Interface Set
A collection that contains no duplicate elements. More formally, sets contain no pair of elements e1 and e2 such that e1.equals(e2) , and at most one null element. As implied by its name, this interface models the mathematical set abstraction.
The Set interface places additional stipulations, beyond those inherited from the Collection interface, on the contracts of all constructors and on the contracts of the add , equals and hashCode methods. Declarations for other inherited methods are also included here for convenience. (The specifications accompanying these declarations have been tailored to the Set interface, but they do not contain any additional stipulations.)
The additional stipulation on constructors is, not surprisingly, that all constructors must create a set that contains no duplicate elements (as defined above).
Note: Great care must be exercised if mutable objects are used as set elements. The behavior of a set is not specified if the value of an object is changed in a manner that affects equals comparisons while the object is an element in the set. A special case of this prohibition is that it is not permissible for a set to contain itself as an element.
Some set implementations have restrictions on the elements that they may contain. For example, some implementations prohibit null elements, and some have restrictions on the types of their elements. Attempting to add an ineligible element throws an unchecked exception, typically NullPointerException or ClassCastException . Attempting to query the presence of an ineligible element may throw an exception, or it may simply return false; some implementations will exhibit the former behavior and some will exhibit the latter. More generally, attempting an operation on an ineligible element whose completion would not result in the insertion of an ineligible element into the set may throw an exception or it may succeed, at the option of the implementation. Such exceptions are marked as «optional» in the specification for this interface.
Unmodifiable Sets
- They are unmodifiable. Elements cannot be added or removed. Calling any mutator method on the Set will always cause UnsupportedOperationException to be thrown. However, if the contained elements are themselves mutable, this may cause the Set to behave inconsistently or its contents to appear to change.
- They disallow null elements. Attempts to create them with null elements result in NullPointerException .
- They are serializable if all elements are serializable.
- They reject duplicate elements at creation time. Duplicate elements passed to a static factory method result in IllegalArgumentException .
- The iteration order of set elements is unspecified and is subject to change.
- They are value-based. Programmers should treat instances that are equal as interchangeable and should not use them for synchronization, or unpredictable behavior may occur. For example, in a future release, synchronization may fail. Callers should make no assumptions about the identity of the returned instances. Factories are free to create new instances or reuse existing ones.
- They are serialized as specified on the Serialized Form page.
This interface is a member of the Java Collections Framework.
Iterate Over a Set in Java
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1. Introduction
Iterating over elements is one of the most fundamental operations we can execute on a collection.
In this tutorial, we’ll take a look at how to iterate over elements of a Set and how it differs from the similar tasks on a List or array.
2. Getting Access to Elements in a Set
A Set, unlike a List and many other collections, isn’t sequential. Their elements aren’t indexed, and depending on the implementation, they may not maintain order.
That means that we can’t ask about a specific element of the set by its number. Because of that, we can’t use a typical for loop or any other index-based method.
2.1. Iterator
The most basic and close-to-metal method of iterating over the set is invoking the iterator method exposed by every Set:
Set names = Sets.newHashSet("Tom", "Jane", "Karen"); Iterator namesIterator = names.iterator();Then we can use the obtained iterator to get elements of that Set, one by one. The most iconic way is checking if the iterator has a next element in the while loop:
while(namesIterator.hasNext())
We can also use the forEachRemaining method added in Java 8:
namesIterator.forEachRemaining(System.out::println);We can also mix these solutions:
String firstName = namesIterator.next(); // save first name to variable namesIterator.forEachRemaining(System.out::println); // print rest of the names All other methods will use an Iterator in some way under the hood.
3. Streams
Every Set exposes the spliterator() method. Because of that, a Set can be easily transformed into a Stream:
names.stream().forEach(System.out::println);We can also leverage the rich Streams API to create more complex pipelines. For example, let’s map, log, and then reduce elements of the set to a single string:
String namesJoined = names.stream() .map(String::toUpperCase) .peek(System.out::println) .collect(Collectors.joining());4. Enhanced Loop
While we can’t use a simple, indexed for loop to iterate over a Set, we can use the enhanced loop feature introduced in Java 5:
5. Iterating with Index
5.1. Converting to Array
Sets aren’t indexed, but we can add an index artificially. One possible solution is to simply convert the Set to some more approachable data structure like an array:
Object[] namesArray = names.toArray(); for (int i = 0; i
Mind that conversion to an array alone will iterate over the Set once. So, in terms of complexity, we’ll be iterating over the Set twice. That may be a problem if performance is crucial.
5.2. Zipping with Index
Another approach is to create an index and zip it with our Set. While we could do it in vanilla Java, there are libraries that provide tools just for that.
For example, we can use Vavr’s streams:
Stream.ofAll(names) .zipWithIndex() .forEach(t -> System.out.println(t._2() + ": " + t._1()));6. Summary
In this tutorial, we looked at various ways of iterating over elements of the Set instance. We explored the usage of iterators, streams, and loops, and the differences between them.
As always, examples are available over GitHub.
Slow MySQL query performance is all too common. Of course it is. A good way to go is, naturally, a dedicated profiler that actually understands the ins and outs of MySQL.
The Jet Profiler was built for MySQL only, so it can do things like real-time query performance, focus on most used tables or most frequent queries, quickly identify performance issues and basically help you optimize your queries.
Critically, it has very minimal impact on your server’s performance, with most of the profiling work done separately — so it needs no server changes, agents or separate services.
Basically, you install the desktop application, connect to your MySQL server, hit the record button, and you’ll have results within minutes:
Class HashSet
Type Parameters: E — the type of elements maintained by this set All Implemented Interfaces: Serializable , Cloneable , Iterable , Collection , Set Direct Known Subclasses: JobStateReasons , LinkedHashSet
This class implements the Set interface, backed by a hash table (actually a HashMap instance). It makes no guarantees as to the iteration order of the set; in particular, it does not guarantee that the order will remain constant over time. This class permits the null element.
This class offers constant time performance for the basic operations ( add , remove , contains and size ), assuming the hash function disperses the elements properly among the buckets. Iterating over this set requires time proportional to the sum of the HashSet instance’s size (the number of elements) plus the «capacity» of the backing HashMap instance (the number of buckets). Thus, it’s very important not to set the initial capacity too high (or the load factor too low) if iteration performance is important.
Note that this implementation is not synchronized. If multiple threads access a hash set concurrently, and at least one of the threads modifies the set, it must be synchronized externally. This is typically accomplished by synchronizing on some object that naturally encapsulates the set. If no such object exists, the set should be «wrapped» using the Collections.synchronizedSet method. This is best done at creation time, to prevent accidental unsynchronized access to the set:
Set s = Collections.synchronizedSet(new HashSet(. ));
The iterators returned by this class’s iterator method are fail-fast: if the set is modified at any time after the iterator is created, in any way except through the iterator’s own remove method, the Iterator throws 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.