
newArray[i] = array[ index( i ) ];
init( newArray, 0, size );
public void addFirst( T element ) {
if( size == array.length )
copyAndSetToNewArray( size * 3 / 2 + 1 );
int i = index( array.length - 1 );
array[ i ] = element;
start = i;
size++;
public void addLast( T element ) {
if( size == array.length )
copyAndSetToNewArray( size * 3 / 2 + 1 );
array[ index( size ) ] = element;
size++;
public T removeFirst() {
if( size == 0 )
throw new NoSuchElementException();
T oldElement = array[ start ];
array[ start ] = null;
start = index( 1 );
size--;
if( array.length > size * 2 + 1 )
copyAndSetToNewArray( size * 11 / 10 + 1 );
return oldElement;
public T removeLast() {
if( size == 0 )
throw new NoSuchElementException();
int i = index( size - 1 );
T oldElement = array[i];
array[i] = null;
size--;
if( array.length > size * 2 + 1 )
copyAndSetToNewArray( size * 11 / 10 + 1 );
return oldElement;
@SuppressWarnings("unchecked")
public int removeAll( ListCondition<T> condition ) {
T[] newArray = (T[]) new Object[ array.length ];
int iNew = 0;
for( int i=0; i < size; i++ ) {
T element = get( i );
if( ! condition.isConditionSatisfied( this, i, element ) )
newArray[ iNew++ ] = element;
int oldSize = size;
init( newArray, 0, iNew );
if( array.length > size * 2 + 1 )
copyAndSetToNewArray( size * 11 / 10 + 1 );
return size - oldSize;
// aux
public boolean equals(Object obj) {
if( obj == this )
return true;
if( obj instanceof List2 ) {
List2 that = (List2) obj;
if( this.size != that.size )
return false;
for( int i=0; i < size; i++ )
if( ! Tools.equals( this.array[ this.index(i) ], that.array[ that.index(i) ] ) )
return false;
return true;
if( obj instanceof List ) {
List that = (List) obj;
if( this.size != that.size() )
return false;
Iterator thatIter = that.iterator();
for( int i=0; i < size; i++ )
if( ! Tools.equals( this.array[ this.index(i) ], thatIter.next() ) )
return false;
return true;
return true;
public int hashCode() {
int hashCode = 1;
for( int i=0; i < size; i++ ) {
T element = array[ index( i ) ];
hashCode = 31*hashCode + ( element==null ? 0 : element.hashCode() );
return hashCode;
public boolean isEmpty() {
return size == 0;
public T getFirst() {
return get( 0 );
public T getLast() {
return get( size() - 1 );
public T getRandom() {
return get( (int) (Math.random() * size) );
public int indexOf( Object element ) {
for( int i=0; i < size; i++ )
if( Tools.equals( array[ index( i ) ], element ) )
return i;
return -1;
public int lastIndexOf( Object element ) {
for( int i=size-1; i >= 0; i-- )
if( Tools.equals( array[ index( i ) ], element ) )
return i;
return -1;
public boolean contains( Object element ) {
return indexOf( element ) != -1;
public boolean add( T element ) {
addLast( element );
return true;
@Deprecated
public void add( int index, T element ) {
throw new UnsupportedOperationException();
public T remove() {
return removeFirst();
@Deprecated
public boolean remove( Object element ) {
throw new UnsupportedOperationException( "use removeAll( Condition ) instead" );
@Deprecated
public T remove( int index ) {
throw new UnsupportedOperationException( "use removeAll( Condition ) instead" );
public void clear() {
init();
public Object[] toArray() {
Object[] result = new Object[ size ];
for( int i=0; i < size; i++ )
result[i] = array[ index( i ) ];
return result;
@SuppressWarnings("unchecked")
public <TT> TT[] toArray( TT[] a ) {
if( a.length < size )
a = (TT[]) java.lang.reflect.Array.newInstance( a.getClass().getComponentType(), size );
for( int i=0; i < size; i++ )
a[i] = (TT) array[ index( i ) ];
if( a.length > size )
a[size] = null;
return a;
@SuppressWarnings("unchecked")
public void sort() {
Object[] a = toArray();
Arrays.sort( a );
for( int i=0; i < size; i++ )
array[ i ] = (T) a[ i ];
start = 0;
@SuppressWarnings("unchecked")
public void sortDesc() {
Object[] a = toArray();
Arrays.sort( a );
for( int i=0, j=size-1; i < size; i++, j-- )
array[ i ] = (T) a[ j ];
start = 0;
@SuppressWarnings("unchecked")
public void sort( Comparator<T> comparator ) {
T[] a = (T[]) toArray();
Arrays.sort( a, comparator );
for( int i=0; i < size; i++ )
array[ i ] = a[ i ];
start = 0;
@SuppressWarnings("unchecked")
public void sortDesc( Comparator<T> comparator ) {
T[] a = (T[]) toArray();
Arrays.sort( a, comparator );
for( int i=0, j=size-1; i < size; i++, j-- )
array[ i ] = a[ j ];
start = 0;
public String toString( String delimiter ) {
return toString( "", delimiter, "", size() );
public String toString( String prefix, String delimiter, String suffix, int max ) {
StringBuffer stringBuffer = new StringBuffer( prefix );
int dest = Math.min( max, size );
for( int i=0; i < dest; i++ ) {
stringBuffer.append( get(i) );
if( i < dest - 1 )
stringBuffer.append( delimiter );
if( size > max )
stringBuffer.append( "...(" ).append( size() - max ).append( " more)" );
stringBuffer.append( suffix );
return stringBuffer.toString();
// batch operation
public boolean containsAll( Collection<?> that ) {
Set<Object> thisSet = new HashSet<Object>( this );
for( Object element : that )
if( ! thisSet.contains( element ) )
return false;
return true;
@SuppressWarnings("unchecked")
public List2<T> subList( int fromIndex, int toIndex ) {
if( fromIndex < 0 || toIndex > size || toIndex < fromIndex )
throw new IndexOutOfBoundsException();
int newSize = toIndex - fromIndex;
T[] newArray = (T[]) new Object[ newSize * 11 / 10 + 1 ];
for( int i=fromIndex, iNew=0; i < toIndex; i++, iNew++ )
newArray[ iNew ] = array[ index( i ) ];
return new List2<T>( newArray, 0, newSize );
public void addV( T... that ) {
for( T element : that )
addLast( element );
public boolean addAll( Collection<? extends T> that ) {
for( T element : that )
addLast( element );
return ! that.isEmpty();
@Deprecated
public boolean addAll( int index, Collection<? extends T> c ) {
throw new UnsupportedOperationException();
public void removeRest( T element ) {
int position = lastIndexOf( element );
if( position == -1 )
return;
while( ! Tools.equals( element, removeLast() ) );
public void removeAllEquals( final T element ) {
removeAll( new ListCondition<T>() { public boolean isConditionSatisfied(List2 list, int index, T currentElement) {
return currentElement.equals( element );
public void removeAllBetween( final T from, final T to ) {
removeAll( new ListCondition<T>() {
@SuppressWarnings("unchecked")
public boolean isConditionSatisfied(List2 list, int index, T element) {
if( from != null && ((Comparable) from).compareTo( element ) > 0 )
return false;
if( to != null && ((Comparable) to).compareTo( element ) <= 0 )
return false;
return true;
public boolean retainAll( Collection<?> that ) {
final Set<Object> thatSet = new HashSet<Object>( that );
int removeCount = removeAll( new ListCondition<T>() { public boolean isConditionSatisfied(List2 list, int index, T element) {
return ! thatSet.contains( element );
return removeCount > 0;
public boolean removeAll( Collection<?> that ) {
final Set<Object> thatSet = new HashSet<Object>( that );
int removeCount = removeAll( new ListCondition<T>() { public boolean isConditionSatisfied(List2 list, int index, T element) {
return thatSet.contains( element );
return removeCount > 0;
// unit test
private static int maxTestCount = 1000 * 1000;
public static void unitTest() throws Exception {
// thest thoese methods for one time
Tools.ensureEquals( new List2(), new ArrayList() );
Tools.ensureNotEquals( List2.create( "abcde" ), new ArrayList() );
Tools.ensureNotEquals( List2.create( "abcde" ), List2.create( "abcdef" ) );
final List<Double> list1 = new ArrayList<Double>();
final List2<Double> list2 = new List2<Double>();
Runnable[] tasks = new Runnable[] {
// test those methods that do NOT change the list
new Runnable() { public void run() {
Tools.ensureEquals( new List2<Double>( list1 ), list1 );
Tools.ensureEquals( List2.createV( list1.toArray() ), list1 );
Tools.ensureEquals( List2.createV( list1.toArray( new Double[0] ) ), list1 );
double[] doubles = new double[ list1.size() ];
int i = 0;
for( double d : list1 )
doubles[i++] = d;
Tools.ensureEquals( List2.create( doubles ), list1 );
Tools.ensure( list1.isEmpty() == list2.isEmpty() );
Arrays.equals( list1.toArray(), list2.toArray() );
Tools.ensureEquals( list1, list2.clone() );
Double notExistElement = -2.0;
Tools.ensure( list1.indexOf( notExistElement ) == -1 );
Tools.ensure( list1.lastIndexOf( notExistElement ) == -1 );
Tools.ensure( list1.contains( notExistElement ) == false );
Tools.ensureEquals( list1.toString(), list2.toString() );
Tools.ensureEquals( list1.toString(), list2.toString() );
Tools.ensureEquals( list1.hashCode(), list2.hashCode() );
if( list1.isEmpty() )
return;
Tools.ensure( list1.get(0).equals( list2.getFirst() ) );
Tools.ensure( list1.get(list1.size()-1).equals( list2.getLast() ) );
Double existRandomElement = list2.getRandom();
Tools.ensure( list1.contains( existRandomElement ) );
Tools.ensure( list2.contains( existRandomElement ) );
Tools.ensure( list1.indexOf( existRandomElement ) == list2.indexOf( existRandomElement ) );
Tools.ensure( list1.indexOf( existRandomElement ) == list2.indexOf( existRandomElement ) );
int from = (int) (Math.random() * list1.size());
int to = (int) (Math.random() * (list1.size()+1));
if( from > to ) {
int t = from;
from = to;
to = t;
Tools.ensureEquals( list1.subList( from, to ), list2.subList( from, to ) );
// test those methods that change the list
new Runnable() { public void run() {
if( list1.isEmpty() )
return;
int i = (int) (Math.random() * list1.size());
double d = Math.random();
list1.set( i, d );
list2.set( i, d );
new Runnable() { public void run() {
if( list1.isEmpty() )
return;
int i = (int) (Math.random() * list1.size());
Tools.ensure( list1.get( i ).equals( list2.get( i ) ) );
new Runnable() { public void run() {
double d = Math.random();
list1.add( 0, d );
list2.addFirst( d );
new Runnable() { public void run() {
double d = Math.random();
list1.add( d );
list2.addLast( d );
new Runnable() { public void run() {
double d = Math.random();
list1.add( d );
list2.addLast( d );
new Runnable() { public void run() {
if( list1.isEmpty() )
return;
Tools.ensure( list1.remove( 0 ).equals( list2.removeFirst() ) );
new Runnable() { public void run() {
if( list1.isEmpty() )
return;
Tools.ensure( list1.remove( list1.size() - 1 ).equals( list2.removeLast() ) );
new Runnable() { public void run() {
if( list1.isEmpty() )
return;
int i = 0;
for( Iterator<Double> iter=list1.iterator(); iter.hasNext(); i++ ) {
iter.next();
if( i % 3 == 0 )
iter.remove();
list2.removeAll( new ListCondition<Double>() { public boolean isConditionSatisfied(List2 list, int index, Double element) {
return index % 3 == 0;
new Runnable() { public void run() {
double d = Math.random();
list1.add( d );
list2.add( d );
new Runnable() { public void run() {
if( list1.isEmpty() )
return;
Tools.ensure( list1.remove(0).equals( list2.remove() ) );
new Runnable() { public void run() {
if( list1.isEmpty() )
return;
int r = (int) (Math.random() * list1.size());
Double element = list1.get( r );
int index = list1.lastIndexOf( element );
for( int i=list1.size()-1; i>=index; i-- )
list1.remove( i );
list2.removeRest( element );
new Runnable() { public void run() {
list2.removeRest( Math.random() - 2 );
new Runnable() { public void run() {
list1.clear();
list2.clear();
new Runnable() { public void run() {
Collections.sort( list1 );
list2.sort();
new Runnable() { public void run() {
Collections.sort( list1 );
Collections.reverse( list1 );
list2.sortDesc();
new Runnable() { public void run() {
Comparator<Double> comparator = new Comparator<Double>() { public int compare(Double o1, Double o2) {
return o1.toString().substring(2).compareTo( o2.toString().substring(2) );
Collections.sort( list1, comparator );
list2.sort( comparator );
new Runnable() { public void run() {
Comparator<Double> comparator = new Comparator<Double>() { public int compare(Double o1, Double o2) {
return o1.toString().substring(2).compareTo( o2.toString().substring(2) );
Collections.sort( list1, comparator );
Collections.reverse( list1 );
list2.sortDesc( comparator );
new Runnable() { public void run() {
Double notExistElement = -2.0;
list2.removeAllEquals( notExistElement );
Tools.ensureEquals( list1, list2 );
list2.removeAllBetween( 0.5, 0.6 );
for( Iterator<Double> iter=list1.iterator(); iter.hasNext(); ) {
double d = iter.next();
if( d >= 0.5 && d < 0.6 )
iter.remove();
Tools.ensureEquals( list1, list2 );
System.out.print( "test List2 " );
for( int i=0; i < maxTestCount; i++ ) {
tasks[ (int) (Math.random() * tasks.length) ].run();
Tools.ensureEquals( list1, list2 );
if( i % (maxTestCount/10) == 0 )
System.out.print( "." );
System.out.println( " ok" );
// source code : ListCondition
public interface ListCondition<T> {
boolean isConditionSatisfied( List2 list, int index, T element );Hi,
I have the following statement:
private List list = new ArrayList();
why not use private ArrayList list = new
ArrayList()?
I know ArrayList implements List, so list has a wide
scope, right? What's the benefit?
Thanks,
JieBy using the interface you are not tied to a specific implementation.
Later, you may determine that using a LinkedList is more efficient to your specific code and then all you need to do is change the statement to:
private List list = new LinkedList();As long as you are not casting the list reference to ArrayList (or something along those lines) you would not need to change anything else. -
Need help sorting 2 arrays linked by their indexes
I have 2 arrays that have been linked by the index of both arrays. The first array has to be sorted alphabetically, so I need a way to sort the 2nd array so that the indices match up again with the first array.
Now, I cannot change my ArrayList (i.e. TreeSet, List, etc.) since there is lots of code that would have to be changed. Here is my test code. Hopefully everyone is a fan of the television show Friends so that you can see how if I sort the firstName Array alphabetically, then I have to have the 2nd array sorted to be "linked" back to the first array based on the index. I am looking for ideas, but if you have simple code, that would be great too.
import java.util.*;
public class Test
public static void main(String[] args)
System.out.println("BEGIN TEST...");
Test test = new Test();
test.beginTest();
System.out.println("\nTEST COMPLETE!");
public Test(){}
public void beginTest()
ArrayList firstName = new ArrayList();
ArrayList lastName = new ArrayList();
//Add to firstName ArrayList
firstName.add("Ross");
firstName.add("Chandler");
firstName.add("Rachel");
firstName.add("Phoebe");
firstName.add("Joey");
//Add to lastName ArrayList
lastName.add("Geller");
lastName.add("Bing");
lastName.add("Green");
lastName.add("Buffay");
lastName.add("Tribbiani");
try
System.out.println("TESTING...\n");
for(int index=0; index<firstName.size(); index++)
String first = firstName.get(index).toString();
String last = lastName.get(index).toString();
System.out.println("Hi, I am " + first + " " + last + ".");
catch(Exception X) { System.out.println("EXCEPTION! "+X); X.printStackTrace(); }
}Keeping the list in synch is conceptually easy.
Basically anything you do to the one List, you have to do to the other in order to keep them consistent.
How are you sorting your "firstnames" list?
Using the Collections.sort method, or some manual coding?
Would it be possible to change your datastructure from "parallel arrays" to "List of people objects"
iepublic class Person {
String firstName;
String lastName;
public Person(String firstName, String lastName){
this.firstName = firstName;
this.lastName = lastName;
// appropriate getter/setter methods
}Then you could build up your list like this:
List friends = new ArrayList();
friends.add(new Person("Ross", "Geller"));
friends.add(new Person("Chandler", "Bing"));
...Now when you sort the list, the last name always goes with the first name because they are in the same object.
You then write different Comparator objects to impose different orderings.
Hope this helps,
evnafets -
i have an array with three fields: id, name and date.
i want to sort it by name first and then date.
the size of my array could be as large as 1 million.
what could be the most efficient way to do so?
tia!It's very inefficient to sort an array on "the fly".
In that case at least use a linked list so you don't
have to move all elements after the insertion point to
make room for a new insert. But it's much better to
sort afterwards when all elements are in the array.Use a TreeSet. A linked list will be slower than a TreeSet, I suspect.
I believe that with a TreeSet, insertions will be somewhat slower (O(logN) vs O(c)), but the subsequent sorting of a linked list will be, what, at least O(NlogN) and possibly O(N^2)? And when you're inserting the first elements into the TreeSet, it should be closer to O(c) anyway, whereas the after-the-fact sorting of a LinkedList wouldn't have an advantage like that.
Then if necessary extract an array out of the TreeSet when you're done creating/inserting elements.
But really, use a database. -
The time complexity of "Sort 1D Array"
Does anyone know the time complexity of "Sort 1D Array"? The document of this function does not say anything about that, so I write test VI to sort an array of 10000 integer elements. It seems that the excution time is less than 1 second, so I guess the complexity is O(n*logn). Any suggestion would be appreciated, thanks.
A few thoughts based on my recollection of that old coding challenge. There are different kinds of algorithmic approaches you may consider.
1. A simple approach to that challenge could involve a lot of search / lookup functions. Each time you parse a word, you perform a search to see if it's in your list already. If so, increment its count. If not, add it to the list with count=1.
This was far too slow for word counting large docs. Too many slow-ish searches. But pretty easy to implement and maybe good enough for smaller docs.
2. I pursued an approach that used hashing instead of searches. It *seemed* too slow, but some months after the challenge when I had another use for the core hash table functionality, I found a really dumb bug that I'd overlooked before. Fixing it improved the speed by something like 100x. (Missed one spot where I needed a shift register to enforce in-placeness).
This is largely the same approach, merely trying to speed up the lookup/search function via a more complex technique (hash tables). As I found out, the complexity makes it more bug-prone. Still, once fixed, lookup speed is in the order of constant with no clear dependency on the number of elements in the hash table. (Note: it *is* still dependent on the "fill ratio". You want enough memory available to size your table for maybe ~25% usage or less.)
3. One idea I tried out was to maintain a separate list of the 10 or 25 most-common words, sorted by count. (The main reason was that there could be a tie for 4th place at the end, and I wanted enough extra candidates around to get the correct tie-breaker result.) Each time I parsed another word and incremented its count, I'd check to see whether its new count qualified it to belong in the special most-common list. I still did a lot of lookup and sort stuff, but it was always on a very
There are surely other kinds of data structures and algorithms available. Do you have a specific app in mind for this? Or just generally curious? I ask because some of the things one might do to optimize for a specific app (or code challenge) wouldn't be so appropriate as a general-purpose routine.
-Kevin P. -
I use the following code to populate a string array:
File dirFile = new File("C:\\somedir");
String fileImport[] = dirFile.list();
This gives me a string array with a whole bunch of files in the following format:
XXXDDMMYYHHMISS.xml
What I need to know is what is the easiest way to sort this array based on this date format, or any date format, in the ascending order, so that when I am loading my XML files, I get the oldest one first.
Appreciate any input.
SamUse the String name of the file (the Date part), together with a java.text.SimpleDateFormat object to parse() the String. You have to set the formatter with a pattern - these are explained fully in the Javadocs for the SimpleDateFormat class. After parsing, you will have java.util.Date objects for each of the files - you can then use these as keys in a java.util.SortedMap (the values would be the files) - the natural ordering of dates will ensure that they are ordered appropriately
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