Class ArrayList
Type Parameters: E — the type of elements in this list All Implemented Interfaces: Serializable , Cloneable , Iterable , Collection , List , RandomAccess Direct Known Subclasses: AttributeList , RoleList , RoleUnresolvedList
Resizable-array implementation of the List interface. Implements all optional list operations, and permits all elements, including null . In addition to implementing the List interface, this class provides methods to manipulate the size of the array that is used internally to store the list. (This class is roughly equivalent to Vector , except that it is unsynchronized.)
The size , isEmpty , get , set , iterator , and listIterator operations run in constant time. The add operation runs in amortized constant time, that is, adding n elements requires O(n) time. All of the other operations run in linear time (roughly speaking). The constant factor is low compared to that for the LinkedList implementation.
Each ArrayList instance has a capacity. The capacity is the size of the array used to store the elements in the list. It is always at least as large as the list size. As elements are added to an ArrayList, its capacity grows automatically. The details of the growth policy are not specified beyond the fact that adding an element has constant amortized time cost.
An application can increase the capacity of an ArrayList instance before adding a large number of elements using the ensureCapacity operation. This may reduce the amount of incremental reallocation.
Note that this implementation is not synchronized. If multiple threads access an ArrayList instance concurrently, and at least one of the threads modifies the list structurally, it must be synchronized externally. (A structural modification is any operation that adds or deletes one or more elements, or explicitly resizes the backing array; merely setting the value of an element is not a structural modification.) This is typically accomplished by synchronizing on some object that naturally encapsulates the list. If no such object exists, the list should be «wrapped» using the Collections.synchronizedList method. This is best done at creation time, to prevent accidental unsynchronized access to the list:
List list = Collections.synchronizedList(new ArrayList(. ));
The iterators returned by this class’s iterator and listIterator methods are fail-fast: if the list is structurally modified at any time after the iterator is created, in any way except through the iterator’s own remove or add methods, the iterator will throw a ConcurrentModificationException . Thus, in the face of concurrent modification, the iterator fails quickly and cleanly, rather than risking arbitrary, non-deterministic behavior at an undetermined time in the future.
Note that the fail-fast behavior of an iterator cannot be guaranteed as it is, generally speaking, impossible to make any hard guarantees in the presence of unsynchronized concurrent modification. Fail-fast iterators throw ConcurrentModificationException on a best-effort basis. Therefore, it would be wrong to write a program that depended on this exception for its correctness: the fail-fast behavior of iterators should be used only to detect bugs.
This class is a member of the Java Collections Framework.
Java select specific item from arraylist in java
And then a list of test cases: Given these test cases we would expect to see ‘Hello’ appear about 10.5% of the time (20 / (80 + 90 + 20)) That way you’ll have only one extra class, and you can reuse it later for other use cases Solution 1: It seems like you are trying to find a specific school in the list of schools.
Selecting objects from an ArrayList based on value of certain fields
As far as I understand it, you want to have a method that returns all courses that match a specified condition.
You might consider looking into lambdas. they look a little funky at first, but they’re really powerful and once you get used to it it’s pretty easy.
For your example, you might have this:
import java.util.ArrayList; import java.util.function.Predicate; class Course < float percentage; char courseLevel; String courseName; boolean takingNow; public static ArrayListallCourses; public ArrayList getCourses(Predicate coursePredicate) < ArrayListtoReturn = new ArrayList<>(); for (Course c : allCourses .stream() .filter(coursePredicate) .toArray(Course[]::new)) < toReturn.add(c); >return toReturn; > > This means we can call it like so:
getCourses(c -> c.courseLevel != 'B'); meaning all that don’t have a course level of ‘B’ will be returned.
What’s going on here? OK. First, we take a collection, and turn it into a » Stream «. This allows us to use Predicate s (which are just conditions to match on) to filter the stream. Finally, we can convert the filtered Stream back to an array, and then add those elements back into an ArrayList .
The lambda defines that condition. It is just a function that accepts a single argument of type Course . In my example, it would be equivalent to the following:
static boolean isMatch(Course c)
The filter method loops over all the elements in the Stream and if applying the Predicate (the lambda, equiv to the above method) returns true , it’s included in its result.
I don’t know if you’re using Java 8 or not, but if you are, you could use streams and filter your ArrayList.
The syntax might look like:
return courseList.stream().filter(c -> c.getCourseName().equals(value));
You could have a separate method for byCourseName, byCourseLevel, etc. The principal would be the same.
If you’re not using Java 8, you could still use separate methods and avoid the need for the switch/case.
If you’re using java 8, you can use Streams and in particular ‘filter’ . see for example the ‘Filter’ section in this tutorial: http://zeroturnaround.com/rebellabs/java-8-explained-applying-lambdas-to-java-collections/
If you were only filtering by name, then you can settle for a lambda expression «filter(p -> p.getName().equals(value)). But since you have a more complex case you’re better off invoking a dedicated method.
Copy one item from an ArrayList to another ArrayList Java 8, ArrayList
Control selected items of an ArrayList
I would recommend that you use Set (such as a HashSet ) and add the selected clients to this set.
This avoids adding a field to Client that doesn’t really belong in that class.
Note that if you’re using a JList for instance, you can use ListModel / ListSelectionModel directly. (You don’t need to create / maintain a data structure for holding the selected clients yourself.)
Add a boolean field selected to the class Client
As you’ve pointed out already, the selected state is not a property of the client, but of the user. If you have multiple users, one user may have selected a client, while another user has not. That’s why the selection should be kept in the user’s scope.
Using a Set , as it was already suggested, seems like the best data structure as it avoids duplicates and thus clients can safely be deselected using remove() .
Extends Client into SelectableClient
This approach has the same problem, the selection state is modelled as property of the client, not the user.
I would recommend you to create a new generic class sth like this:
class SelectableItem < private T item; private boolean selected; public SelectableItem(T item, boolean selected) < . >// getters setters > and create array or set of this type. That way you’ll have only one extra class, and you can reuse it later for other use cases
How to Find an Element in a List with Java, List
Get a specific object in arrayList
It seems like you are trying to find a specific school in the list of schools. If this is not what you are trying to do, please let me know.
public School findSchool(String schoolName) < // Goes through the List of schools. for (School i : schools) < if (i.getName.equals(schoolname)) < return i; >> return null; > Java 8’s streaming API gives you a pretty neat syntax to doing so, by filtering. If you can assume that there’s only one school with a given name, you could use the findFirst() method:
School aaaSchool = schools.stream() .filter(x -> x.getName().equals("aaa")) .findFirst() .orElse(null); If you can’t, you’ll have to do with a sub-list of schools:
List aaaSchools = schools.stream() .filter(x -> x.getName().equals("aaa")) .collect(Collectors.toList()); Select Random element from ArrayList print then remove It, Fast and simple, make use of Collections.shuffle import java.util.ArrayList; import java.util.Collections; import java.util.
Select value from arraylist based on its percentage [duplicate]
Alright, I wanted to take a stab at this without looking at someone else’s code. Here’s what I came up with.
Btw, I hope it was the Java tag that was in error and not the C# tag :D.
Here’s the entire program. What follows below is an explanation of each piece
I chose to take each element to be a portion in a ratio. Therefore, in your example, your total is 100 (20 + 80) meaning that the 20 content model should get chosen 20% of the time. If you want to constrain your content models such that their total weights add up to 100, that should be done at the time that you create them.
So here’s my solution.
First the content models:
class ContentModel < public string Content < get; set; >public int Weight < get; set; >> And then a list of test cases:
static List contentOptions = new List < new ContentModel < Content = "hello", Weight = 20 >, new ContentModel < Content = "hey", Weight = 80 >, new ContentModel < Content = "yo dawg", Weight = 90 >>; Given these test cases we would expect to see ‘Hello’ appear about 10.5% of the time (20 / (80 + 90 + 20)) * 100. And so on for the rest of the test cases.
Here’s the generator that makes that happen:
Here all we’re going to do is figure out what the total weight is that we’re working with. Then we’re going to pick a random number and go through each model asking «Is this number from this content model?» If no, then subtract that content model’s weight and move to the next one until we get to a model where the selection — weight is < 0. In this case we have the model that was chosen. I hope that makes sense.
(Note: I chose to recalculate the total weight every time in case you change the source list of options. If you make that list readonly, then you could move that .Sum() call outside the while loop.)
static IEnumerable GetGreetings() < Random generator = new Random(); while (true) < int totalWeight = contentOptions.Sum(x =>x.Weight); int selection = generator.Next(0, totalWeight); foreach (ContentModel model in contentOptions) < if (selection - model.Weight >0) selection -= model.Weight; else < yield return model.Content; break; >> > > And finally, here’s the main method which will also test this whole thing:
static void Main(string[] args) < ListselectedGreetings = new List(); /* This will get 1000 greetings, * which are the Content property of the models, group them by the greeting, * count them, and then print the count along with the greeting to the Console. */ GetGreetings() .Take(1000) .GroupBy(x => x) .Select(x => new < Count = x.Count(), Content = x.Key >) .ToList() .ForEach(x => Console.WriteLine(" : ", x.Content, x.Count)); Console.ReadLine(); > Here are my results from a run through:
ArrayList in Java, After adding the elements, if we wish to change the element, it can be done using the set() method. Since an ArrayList is indexed, the element