Why isn't Collections.sort() optimized for ArrayList, but is for LinkedList?

Why does Collections.sort() creates an extra object array and performs Tim sort on the array and then finally copies the sorted array back into List object? I know this call is optimized for LinkedList, but won't we lose out on performance for ArrayList?

We could have avoided 2n number of operations in converting it into an object array and adding them back to the list. I know that these extra operations wouldn't affect the Big-O of the whole sorting operation, but I believe it could've been further optimised for ArrayList.

Am I missing something here? I'm just trying to understand why the architecture is laid out as such. Thanks.

https://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Collections.java#l164

edited Jan 13 at 10:49

Slaw

7,80831033

asked Jan 13 at 10:41

Vineeth Chitteti

713522

Please PM here if the question is a possible duplicate. I will be glad to know that there is already an answer to this question.

– Vineeth Chitteti
Jan 13 at 10:42

add a comment |

Am I missing something here? I'm just trying to understand why the architecture is laid out as such. Thanks.

https://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Collections.java#l164

edited Jan 13 at 10:49

Slaw

7,80831033

asked Jan 13 at 10:41

Vineeth Chitteti

713522

Please PM here if the question is a possible duplicate. I will be glad to know that there is already an answer to this question.

– Vineeth Chitteti
Jan 13 at 10:42

add a comment |

Am I missing something here? I'm just trying to understand why the architecture is laid out as such. Thanks.

https://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Collections.java#l164

edited Jan 13 at 10:49

Slaw

7,80831033

asked Jan 13 at 10:41

Vineeth Chitteti

713522

Am I missing something here? I'm just trying to understand why the architecture is laid out as such. Thanks.

https://hg.openjdk.java.net/jdk8/jdk8/jdk/file/687fd7c7986d/src/share/classes/java/util/Collections.java#l164

java algorithm sorting timsort

edited Jan 13 at 10:49

Slaw

7,80831033

asked Jan 13 at 10:41

Vineeth Chitteti

713522

edited Jan 13 at 10:49

Slaw

7,80831033

asked Jan 13 at 10:41

Vineeth Chitteti

713522

edited Jan 13 at 10:49

Slaw

7,80831033

edited Jan 13 at 10:49

Slaw

7,80831033

edited Jan 13 at 10:49

Slaw

7,80831033

asked Jan 13 at 10:41

Vineeth Chitteti

713522

asked Jan 13 at 10:41

Vineeth Chitteti

713522

asked Jan 13 at 10:41

Vineeth Chitteti

713522

Please PM here if the question is a possible duplicate. I will be glad to know that there is already an answer to this question.

– Vineeth Chitteti
Jan 13 at 10:42

add a comment |

Please PM here if the question is a possible duplicate. I will be glad to know that there is already an answer to this question.

– Vineeth Chitteti
Jan 13 at 10:42

Please PM here if the question is a possible duplicate. I will be glad to know that there is already an answer to this question.

– Vineeth Chitteti
Jan 13 at 10:42

add a comment |

1 Answer
1

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You are looking at an older JDK version. At least since since JDK 1.8.0_162 Collections.sort() calls List's sort(Comparator<? super E> c). And while the default implementation creates an array from the List and sorts that array, ArrayList overrides that default implementation, and sorts the backing array directly.

Collections's sort:

public static <T extends Comparable<? super T>> void sort(List<T> list) {

    list.sort(null);

}

List's sort:

default void sort(Comparator<? super E> c) {

    Object a = this.toArray();

    Arrays.sort(a, (Comparator) c);

    ListIterator<E> i = this.listIterator();

    for (Object e : a) {

        i.next();

        i.set((E) e);

    }

}

ArrayList's sort:

public void sort(Comparator<? super E> c) {

    final int expectedModCount = modCount;

    Arrays.sort((E) elementData, 0, size, c);

    if (modCount != expectedModCount) {

        throw new ConcurrentModificationException();

    }

    modCount++;

}

In other words, this has been addressed in JDK versions newer than the one you are looking at.

You can look at the source here (thanks to eckes's link).

edited Jan 13 at 11:14

answered Jan 13 at 10:44

Eran

283k37458546

1

The link to the source file (supposedly for JDK8, though I don't fully understand the repo's organization) does show Collections.sort using the default List.sort implementation directly.

– Slaw
Jan 13 at 10:48

Not what I see; I see sorting done on the toArray() on line 230 (perhaps you're looking at the synchronized wrapper?). That said, OpenJDK11 is showing what you're saying in its source.

– Joe C
Jan 13 at 10:56

1

@Slaw the linked source file seems to be jdk8-b132. My answer contains code of JDK 1.8.0_162

– Eran
Jan 13 at 10:57

Here's the JDK9 version - hg.openjdk.java.net/jdk9/jdk9/jdk/file/d966fc5a7be5/src/…. I'll link to JDK 1.8.0_162 if I can find it

– Eran
Jan 13 at 11:00

@Eran hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/c00bdbbd9a77/src/share/…

– eckes
Jan 13 at 11:09

add a comment |

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1 Answer
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1 Answer
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Collections's sort:

public static <T extends Comparable<? super T>> void sort(List<T> list) {

    list.sort(null);

}

List's sort:

default void sort(Comparator<? super E> c) {

    Object a = this.toArray();

    Arrays.sort(a, (Comparator) c);

    ListIterator<E> i = this.listIterator();

    for (Object e : a) {

        i.next();

        i.set((E) e);

    }

}

ArrayList's sort:

public void sort(Comparator<? super E> c) {

    final int expectedModCount = modCount;

    Arrays.sort((E) elementData, 0, size, c);

    if (modCount != expectedModCount) {

        throw new ConcurrentModificationException();

    }

    modCount++;

}

In other words, this has been addressed in JDK versions newer than the one you are looking at.

You can look at the source here (thanks to eckes's link).

edited Jan 13 at 11:14

answered Jan 13 at 10:44

Eran

283k37458546

1

The link to the source file (supposedly for JDK8, though I don't fully understand the repo's organization) does show Collections.sort using the default List.sort implementation directly.

– Slaw
Jan 13 at 10:48

Not what I see; I see sorting done on the toArray() on line 230 (perhaps you're looking at the synchronized wrapper?). That said, OpenJDK11 is showing what you're saying in its source.

– Joe C
Jan 13 at 10:56

1

@Slaw the linked source file seems to be jdk8-b132. My answer contains code of JDK 1.8.0_162

– Eran
Jan 13 at 10:57

Here's the JDK9 version - hg.openjdk.java.net/jdk9/jdk9/jdk/file/d966fc5a7be5/src/…. I'll link to JDK 1.8.0_162 if I can find it

– Eran
Jan 13 at 11:00

@Eran hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/c00bdbbd9a77/src/share/…

– eckes
Jan 13 at 11:09

add a comment |

Collections's sort:

public static <T extends Comparable<? super T>> void sort(List<T> list) {

    list.sort(null);

}

List's sort:

default void sort(Comparator<? super E> c) {

    Object a = this.toArray();

    Arrays.sort(a, (Comparator) c);

    ListIterator<E> i = this.listIterator();

    for (Object e : a) {

        i.next();

        i.set((E) e);

    }

}

ArrayList's sort:

public void sort(Comparator<? super E> c) {

    final int expectedModCount = modCount;

    Arrays.sort((E) elementData, 0, size, c);

    if (modCount != expectedModCount) {

        throw new ConcurrentModificationException();

    }

    modCount++;

}

In other words, this has been addressed in JDK versions newer than the one you are looking at.

You can look at the source here (thanks to eckes's link).

edited Jan 13 at 11:14

answered Jan 13 at 10:44

Eran

283k37458546

1

The link to the source file (supposedly for JDK8, though I don't fully understand the repo's organization) does show Collections.sort using the default List.sort implementation directly.

– Slaw
Jan 13 at 10:48

Not what I see; I see sorting done on the toArray() on line 230 (perhaps you're looking at the synchronized wrapper?). That said, OpenJDK11 is showing what you're saying in its source.

– Joe C
Jan 13 at 10:56

1

@Slaw the linked source file seems to be jdk8-b132. My answer contains code of JDK 1.8.0_162

– Eran
Jan 13 at 10:57

Here's the JDK9 version - hg.openjdk.java.net/jdk9/jdk9/jdk/file/d966fc5a7be5/src/…. I'll link to JDK 1.8.0_162 if I can find it

– Eran
Jan 13 at 11:00

@Eran hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/c00bdbbd9a77/src/share/…

– eckes
Jan 13 at 11:09

add a comment |

Collections's sort:

public static <T extends Comparable<? super T>> void sort(List<T> list) {

    list.sort(null);

}

List's sort:

default void sort(Comparator<? super E> c) {

    Object a = this.toArray();

    Arrays.sort(a, (Comparator) c);

    ListIterator<E> i = this.listIterator();

    for (Object e : a) {

        i.next();

        i.set((E) e);

    }

}

ArrayList's sort:

public void sort(Comparator<? super E> c) {

    final int expectedModCount = modCount;

    Arrays.sort((E) elementData, 0, size, c);

    if (modCount != expectedModCount) {

        throw new ConcurrentModificationException();

    }

    modCount++;

}

In other words, this has been addressed in JDK versions newer than the one you are looking at.

You can look at the source here (thanks to eckes's link).

edited Jan 13 at 11:14

answered Jan 13 at 10:44

Eran

283k37458546

Collections's sort:

public static <T extends Comparable<? super T>> void sort(List<T> list) {

    list.sort(null);

}

List's sort:

default void sort(Comparator<? super E> c) {

    Object a = this.toArray();

    Arrays.sort(a, (Comparator) c);

    ListIterator<E> i = this.listIterator();

    for (Object e : a) {

        i.next();

        i.set((E) e);

    }

}

ArrayList's sort:

public void sort(Comparator<? super E> c) {

    final int expectedModCount = modCount;

    Arrays.sort((E) elementData, 0, size, c);

    if (modCount != expectedModCount) {

        throw new ConcurrentModificationException();

    }

    modCount++;

}

In other words, this has been addressed in JDK versions newer than the one you are looking at.

You can look at the source here (thanks to eckes's link).

edited Jan 13 at 11:14

answered Jan 13 at 10:44

Eran

283k37458546

edited Jan 13 at 11:14

answered Jan 13 at 10:44

Eran

283k37458546

answered Jan 13 at 10:44

Eran

283k37458546

answered Jan 13 at 10:44

Eran

283k37458546

1

The link to the source file (supposedly for JDK8, though I don't fully understand the repo's organization) does show Collections.sort using the default List.sort implementation directly.

– Slaw
Jan 13 at 10:48

Not what I see; I see sorting done on the toArray() on line 230 (perhaps you're looking at the synchronized wrapper?). That said, OpenJDK11 is showing what you're saying in its source.

– Joe C
Jan 13 at 10:56

1

@Slaw the linked source file seems to be jdk8-b132. My answer contains code of JDK 1.8.0_162

– Eran
Jan 13 at 10:57

Here's the JDK9 version - hg.openjdk.java.net/jdk9/jdk9/jdk/file/d966fc5a7be5/src/…. I'll link to JDK 1.8.0_162 if I can find it

– Eran
Jan 13 at 11:00

@Eran hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/c00bdbbd9a77/src/share/…

– eckes
Jan 13 at 11:09

add a comment |

1

The link to the source file (supposedly for JDK8, though I don't fully understand the repo's organization) does show Collections.sort using the default List.sort implementation directly.

– Slaw
Jan 13 at 10:48

Not what I see; I see sorting done on the toArray() on line 230 (perhaps you're looking at the synchronized wrapper?). That said, OpenJDK11 is showing what you're saying in its source.

– Joe C
Jan 13 at 10:56

1

@Slaw the linked source file seems to be jdk8-b132. My answer contains code of JDK 1.8.0_162

– Eran
Jan 13 at 10:57

Here's the JDK9 version - hg.openjdk.java.net/jdk9/jdk9/jdk/file/d966fc5a7be5/src/…. I'll link to JDK 1.8.0_162 if I can find it

– Eran
Jan 13 at 11:00

@Eran hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/c00bdbbd9a77/src/share/…

– eckes
Jan 13 at 11:09

The link to the source file (supposedly for JDK8, though I don't fully understand the repo's organization) does show Collections.sort using the default List.sort implementation directly.

– Slaw
Jan 13 at 10:48

Not what I see; I see sorting done on the toArray() on line 230 (perhaps you're looking at the synchronized wrapper?). That said, OpenJDK11 is showing what you're saying in its source.

– Joe C
Jan 13 at 10:56

@Slaw the linked source file seems to be jdk8-b132. My answer contains code of JDK 1.8.0_162

– Eran
Jan 13 at 10:57

Here's the JDK9 version - hg.openjdk.java.net/jdk9/jdk9/jdk/file/d966fc5a7be5/src/…. I'll link to JDK 1.8.0_162 if I can find it

– Eran
Jan 13 at 11:00

@Eran hg.openjdk.java.net/jdk8u/jdk8u/jdk/file/c00bdbbd9a77/src/share/…

– eckes
Jan 13 at 11:09

add a comment |

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