Mixing
Weird Java Concurrent modification exception example
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up vote
6
down vote
favorite
If we write like this, there is a concurrent modification exception :
public static void main(String... args)
List<String> listOfBooks = new ArrayList<>();
listOfBooks.add("Programming Pearls");
listOfBooks.add("Clean Code");
listOfBooks.add("Effective Java");
listOfBooks.add("Code Complete");
System.err.println("Before deleting : " + listOfBooks);
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
System.err.println("After deleting : " + listOfBooks);
On the other hand, if we write like this, there is NO concurrent modification exception !
Notice that code is exact the same, except the strings for compare, in first example it is a Code, and in second it is a Java
public static void main(String... args)
List<String> listOfBooks = new ArrayList<>();
listOfBooks.add("Programming Pearls");
listOfBooks.add("Clean Code");
listOfBooks.add("Effective Java");
listOfBooks.add("Code Complete");
System.err.println("Before deleting : " + listOfBooks);
for (String book : listOfBooks)
if (book.contains("Java"))
listOfBooks.remove(book);
System.err.println("After deleting : " + listOfBooks);
I'm using Netbeans 8.2, Windows 7 32bit, with JDK 1.8.0_131
What's wrong ?
java exception
asked 4 hours ago
dobrivoje
876
add a comment |Â
up vote
6
down vote
favorite
If we write like this, there is a concurrent modification exception :
public static void main(String... args)
List<String> listOfBooks = new ArrayList<>();
listOfBooks.add("Programming Pearls");
listOfBooks.add("Clean Code");
listOfBooks.add("Effective Java");
listOfBooks.add("Code Complete");
System.err.println("Before deleting : " + listOfBooks);
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
System.err.println("After deleting : " + listOfBooks);
On the other hand, if we write like this, there is NO concurrent modification exception !
Notice that code is exact the same, except the strings for compare, in first example it is a Code, and in second it is a Java
public static void main(String... args)
List<String> listOfBooks = new ArrayList<>();
listOfBooks.add("Programming Pearls");
listOfBooks.add("Clean Code");
listOfBooks.add("Effective Java");
listOfBooks.add("Code Complete");
System.err.println("Before deleting : " + listOfBooks);
for (String book : listOfBooks)
if (book.contains("Java"))
listOfBooks.remove(book);
System.err.println("After deleting : " + listOfBooks);
I'm using Netbeans 8.2, Windows 7 32bit, with JDK 1.8.0_131
What's wrong ?
java exception
asked 4 hours ago
dobrivoje
876
add a comment |Â
up vote
6
down vote
favorite
up vote
6
down vote
favorite
If we write like this, there is a concurrent modification exception :
public static void main(String... args)
List<String> listOfBooks = new ArrayList<>();
listOfBooks.add("Programming Pearls");
listOfBooks.add("Clean Code");
listOfBooks.add("Effective Java");
listOfBooks.add("Code Complete");
System.err.println("Before deleting : " + listOfBooks);
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
System.err.println("After deleting : " + listOfBooks);
On the other hand, if we write like this, there is NO concurrent modification exception !
Notice that code is exact the same, except the strings for compare, in first example it is a Code, and in second it is a Java
public static void main(String... args)
List<String> listOfBooks = new ArrayList<>();
listOfBooks.add("Programming Pearls");
listOfBooks.add("Clean Code");
listOfBooks.add("Effective Java");
listOfBooks.add("Code Complete");
System.err.println("Before deleting : " + listOfBooks);
for (String book : listOfBooks)
if (book.contains("Java"))
listOfBooks.remove(book);
System.err.println("After deleting : " + listOfBooks);
I'm using Netbeans 8.2, Windows 7 32bit, with JDK 1.8.0_131
What's wrong ?
java exception
asked 4 hours ago
dobrivoje
876
If we write like this, there is a concurrent modification exception :
public static void main(String... args)
List<String> listOfBooks = new ArrayList<>();
listOfBooks.add("Programming Pearls");
listOfBooks.add("Clean Code");
listOfBooks.add("Effective Java");
listOfBooks.add("Code Complete");
System.err.println("Before deleting : " + listOfBooks);
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
System.err.println("After deleting : " + listOfBooks);
On the other hand, if we write like this, there is NO concurrent modification exception !
Notice that code is exact the same, except the strings for compare, in first example it is a Code, and in second it is a Java
public static void main(String... args)
List<String> listOfBooks = new ArrayList<>();
listOfBooks.add("Programming Pearls");
listOfBooks.add("Clean Code");
listOfBooks.add("Effective Java");
listOfBooks.add("Code Complete");
System.err.println("Before deleting : " + listOfBooks);
for (String book : listOfBooks)
if (book.contains("Java"))
listOfBooks.remove(book);
System.err.println("After deleting : " + listOfBooks);
I'm using Netbeans 8.2, Windows 7 32bit, with JDK 1.8.0_131
What's wrong ?
java exception
java exception
asked 4 hours ago
dobrivoje
876
asked 4 hours ago
dobrivoje
876
asked 4 hours ago
dobrivoje
876
asked 4 hours ago
dobrivoje
876
asked 4 hours ago
dobrivoje
876
876
add a comment |Â
add a comment |Â
3 Answers
3
active
oldest
votes
up vote
5
down vote
List.remove() will not throw ConcurrentModificationException when it removes the second last element from the list.
Quoting from this Java Bug (JDK-4902078) .
When the Collections Framework was added to the platform it was deemed too expensive to check for comodification once rather than twice per iteration; the check was made on Iterator.next rather than Iterator.hasNext. Expert reviewers thought this was sufficient. They were unaware that it fails to detect one important case: if an element is removed from the list immediately prior to the final call to hasNext in an iteration, the call returns false and the iteration terminates, silently ignoring the last element on the list.
You can also check this answer :-
https://stackoverflow.com/a/8189786/1992276
answered 3 hours ago
GanitK
311315
What i concluded is that this bug Java Bug (JDK-4902078) still persists. I simply added more "add()" lines, and CM exception occurs only on elements not equals to before-last-element. If I correctly understood this bug status, this is closed and won't fixed task ? This means that decision is made that this bug will stay present, right ?
– dobrivoje
1 hour ago
add a comment |Â
up vote
3
down vote
There are two ways used to iterate over an collection: enumeration and iterator.
First one allows for the collection to be modified during iteration (fail slow), second does not (fail fast). In a for-each loop you are using an iterator, so any modification to the collection, during it's iteration would cause an exception.
You have 3 choices, to solve this problem:
Use an iterator instead:
Iterator<String> bookIt = listOfBooks.iterator();
while(bookIt.hasNext())
String book = bookIt.next();
if (book.contains("Java"))
bookIt.remove();
Create a new list with only acceptable elements (filter out the unwanted):
List<String> booksWithNoCode = listOfBooks.stream()
.filter(book-> !book.contains("Code"))
.collect(toList())
Use Collection.removeIf(), you will remove all elements from the list, that are matching given criteria.
listOfBooks.removeIf(book-> book.contains("Code"))
You can find more information in this post and here.
answered 4 hours ago
Beri
7,82531638
2
I agree on your proposals, I'm aware that the Iterator solution is the most correct one along with a stream solution (which works as it is unmodifiable) ,but that's not my question. I expect in both situations CM exception, and I get it only in case of "Code", not in case of "Java"
– dobrivoje
4 hours ago
What do you mean by "enumerator"? That isn't a term used in Java.
– chrylis
3 hours ago
Sorry, I have fixed that.
– Beri
2 hours ago
add a comment |Â
up vote
2
down vote
You can't modify the listOfBooks while you are iterating though it with the for each loop.
edit:
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
Is the same as:
for (Iterator<String> i = listOfBooks.iterator(); i.hasNext();)
String book = i.next();
if (book.contains("Code"))
listOfBooks.remove(book);
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/tip/src/share/classes/java/util/ArrayList.java
The key in the arraylist code is:
public boolean remove(Object o)
if (o == null)
for (int index = 0; index < size; index++)
if (elementData[index] == null)
fastRemove(index);
return true;
else
for (int index = 0; index < size; index++)
if (o.equals(elementData[index]))
fastRemove(index);
return true;
return false;
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index)
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
and the iterator code:
public boolean hasNext()
return cursor != size;
@SuppressWarnings("unchecked")
public E next()
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
The cursor always points to the next element so when you get the "Effective Java" i = 2 but cursor is 3.
When you call the remove the cursor is at 3 and the size is 4.
The size is then decremented by the remove and now cursor == size and the next hasNext() returns false ending the loop.
answered 4 hours ago
Charles
32916
that's true, however, the only difference are the data theme self, so I expected in both cases to see exception, don't you agree ?
– dobrivoje
4 hours ago
2
hmm thats interesting... It seems you can only remove the second to the last element safely... I will dig into it a bit see if I find something :)
– Charles
4 hours ago
add a comment |Â
3 Answers
3
active
oldest
votes
3 Answers
3
active
oldest
votes
active
oldest
votes
active
oldest
votes
up vote
5
down vote
List.remove() will not throw ConcurrentModificationException when it removes the second last element from the list.
Quoting from this Java Bug (JDK-4902078) .
When the Collections Framework was added to the platform it was deemed too expensive to check for comodification once rather than twice per iteration; the check was made on Iterator.next rather than Iterator.hasNext. Expert reviewers thought this was sufficient. They were unaware that it fails to detect one important case: if an element is removed from the list immediately prior to the final call to hasNext in an iteration, the call returns false and the iteration terminates, silently ignoring the last element on the list.
You can also check this answer :-
https://stackoverflow.com/a/8189786/1992276
answered 3 hours ago
GanitK
311315
What i concluded is that this bug Java Bug (JDK-4902078) still persists. I simply added more "add()" lines, and CM exception occurs only on elements not equals to before-last-element. If I correctly understood this bug status, this is closed and won't fixed task ? This means that decision is made that this bug will stay present, right ?
– dobrivoje
1 hour ago
add a comment |Â
up vote
5
down vote
List.remove() will not throw ConcurrentModificationException when it removes the second last element from the list.
Quoting from this Java Bug (JDK-4902078) .
When the Collections Framework was added to the platform it was deemed too expensive to check for comodification once rather than twice per iteration; the check was made on Iterator.next rather than Iterator.hasNext. Expert reviewers thought this was sufficient. They were unaware that it fails to detect one important case: if an element is removed from the list immediately prior to the final call to hasNext in an iteration, the call returns false and the iteration terminates, silently ignoring the last element on the list.
You can also check this answer :-
https://stackoverflow.com/a/8189786/1992276
answered 3 hours ago
GanitK
311315
What i concluded is that this bug Java Bug (JDK-4902078) still persists. I simply added more "add()" lines, and CM exception occurs only on elements not equals to before-last-element. If I correctly understood this bug status, this is closed and won't fixed task ? This means that decision is made that this bug will stay present, right ?
– dobrivoje
1 hour ago
add a comment |Â
up vote
5
down vote
up vote
5
down vote
List.remove() will not throw ConcurrentModificationException when it removes the second last element from the list.
Quoting from this Java Bug (JDK-4902078) .
When the Collections Framework was added to the platform it was deemed too expensive to check for comodification once rather than twice per iteration; the check was made on Iterator.next rather than Iterator.hasNext. Expert reviewers thought this was sufficient. They were unaware that it fails to detect one important case: if an element is removed from the list immediately prior to the final call to hasNext in an iteration, the call returns false and the iteration terminates, silently ignoring the last element on the list.
You can also check this answer :-
https://stackoverflow.com/a/8189786/1992276
answered 3 hours ago
GanitK
311315
List.remove() will not throw ConcurrentModificationException when it removes the second last element from the list.
Quoting from this Java Bug (JDK-4902078) .
When the Collections Framework was added to the platform it was deemed too expensive to check for comodification once rather than twice per iteration; the check was made on Iterator.next rather than Iterator.hasNext. Expert reviewers thought this was sufficient. They were unaware that it fails to detect one important case: if an element is removed from the list immediately prior to the final call to hasNext in an iteration, the call returns false and the iteration terminates, silently ignoring the last element on the list.
You can also check this answer :-
https://stackoverflow.com/a/8189786/1992276
answered 3 hours ago
GanitK
311315
answered 3 hours ago
GanitK
311315
answered 3 hours ago
GanitK
311315
answered 3 hours ago
GanitK
311315
311315
What i concluded is that this bug Java Bug (JDK-4902078) still persists. I simply added more "add()" lines, and CM exception occurs only on elements not equals to before-last-element. If I correctly understood this bug status, this is closed and won't fixed task ? This means that decision is made that this bug will stay present, right ?
– dobrivoje
1 hour ago
add a comment |Â
What i concluded is that this bug Java Bug (JDK-4902078) still persists. I simply added more "add()" lines, and CM exception occurs only on elements not equals to before-last-element. If I correctly understood this bug status, this is closed and won't fixed task ? This means that decision is made that this bug will stay present, right ?
– dobrivoje
1 hour ago
What i concluded is that this bug Java Bug (JDK-4902078) still persists. I simply added more "add()" lines, and CM exception occurs only on elements not equals to before-last-element. If I correctly understood this bug status, this is closed and won't fixed task ? This means that decision is made that this bug will stay present, right ?
– dobrivoje
1 hour ago
What i concluded is that this bug Java Bug (JDK-4902078) still persists. I simply added more "add()" lines, and CM exception occurs only on elements not equals to before-last-element. If I correctly understood this bug status, this is closed and won't fixed task ? This means that decision is made that this bug will stay present, right ?
– dobrivoje
1 hour ago
add a comment |Â
up vote
3
down vote
There are two ways used to iterate over an collection: enumeration and iterator.
First one allows for the collection to be modified during iteration (fail slow), second does not (fail fast). In a for-each loop you are using an iterator, so any modification to the collection, during it's iteration would cause an exception.
You have 3 choices, to solve this problem:
Use an iterator instead:
Iterator<String> bookIt = listOfBooks.iterator();
while(bookIt.hasNext())
String book = bookIt.next();
if (book.contains("Java"))
bookIt.remove();
Create a new list with only acceptable elements (filter out the unwanted):
List<String> booksWithNoCode = listOfBooks.stream()
.filter(book-> !book.contains("Code"))
.collect(toList())
Use Collection.removeIf(), you will remove all elements from the list, that are matching given criteria.
listOfBooks.removeIf(book-> book.contains("Code"))
You can find more information in this post and here.
answered 4 hours ago
Beri
7,82531638
2
I agree on your proposals, I'm aware that the Iterator solution is the most correct one along with a stream solution (which works as it is unmodifiable) ,but that's not my question. I expect in both situations CM exception, and I get it only in case of "Code", not in case of "Java"
– dobrivoje
4 hours ago
What do you mean by "enumerator"? That isn't a term used in Java.
– chrylis
3 hours ago
Sorry, I have fixed that.
– Beri
2 hours ago
add a comment |Â
up vote
3
down vote
There are two ways used to iterate over an collection: enumeration and iterator.
First one allows for the collection to be modified during iteration (fail slow), second does not (fail fast). In a for-each loop you are using an iterator, so any modification to the collection, during it's iteration would cause an exception.
You have 3 choices, to solve this problem:
Use an iterator instead:
Iterator<String> bookIt = listOfBooks.iterator();
while(bookIt.hasNext())
String book = bookIt.next();
if (book.contains("Java"))
bookIt.remove();
Create a new list with only acceptable elements (filter out the unwanted):
List<String> booksWithNoCode = listOfBooks.stream()
.filter(book-> !book.contains("Code"))
.collect(toList())
Use Collection.removeIf(), you will remove all elements from the list, that are matching given criteria.
listOfBooks.removeIf(book-> book.contains("Code"))
You can find more information in this post and here.
answered 4 hours ago
Beri
7,82531638
2
I agree on your proposals, I'm aware that the Iterator solution is the most correct one along with a stream solution (which works as it is unmodifiable) ,but that's not my question. I expect in both situations CM exception, and I get it only in case of "Code", not in case of "Java"
– dobrivoje
4 hours ago
What do you mean by "enumerator"? That isn't a term used in Java.
– chrylis
3 hours ago
Sorry, I have fixed that.
– Beri
2 hours ago
add a comment |Â
up vote
3
down vote
up vote
3
down vote
There are two ways used to iterate over an collection: enumeration and iterator.
First one allows for the collection to be modified during iteration (fail slow), second does not (fail fast). In a for-each loop you are using an iterator, so any modification to the collection, during it's iteration would cause an exception.
You have 3 choices, to solve this problem:
Use an iterator instead:
Iterator<String> bookIt = listOfBooks.iterator();
while(bookIt.hasNext())
String book = bookIt.next();
if (book.contains("Java"))
bookIt.remove();
Create a new list with only acceptable elements (filter out the unwanted):
List<String> booksWithNoCode = listOfBooks.stream()
.filter(book-> !book.contains("Code"))
.collect(toList())
Use Collection.removeIf(), you will remove all elements from the list, that are matching given criteria.
listOfBooks.removeIf(book-> book.contains("Code"))
You can find more information in this post and here.
answered 4 hours ago
Beri
7,82531638
There are two ways used to iterate over an collection: enumeration and iterator.
First one allows for the collection to be modified during iteration (fail slow), second does not (fail fast). In a for-each loop you are using an iterator, so any modification to the collection, during it's iteration would cause an exception.
You have 3 choices, to solve this problem:
Use an iterator instead:
Iterator<String> bookIt = listOfBooks.iterator();
while(bookIt.hasNext())
String book = bookIt.next();
if (book.contains("Java"))
bookIt.remove();
Create a new list with only acceptable elements (filter out the unwanted):
List<String> booksWithNoCode = listOfBooks.stream()
.filter(book-> !book.contains("Code"))
.collect(toList())
Use Collection.removeIf(), you will remove all elements from the list, that are matching given criteria.
listOfBooks.removeIf(book-> book.contains("Code"))
You can find more information in this post and here.
answered 4 hours ago
Beri
7,82531638
edited 3 hours ago
answered 4 hours ago
Beri
7,82531638
answered 4 hours ago
Beri
7,82531638
answered 4 hours ago
Beri
7,82531638
7,82531638
2
I agree on your proposals, I'm aware that the Iterator solution is the most correct one along with a stream solution (which works as it is unmodifiable) ,but that's not my question. I expect in both situations CM exception, and I get it only in case of "Code", not in case of "Java"
– dobrivoje
4 hours ago
What do you mean by "enumerator"? That isn't a term used in Java.
– chrylis
3 hours ago
Sorry, I have fixed that.
– Beri
2 hours ago
add a comment |Â
2
I agree on your proposals, I'm aware that the Iterator solution is the most correct one along with a stream solution (which works as it is unmodifiable) ,but that's not my question. I expect in both situations CM exception, and I get it only in case of "Code", not in case of "Java"
– dobrivoje
4 hours ago
What do you mean by "enumerator"? That isn't a term used in Java.
– chrylis
3 hours ago
Sorry, I have fixed that.
– Beri
2 hours ago
2
2
I agree on your proposals, I'm aware that the Iterator solution is the most correct one along with a stream solution (which works as it is unmodifiable) ,but that's not my question. I expect in both situations CM exception, and I get it only in case of "Code", not in case of "Java"
– dobrivoje
4 hours ago
I agree on your proposals, I'm aware that the Iterator solution is the most correct one along with a stream solution (which works as it is unmodifiable) ,but that's not my question. I expect in both situations CM exception, and I get it only in case of "Code", not in case of "Java"
– dobrivoje
4 hours ago
What do you mean by "enumerator"? That isn't a term used in Java.
– chrylis
3 hours ago
What do you mean by "enumerator"? That isn't a term used in Java.
– chrylis
3 hours ago
Sorry, I have fixed that.
– Beri
2 hours ago
Sorry, I have fixed that.
– Beri
2 hours ago
add a comment |Â
up vote
2
down vote
You can't modify the listOfBooks while you are iterating though it with the for each loop.
edit:
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
Is the same as:
for (Iterator<String> i = listOfBooks.iterator(); i.hasNext();)
String book = i.next();
if (book.contains("Code"))
listOfBooks.remove(book);
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/tip/src/share/classes/java/util/ArrayList.java
The key in the arraylist code is:
public boolean remove(Object o)
if (o == null)
for (int index = 0; index < size; index++)
if (elementData[index] == null)
fastRemove(index);
return true;
else
for (int index = 0; index < size; index++)
if (o.equals(elementData[index]))
fastRemove(index);
return true;
return false;
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index)
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
and the iterator code:
public boolean hasNext()
return cursor != size;
@SuppressWarnings("unchecked")
public E next()
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
The cursor always points to the next element so when you get the "Effective Java" i = 2 but cursor is 3.
When you call the remove the cursor is at 3 and the size is 4.
The size is then decremented by the remove and now cursor == size and the next hasNext() returns false ending the loop.
answered 4 hours ago
Charles
32916
that's true, however, the only difference are the data theme self, so I expected in both cases to see exception, don't you agree ?
– dobrivoje
4 hours ago
2
hmm thats interesting... It seems you can only remove the second to the last element safely... I will dig into it a bit see if I find something :)
– Charles
4 hours ago
add a comment |Â
up vote
2
down vote
You can't modify the listOfBooks while you are iterating though it with the for each loop.
edit:
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
Is the same as:
for (Iterator<String> i = listOfBooks.iterator(); i.hasNext();)
String book = i.next();
if (book.contains("Code"))
listOfBooks.remove(book);
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/tip/src/share/classes/java/util/ArrayList.java
The key in the arraylist code is:
public boolean remove(Object o)
if (o == null)
for (int index = 0; index < size; index++)
if (elementData[index] == null)
fastRemove(index);
return true;
else
for (int index = 0; index < size; index++)
if (o.equals(elementData[index]))
fastRemove(index);
return true;
return false;
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index)
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
and the iterator code:
public boolean hasNext()
return cursor != size;
@SuppressWarnings("unchecked")
public E next()
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
The cursor always points to the next element so when you get the "Effective Java" i = 2 but cursor is 3.
When you call the remove the cursor is at 3 and the size is 4.
The size is then decremented by the remove and now cursor == size and the next hasNext() returns false ending the loop.
answered 4 hours ago
Charles
32916
that's true, however, the only difference are the data theme self, so I expected in both cases to see exception, don't you agree ?
– dobrivoje
4 hours ago
2
hmm thats interesting... It seems you can only remove the second to the last element safely... I will dig into it a bit see if I find something :)
– Charles
4 hours ago
add a comment |Â
up vote
2
down vote
up vote
2
down vote
You can't modify the listOfBooks while you are iterating though it with the for each loop.
edit:
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
Is the same as:
for (Iterator<String> i = listOfBooks.iterator(); i.hasNext();)
String book = i.next();
if (book.contains("Code"))
listOfBooks.remove(book);
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/tip/src/share/classes/java/util/ArrayList.java
The key in the arraylist code is:
public boolean remove(Object o)
if (o == null)
for (int index = 0; index < size; index++)
if (elementData[index] == null)
fastRemove(index);
return true;
else
for (int index = 0; index < size; index++)
if (o.equals(elementData[index]))
fastRemove(index);
return true;
return false;
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index)
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
and the iterator code:
public boolean hasNext()
return cursor != size;
@SuppressWarnings("unchecked")
public E next()
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
The cursor always points to the next element so when you get the "Effective Java" i = 2 but cursor is 3.
When you call the remove the cursor is at 3 and the size is 4.
The size is then decremented by the remove and now cursor == size and the next hasNext() returns false ending the loop.
answered 4 hours ago
Charles
32916
You can't modify the listOfBooks while you are iterating though it with the for each loop.
edit:
for (String book : listOfBooks)
if (book.contains("Code"))
listOfBooks.remove(book);
Is the same as:
for (Iterator<String> i = listOfBooks.iterator(); i.hasNext();)
String book = i.next();
if (book.contains("Code"))
listOfBooks.remove(book);
http://hg.openjdk.java.net/jdk8/jdk8/jdk/file/tip/src/share/classes/java/util/ArrayList.java
The key in the arraylist code is:
public boolean remove(Object o)
if (o == null)
for (int index = 0; index < size; index++)
if (elementData[index] == null)
fastRemove(index);
return true;
else
for (int index = 0; index < size; index++)
if (o.equals(elementData[index]))
fastRemove(index);
return true;
return false;
/*
* Private remove method that skips bounds checking and does not
* return the value removed.
*/
private void fastRemove(int index)
modCount++;
int numMoved = size - index - 1;
if (numMoved > 0)
System.arraycopy(elementData, index+1, elementData, index,
numMoved);
elementData[--size] = null; // clear to let GC do its work
and the iterator code:
public boolean hasNext()
return cursor != size;
@SuppressWarnings("unchecked")
public E next()
checkForComodification();
int i = cursor;
if (i >= size)
throw new NoSuchElementException();
Object elementData = ArrayList.this.elementData;
if (i >= elementData.length)
throw new ConcurrentModificationException();
cursor = i + 1;
return (E) elementData[lastRet = i];
The cursor always points to the next element so when you get the "Effective Java" i = 2 but cursor is 3.
When you call the remove the cursor is at 3 and the size is 4.
The size is then decremented by the remove and now cursor == size and the next hasNext() returns false ending the loop.
answered 4 hours ago
Charles
32916
edited 3 hours ago
answered 4 hours ago
Charles
32916
answered 4 hours ago
Charles
32916
answered 4 hours ago
Charles
32916
32916
that's true, however, the only difference are the data theme self, so I expected in both cases to see exception, don't you agree ?
– dobrivoje
4 hours ago
2
hmm thats interesting... It seems you can only remove the second to the last element safely... I will dig into it a bit see if I find something :)
– Charles
4 hours ago
add a comment |Â
that's true, however, the only difference are the data theme self, so I expected in both cases to see exception, don't you agree ?
– dobrivoje
4 hours ago
2
hmm thats interesting... It seems you can only remove the second to the last element safely... I will dig into it a bit see if I find something :)
– Charles
4 hours ago
that's true, however, the only difference are the data theme self, so I expected in both cases to see exception, don't you agree ?
– dobrivoje
4 hours ago
that's true, however, the only difference are the data theme self, so I expected in both cases to see exception, don't you agree ?
– dobrivoje
4 hours ago
2
2
hmm thats interesting... It seems you can only remove the second to the last element safely... I will dig into it a bit see if I find something :)
– Charles
4 hours ago
hmm thats interesting... It seems you can only remove the second to the last element safely... I will dig into it a bit see if I find something :)
– Charles
4 hours ago
add a comment |Â
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