In the conventional loop, we could have as below, making single nested layer.
for (int i=0; listObject != null && i < listObject.size(); i++) {
// Do whatever we want
}
However, using the below style for each loop, I'll need a double nested code: -
if (listObject != null) {
for (Object object: listObject) {
// Do whatever we want
}
}
Is it possible to embed the listObject != null condition into the for-loop statement to make it single nested code?
Your second example is clear, easily understood code. There is nothing wrong with nesting a for loop in an if block. It's even more clear than your first example.
But if you insist on combining them, you can use the ternary operator to supply an empty list if listObject is null. Using Collections.emptyList means no iterations will take place and no NullPointerException will be thrown.
for (Object object : listObject == null ? Collections.emptyList() : listObject)
I don't think I would use code like this when a clear example such as your second example already exists, but this code does provide a way to get two colons inside a for loop.
To make it concise on, while having a single nested loop, I decided to make it into function as below
void checkCondition(List<Object> listObject) {
if (listObject == null) return;
for (Object object: listObject) {
// Do whatever
}
}
Related
String var;
while((var = "abc") == "abc"){
System.out.println("In loop");
}
What is the advantage of assigning a variable while doing a check for the condition in the while loop.
In that example, there is none, but I assume you're talking about something like:
while ((var = obj.someMethod()) != null) {
// ...use var...
}
...where null is any of several marker values depending on what obj and someMethod are. For instance, using BufferedReader's readLine, you might loop through lines like this:
while ((line = reader.nextLine()) != null) {
// ...use the line...
}
This is a fairly common idiom when dealing with objects that have a method that keeps returning something useful until/unless it reaches the "end" of what it's working through, at which point it returns a marker value saying it's done (null is a common choice). The idiom is useful because it advances to the "next" thing, remembers the "next" thing, and checks for whether it's done.
But in your example, there'd be no point whatsoever. Also, it compares strings incorrectly. :-)
In Java it is possible to declare a variable in the initialization part of a for-loop:
for ( int i=0; i < 10; i++) {
// do something (with i)
}
But with the while statement this seems not to be possible.
Quite often I see code like this, when the conditional for the while loop needs to be updated after every iteration:
List<Object> processables = retrieveProcessableItems(..); // initial fetch
while (processables.size() > 0) {
// process items
processables = retrieveProcessableItems(..); // update
}
Here on stackoverflow I found at least a solution to prevent the duplicate code of fetching the processable items:
List<Object> processables;
while ((processables = retrieveProcessableItems(..)).size() > 0) {
// process items
}
But the variable still has to be declared outside the while-loop.
So, as I want to keep my variable scopes clean, is it possible to declare a variable within the while conditional, or is there some other solution for such cases?
You can write a while loop using a for loop:
while (condition) { ... }
is the same as
for (; condition; ) { ... }
since all three bits in the brackets of the basic for statement declaration are optional:
BasicForStatement:
for ( [ForInit] ; [Expression] ; [ForUpdate] ) Statement
Similarly, you can just rewrite your while loop as a for loop:
for (List<Object> processables;
(processables = retrieveProcessableItems(..)).size() > 0;) {
// ... Process items.
}
Note that some static analysis tools (e.g. eclipse 4.5) might demand that an initial value is assigned to processables, e.g. List<Object> processables = null. This is incorrect, according to JLS; my version of javac does not complain if the variable is left initially unassigned.
No it's not possible.
It doesn't really make too much sense either: unlike a for loop where you can set up the initial state of the "looping variable", in a while loop you test the value of an existing variable, akin to the conditional check of the for loop.
Of course, if you're concerned about variables "leaking" into other parts of your code, you could enclose the whole thing in an extra scope block:
{
/*declare variable here*/
while(...){...}
}
Alternatively, convert the while loop into a for loop.
Make a do/while:
String value;
do {
value = getValue();
...your processing
} while (value != null && !value.isEmpty());
I am attempting to write code to traverse a collection of type InstallationComponentSetup:
java.util.Collection<InstallationComponentSetup> components= context.getInstallationComponents();
Iterator it = components.iterator();
while (it.hasNext())
{
if (((InstallationComponentSetup)it).getName() == "ACQ")
{
return true;
}
}
The cast in the if-statement fails, but I don't really know why (I am a C++ programmer!).
If someone could give me some pointers as to what I am doing wrong I would be grateful.
it is an Iterator, whereas it.next() is an InstallationComponentSetup.
The error results from the fact that an Iterator cannot be cast as an InstallationComponentSetup.
Also, you shouldn't even need to cast if you parametrize the Iterator appropriately:
Iterator<InstallationComponentSetup> it = components.iterator();
Finally, don't compare strings with something like a == b, instead use a.equals(b). See "How do I compare strings in Java" for further details.
You might also want to look into the for-each loop if all you want to do is iterate over the collection. Your code can be rewritten as:
for (InstallationComponentSetup component : components)
if (component.getName().equals("ACQ"))
return true;
If you are comparing String , use equals() method .
Even your casting is wrong.You have to invoke next() on the iterator to get the next element . Hence it.next() gives you the next element which will be an object of InstallationComponentSetup, it is not of type InstallationComponentSetup hence the cast will fail.
Here you are casting the Iterator to your class type which will fail.
if (((InstallationComponentSetup)it).getName() == "ACQ")
{
return true;
}
I believe there is no need of cast here as you have defined the Collection to hold the specific type of element and also if you declare the Iterator of a specific type.
You can simply do :
// define Iterator of InstallationComponentSetup
Iterator<InstallationComponentSetup> it = components.iterator();
if("ACQ".equals(it.next().getName())) {
return true;
}
You can also consider using the enhanced for loop in Java , if your purpose is only to read the elements .
for(InstallationComponentSetup component: components) {
if("ACQ".equals(component.getName())) {
return true;
}
}
You have to retrieve the next element in the iteration before you compare:
InstallationComponentSetup next = it.next();
if (next.getName() == "ACQ")
{
return true;
}
Try to use the following code. It is more concise and easier to understand.
Collection<InstallationComponentSetup> components= context.getInstallationComponents();
for(InstallationComponentSetup comp : components){
if("ACQ".equals(comp.getName()){
return;
}
}
I think you had two problems in you code.
Cast the iterator to an object doesn't work like that. You need to use it.next() to get the object and move the iterator.
like already mentioned you need equals to compare Strings. == compares "memory locations" (in C++ terms).
Use it.next() to get the next element.
Also, use the .equals() method to compare strings in Java. Otherwise, the references are compared.
Finally, the cast should be unnecessary with a type-parameterized Iterator.
while (it.hasNext())
{
if ( it.next().getName().equals("ACQ") ) {
...
}
}
You have to retrieve the next element in the iteration before you compare:
java.util.Collection<InstallationComponentSetup> components= context.getInstallationComponents();
Iterator<InstallationComponentSetup> it = components.iterator();
while (it.hasNext()) {
if ("ACQ".equals(it.next().getName())) {
return true;
}
}
It would be easier to use foreach loop, make use of generic type, use equals for String and change string comparison order to be null secure.
Collection<InstallationComponentSetup> components= context.getInstallationComponents();
for (InstallationComponentSetup setup : components)
{
if ("ACQ".equals(setup.getName()))
{
return true;
}
}
The install4j API is still for Java 1.4, so there are no generics yet. This will work:
for (Object o : context.getInstallationComponents()) {
InstallationComponentSetup component = (InstallationComponentSetup)o;
if (component.getName().equals("ACQ")) {
return true;
}
}
if have the following problem:
I have a List which i am going through using the enhanced for loop. Every time i want to remove sth, out of the list, i get a ConcurrentModificationException. I already found out why this exception is thrown, but i don`t know how i can modify my code, so that its working. This is my code:
for(Subject s : SerData.schedule)
{
//Checking of the class is already existing
for(Classes c : s.classes)
{
if(c.day == day &c.which_class == which_class)
{
int index = getclassesindex(s.classes, new Classes(day, which_class));
synchronized (s) {
s.classes.remove(index);
}
}
}
//More code....
}
I also tried out this implementation.
for(Subject s : SerData.schedule)
{
//Checking of the class is already existing
Iterator<Classes> x = s.classes.iterator();
while(x.hasNext())
{
Classes c = x.next();
if(c.day == day &c.which_class == which_class)
{
int index = getclassesindex(s.classes, new Classes(day, which_class));
synchronized (s) {
s.classes.remove(index);
}
}
}
//More code....
}
not working either...
Is there a common used, standard solution? (Hopefully sth. that is not obvious :D )
The main reason this issue occurs is because of the semantic meaning of your for-each loop.
When you use for-each loops, the data structure that is being traversed cannot be modified.
Essentially anything of this form will throw this exception:
for( Object o : objCollection )
{
// ...
if ( satisfiesSomeProperty ( o ) )
objList.remove(o); // This is an error!!
// ...
}
As a side note, you can't add or replace elements in the collection either.
There are a few ways to perform this operation.
One way is to use an iterator and call the remove() method when the object is to be removed.
Iterator <Object> objItr = objCollection.iterator();
while(objItr.hasNext())
{
Object o = objItr.next();
// ...
if ( satifiesSomeProperty ( o ) )
objItr.remove(); // This is okay
// ...
}
This option has the property that removal of the object is done in time proportional to the iterator's remove method.
The next option is to store the objects you want to remove, and then remove them after traversing the list. This may be useful in situations where removal during iteration may produce inconsistent results.
Collection <Object> objsToRemove = // ...
for( Object o : objCollection )
{
// ...
if ( satisfiesSomeProperty ( o ) )
objsToRemove.add (o);
// ...
}
objCollection.removeAll ( objsToRemove );
These two methods work for general Collection types, but for lists, you could use a standard for loop and walk the list from the end of the list to the front, removing what you please.
for (int i = objList.size() - 1; i >= 0; i--)
{
Object o = objList.get(i);
// ...
if ( satisfiesSomeProperty(o) )
objList.remove(i);
// ...
}
Walking in the normal direction and removing could also be done, but you would have to take care of how incrementation occurs; specifically, you don't want to increment i when you remove, since the next element is shifted down to the same index.
for (int i = 0; i < objList.size(); i++)
{
Object o = objList.get(i);
// ...
if ( satisfiesSomeProperty(o) )
{
objList.remove(i);
i--;
}
//caveat: only works if you don't use `i` later here
// ...
}
Hope this provides a good overview of the concepts and helps!
Using Iterator.remove() should prevent the exception from being thrown.
Hm if I get it right you are iterating over a collection of classes and if a given class matches some criteria you are looking for the its index and try to remove it?
Why not just do:
Iterator<Classes> x = s.classes.iterator();
while(x.hasNext()){
Classes c = x.next();
if(c.day == day && c.which_class == which_class) {
x.remove();
}
}
Add synchronization if need be (but I would prefer a concurrent collection if I were you), preferably change the "==" to equals(), add getters/setters etc. Also the convention in java is to name variables and methods using camelCase (and not separating them with "_").
Actually this is one of the cases when you have to use an iterator.
From the javadoc on ConcurrentModificationException:
"if a thread modifies a collection directly while it is iterating over the collection with a fail-fast iterator, the iterator will throw this exception."
So within your
for (Classes c : s.classes)
you are executing
s.classes.remove(index)
and the iterator is doing just what its contract says. Declare the index(es) in a scope outside the loop and remove your target after the loop is done.
Iterator<Classes> classesIterator = s.classes.iterator();
while (classesIterator.hasNext()) {
Classes c = classesIterator.next();
if (c.day == day && c.which_class == which_class) {
classesIterator.remove();
}
}
There is no general solution for Collection subclasses in general - most iterators will become invalid if the collection is modified, unless the modification happens through the iterator itself via Iterator.remove().
There is a potential solution when it comes to List implementations: the List interface has index-based add/get/set/remove operations. Rather than use an Iterator instance, you can iterate through the list explicitly with a counter-based loop, much like with arrays. You should take care, however, to update the loop counter appropriately when inserting or deleting elements.
Your for-each iterator is fail-fast and this is why remove operation fails as it would change the collection while traversing it.
What implementation of List interface are you using?
Noticed synchronisation on Subject, are you using this code concurrently?
If concurrency is the case, then I would recommend using CopyOnWriteArrayList. It doesn't need synchronisation and its for-each iterator doesn't throw ConcurrentModificationException.
Say I have already created an iterator called "iter" and an arraylist called "database". I want to be able to look through the arraylist and see if any element in the arraylist is equal to a String called "test". If it is, then I would like to add the element to another list.
while(iter.hasNext()) {
if(database.next() == test) {
database.next().add(another_list);
}
}
What am I doing wrong? I'm completely new to iterators in java. Do I need to write my own iterator class? Any code examples would be greatly appreciated. Thanks
The problem with your code is that every time you call .next(), it advances the iterator forward to the next position. This means that this code
if(database.next() == test) {
database.next().add(another_list);
}
Won't work as intended, because the first call to database.next() will not give back the same value as the second call to database.next(). To fix this, you'll want to make a temporary variable to hold on to the new value, as seen here:
while(iter.hasNext()) {
/* type */ curr = iter.next();
if(curr == test) {
curr.add(another_list);
}
}
(Filling in the real type of what's being iterated over in place of /* type */)
In many cases, though, you don't need to use iterators explicitly. Most of the Collections types implement the Iterable interface, in which case you can just write
/* container */ c;
for(/* type */ curr: c) {
if(curr == test) {
curr.add(another_list);
}
}
Hope this helps!
if(database.contains("test"))
{
another_list.add("test");
}
you can use the built in method contains(...)
you should use equals(...) for data comparisions
look at the javadoc to see if there is already a method present for your purpose