Questions tagged [collections]
Collections APIs provide developers with a set of classes and interfaces that make it easier to handle collections of objects.
24,103
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What are the differences between a HashMap and a Hashtable in Java?
What are the differences between a HashMap and a Hashtable in Java?
Which is more efficient for non-threaded applications?
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46
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How do I efficiently iterate over each entry in a Java Map?
If I have an object implementing the Map interface in Java and I wish to iterate over every pair contained within it, what is the most efficient way of going through the map?
Will the ordering of ...
- 38.9k
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votes
34
answers
1.4m
views
When to use LinkedList over ArrayList in Java?
I've always been one to simply use:
List<String> names = new ArrayList<>();
I use the interface as the type name for portability, so that when I ask questions such as this, I can rework ...
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votes
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Initialization of an ArrayList in one line
I wanted to create a list of options for testing purposes. At first, I did this:
ArrayList<String> places = new ArrayList<String>();
places.add("Buenos Aires");
places.add("Córdoba");
...
- 92.5k
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votes
65
answers
1.8m
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Sort a Map<Key, Value> by values
I need to sort a Map<Key, Value> on the values.
Since the values are not unique, I find myself converting the keySet into an array, and sorting that array through array sort with a custom ...
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votes
22
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How to directly initialize a HashMap (in a literal way)?
Is there some way of initializing a Java HashMap like this?:
Map<String,String> test =
new HashMap<String, String>{"test":"test","test":"test"};
What would be the correct syntax? I ...
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1301
votes
31
answers
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views
Iterating through a Collection, avoiding ConcurrentModificationException when removing objects in a loop
We all know you can't do the following because of ConcurrentModificationException:
for (Object i : l) {
if (condition(i)) {
l.remove(i);
}
}
But this apparently works sometimes, but ...
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votes
17
answers
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views
Converting 'ArrayList<String> to 'String[]' in Java
How might I convert an ArrayList<String> object to a String[] array in Java?
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43
answers
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How can I initialise a static Map?
How would you initialise a static Map in Java?
Method one: static initialiser
Method two: instance initialiser (anonymous subclass)
or
some other method?
What are the pros and cons of each?
Here ...
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votes
13
answers
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Difference between <? super T> and <? extends T> in Java [duplicate]
What is the difference between List<? super T> and List<? extends T> ?
I used to use List<? extends T>, but it does not allow me to add elements to it list.add(e), whereas the List&...
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votes
25
answers
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How to initialize HashSet values by construction?
I need to create a Set with initial values.
Set<String> h = new HashSet<String>();
h.add("a");
h.add("b");
Is there a way to do this in one line of code? For instance, it's useful for a ...
- 13.8k
914
votes
25
answers
2.7m
views
How to make a new List in Java
We create a Set as:
Set myset = new HashSet()
How do we create a List in Java?
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911
votes
15
answers
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views
Efficiency of Java "Double Brace Initialization"?
In Hidden Features of Java the top answer mentions Double Brace Initialization, with a very enticing syntax:
Set<String> flavors = new HashSet<String>() {{
add("vanilla");
add("...
- 30.8k
879
votes
10
answers
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views
How to convert Set to Array?
Set seems like a nice way to create Arrays with guaranteed unique elements, but it does not expose any good way to get properties, except for generator [Set].values, which is called in an awkward way ...
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843
votes
12
answers
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How can I turn a List of Lists into a List in Java 8?
If I have a List<List<Object>>, how can I turn that into a List<Object> that contains all the objects in the same iteration order by using the features of Java 8?
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19
answers
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How to convert an Array to a Set in Java
I would like to convert an array to a Set in Java. There are some obvious ways of doing this (i.e. with a loop) but I would like something a bit neater, something like:
java.util.Arrays.asList(Object[...
user130076
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13
answers
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How do I convert a Map to List in Java?
How do I convert a Map<key,value> to a List<value>? Should I iterate over all map values and insert them into a list?
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votes
16
answers
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How can I convert List<Integer> to int[] in Java? [duplicate]
How can I convert a List<Integer> to int[] in Java?
I'm confused because List.toArray() actually returns an Object[], which can be cast to neither Integer[] nor int[].
Right now I'm using a loop ...
unknown
773
votes
17
answers
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views
Easiest way to convert a List to a Set in Java
What is the easiest way to convert a List to a Set in Java?
- 7,985
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31
answers
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How to filter a Java Collection (based on predicate)?
I want to filter a java.util.Collection based on a predicate.
- 14.8k
763
votes
20
answers
1.3m
views
How do I remove an array item in TypeScript?
I have an array that I've created in TypeScript and it has a property that I use as a key. If I have that key, how can I remove an item from it?
- 12.4k
717
votes
13
answers
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Ways to iterate over a list in Java
Being somewhat new to the Java language I'm trying to familiarize myself with all the ways (or at least the non-pathological ones) that one might iterate through a list (or perhaps other collections) ...
- 11.4k
698
votes
11
answers
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views
Why there is no ConcurrentHashSet against ConcurrentHashMap
HashSet is based on HashMap.
If we look at HashSet<E> implementation, everything is been managed under HashMap<E,Object>.
<E> is used as a key of HashMap.
And we know that HashMap ...
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684
votes
35
answers
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views
Java 8 Distinct by property
In Java 8 how can I filter a collection using the Stream API by checking the distinctness of a property of each object?
For example I have a list of Person object and I want to remove people with the ...
- 11.6k
660
votes
28
answers
1.3m
views
How to convert comma-separated String to List?
Is there any built-in method in Java which allows us to convert comma separated String to some container (e.g array, List or Vector)? Or do I need to write custom code for that?
String commaSeparated =...
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626
votes
15
answers
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Why is there no SortedList in Java?
In Java there are the SortedSet and SortedMap interfaces. Both belong to the Java Collections framework and provide a sorted way to access the elements.
However, in my understanding there is no ...
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625
votes
6
answers
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views
Is there a short contains function for lists?
Given a list xs and a value item, how can I check whether xs contains item (i.e., if any of the elements of xs is equal to item)? Is there something like xs.contains(item)?
For performance ...
- 324k
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votes
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answers
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How do I remove repeated elements from ArrayList?
I have an ArrayList<String>, and I want to remove repeated strings from it. How can I do this?
- 6,011
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votes
21
answers
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How to convert int[] into List<Integer> in Java?
How do I convert int[] into List<Integer> in Java?
Of course, I'm interested in any other answer than doing it in a loop, item by item. But if there's no other answer, I'll pick that one as the ...
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votes
7
answers
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C# Set collection?
Does anyone know if there is a good equivalent to Java's Set collection in C#? I know that you can somewhat mimic a set using a Dictionary or a HashTable by populating but ignoring the values, but ...
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552
votes
12
answers
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HashSet vs. List performance
It's clear that a search performance of the generic HashSet<T> class is higher than of the generic List<T> class. Just compare the hash-based key with the linear approach in the List<T&...
- 15.5k
518
votes
28
answers
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views
Most efficient way to increment a Map value in Java
I hope this question is not considered too basic for this forum, but we'll see. I'm wondering how to refactor some code for better performance that is getting run a bunch of times.
Say I'm creating a ...
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votes
47
answers
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Get nth character of a string in Swift
How can I get the nth character of a string? I tried bracket([]) accessor with no luck.
var string = "Hello, world!"
var firstChar = string[0] // Throws error
ERROR: 'subscript' is unavailable: ...
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502
votes
15
answers
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Retrieving a List from a java.util.stream.Stream in Java 8
I was playing around with Java 8 lambdas to easily filter collections. But I did not find a concise way to retrieve the result as a new list within the same statement. Here is my most concise approach ...
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votes
6
answers
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LINQ .Any VS .Exists - What's the difference?
Using LINQ on collections, what is the difference between the following lines of code?
if(!coll.Any(i => i.Value))
and
if(!coll.Exists(i => i.Value))
When I disassemble .Exists, it looks like ...
- 11.1k
493
votes
21
answers
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views
Easy way to convert Iterable to Collection
In my application I use 3rd party library (Spring Data for MongoDB to be exact).
Methods of this library return Iterable<T>, while the rest of my code expects Collection<T>.
Is there any ...
- 12.8k
488
votes
19
answers
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views
Is it better to return null or empty collection?
That's kind of a general question (but I'm using C#), what's the best way (best practice), do you return null or empty collection for a method that has a collection as a return type ?
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votes
7
answers
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defaultdict of defaultdict?
Is there a way to have a defaultdict(defaultdict(int)) in order to make the following code work?
for x in stuff:
d[x.a][x.b] += x.c_int
d needs to be built ad-hoc, depending on x.a and x.b ...
- 104k
465
votes
21
answers
1.3m
views
How to sort a List/ArrayList?
I have a List of doubles in java and I want to sort ArrayList in descending order.
Input ArrayList is as below:
List<Double> testList = new ArrayList();
testList.add(0.5);
testList.add(0.2);
...
- 4,831
465
votes
3
answers
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views
Is there an AddRange equivalent for a HashSet in C#
With a list you can do:
list.AddRange(otherCollection);
There is no add range method in a HashSet.
What is the best way to add another ICollection to a HashSet?
- 10.7k
454
votes
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answers
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What are the reasons why Map.get(Object key) is not (fully) generic
What are the reasons behind the decision to not have a fully generic get method
in the interface of java.util.Map<K, V>.
To clarify the question, the signature of the method is
V get(Object ...
- 12.9k
438
votes
22
answers
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views
Getting an element from a Set
Why doesn't Set provide an operation to get an element that equals another element?
Set<Foo> set = ...;
...
Foo foo = new Foo(1, 2, 3);
Foo bar = set.get(foo); // get the Foo element from the ...
- 5,028
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votes
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answers
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How to get the first element of the List or Set? [duplicate]
I'd like to know if I can get the first element of a list or set. Which method to use?
- 84.8k
424
votes
6
answers
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views
Cost of len() function
What is the cost of len() function for Python built-ins? (list/tuple/string/dictionary)
- 89.2k
420
votes
11
answers
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Java - How to create new Entry (key, value)
I'd like to create new item that similarly to Util.Map.Entry that will contain the structure key, value.
The problem is that I can't instantiate a Map.Entry because it's an interface.
Does anyone ...
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votes
8
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How to quickly and conveniently create a one element arraylist [duplicate]
Is there a Utility method somewhere that can do this in 1 line? I can't find it anywhere in Collections, or List.
public List<String> stringToOneElementList(String s) {
List<String> ...
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409
votes
8
answers
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views
How to easily initialize a list of Tuples?
I love tuples. They allow you to quickly group relevant information together without having to write a struct or class for it. This is very useful while refactoring very localized code.
Initializing a ...
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396
votes
10
answers
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Java Ordered Map
Is there an object in Java that acts like a Map for storing and accessing key/value pairs, but can return an ordered list of keys and an ordered list of values, such that the key and value lists are ...
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381
votes
5
answers
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Difference between a Seq and a List in Scala
I've seen in many examples that sometimes a Seq is being used, while other times is the List...
Is there any difference, other than the former one being a Scala type and the List coming from Java?
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What is the Simplest Way to Reverse an ArrayList?
What is the simplest way to reverse this ArrayList?
ArrayList<Integer> aList = new ArrayList<>();
//Add elements to ArrayList object
aList.add("1");
aList.add("2");
aList.add("3");
aList....
- 5,397