Collection details in Java

How can you get synchronized standard collection objects?

Using the static methods synchronizedMap() and synchronizedList() of the Collections class. These methods return the synchronized decorator of the passed collection. However, manual synchronization is still required in the case of a collection traversal. 

Map m = Collections.synchronizedMap(new HashMap());
List l = Collections.synchronizedList(new ArrayList());

Since Java 6, JCF has been extended with special collections that support multithreading, such as CopyOnWriteArrayList and ConcurrentHashMap.

How do I get a read-only collection?

With help:

  • Collections.unmodifiableList(list);
  • Collections.unmodifiableSet(set);
  • Collections.unmodifiableMap(map).

These methods take a collection as a parameter and return a read-only collection with the same elements inside.

Single threaded program that causes the collection to throw a ConcurrentModificationException.

public static void main(String[] args) {
    List<Integer> list = new ArrayList<>();
    list.add(1);
    list.add(2);
    list.add(3);

    for (Integer integer : list) {
        list.remove(1);
    }
}

Give an example where a collection is throwing an UnsupportedOperationException.

public static void main(String[] args) {
    List<Integer> list = Collections.emptyList();
    list.add(0);
}

Implementation the symmetric difference of the two collections using the Collection (addAll(...), removeAll(...), retainAll(...)) methods.

The symmetric difference of two collections is the set of items that do not belong to both of the original collections at the same time.

<T> Collection<T> symmetricDifference(Collection<T> a, Collection<T> b) {    
    // Merge collections.
    Collection<T> result = new ArrayList<>(a);
    result.addAll(b);
    
    // Get the intersection of the collections.
    Collection<T> intersection = new ArrayList<>(a);
    intersection.retainAll(b);
    
    // Delete the items located in both collections.
    result.removeAll(intersection);

    return result;
}

How to make cache with "invalidation policy" using LinkedHashMap?

It is necessary to use LRU (Least Recently Used algorithm) and LinkedHashMap with access-order. In this case, when accessing an element, it will move to the end of the list, and the least used elements will gradually group at the beginning of the list. Also in the standard implementation of LinkedHashMap there is a removeEldestEntries() method that returns true if the current LinkedHashMap object should remove the least used element from the collection using the put() and putAll() methods.

public class LRUCache<K, V> extends LinkedHashMap<K, V> {
    private static final int MAX_ENTRIES = 10;

    public LRUCache(int initialCapacity) {
        super(initialCapacity, 0.85f, true);
    }

    @Override
    protected boolean removeEldestEntry(Map.Entry<K, V> eldest) {
        return size() > MAX_ENTRIES;
    }
}

It is worth noting that LinkedHashMap does not allow you to fully implement the LRU algorithm, since when you insert an element already in the collection, the order of iteration over the elements does not change.

How to copy the elements of any collection into an array in one line?

Object[] array = collection.toArray();

How to get a List with all the elements except the first and last 3 in one call from List?

List<Integer> subList = list.subList(3, list.size() - 3);

How can I convert a HashSet to an ArrayList in one line?

ArrayList<Integer> list = new ArrayList<>(new HashSet<>());

How to convert ArrayList to HashSet in one line?

HashSet<Integer> set = new HashSet<>(new ArrayList<>());

Make a HashSet from the keys of the HashMap.

HashSet<Object> set = new HashSet<>(map.keySet());

Make HashMap from HashSet <Map.Entry<K, V>>.

HashMap<K, V> map = new HashMap<>(set.size());
for (Map.Entry<K, V> entry : set) {
    map.put(entry.getKey(), entry.getValue());
}

Read also:


Comments

Post a Comment

Popular posts from this blog

Methods for reading XML in Java

Image
DOM (Document Object Model) - object - reads XML, recreating it in memory in the form of an object structure, while the XML document is represented as a set of tags - nodes. Each node can have an unlimited number of child nodes. Each child can also contain several levels of descendants or none at all. Thus, the result is a kind of tree. Pluses: Easy to program. If there are many objects in XML with cross-references to each other, it is enough to go through the document twice: first time create objects without references and fill in the name-object dictionary, the second time - restore links. If an error occurs in XML, a half-created XML structure remains in memory, which will be automatically destroyed. Suitable for both reading and writing. Minuses: Low speed. Wastes a lot of memory. SAX (Simple API for XML) - event-driven - reads an XML document, responding to emerging events (opening or closing tag, string, attribute) by calling the event handlers provided by the appl...
Read more

XML, well-formed XML and valid XML

Image
XML, the eXtensible Markup Language, is a language with a simple formal syntax, well suited for creating and manipulating documents with programs, and at the same time easy to read and create documents by humans. XML is extensible, it does not fix the markup used in documents, and the developer is free to create markup in accordance with the needs of a particular area, being limited only by the syntax rules of the language. DTD A DTD, Document Type Definition, is a predefined set of rules that defines the relationships between elements and attributes. For example, the DTD for HTML says that the DIV tag must be inside the BODY tag and can appear multiple times, TITLE - in HEAD and only once, and SCRIPT - both there and there as many times as you want. A DTD is usually described directly in the document as a formulation line starting with <!DOCTYPE ... > or in a separate file. Difference between well-formed XML and valid XML Depending on the level of compliance, the document...
Read more

ArrayList and LinkedList in Java, memory usage and speed

Image
How much extra memory is needed when calling ArrayList.add()? If there is enough space in the array to accommodate a new element, then no additional memory is required. Otherwise, a new array is created, 1.5 times larger than the existing one (this is true for JDK above 1.7, in earlier versions the size of the increase is different). How much extra memory is allocated when you call LinkedList.add()? One new instance of the nested Node class is created. How much memory to store one byte primitive in LinkedList? Each LinkedList element stores a link to the previous element, the next element, and a data link. private static class Node<E> { E item; Node<E> next; Node<E> prev; //... } For 32-bit systems, each link is 32 bits (4 bytes). The object (header) of the nested Node class itself takes 8 bytes. 4 + 4 + 4 + 8 = 20 bytes, and since the size of each object in Java is a multiple of 8, so we get 24 bytes. A byte primitive occupies 1 by...
Read more