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What are Java Collections? Get started with the framework

Why do we need collections in Java?

Most likely, you will store them on a List or Hash table, and they are the classes that are provided by Java Collection Framework. If you have been programming in Java, then you know that Collections and Streams are two of the most important Java APIs.

  • A developer can mix classes with other types that result in increasing the reusability of code.
  • Here the rule is objects stored in the collection are also should be Serializable types to store collections in the file.
  • Returns true if this list contains the specified element.
  • You can find out how many elements are in the queue, but you can’t find out what, say, the “third” element is.
  • The collection never stores any primitive values, but if we want to store primitive values we have to represent primitive values as objects.
  • Hashtable class does not allow null key as well as value otherwise it will throw NullPointerException.
  • OnJava Interview Questionswhere I have listed top 75 interview questions and answers which will help you set apart in the interview process.

This class makes no guarantees for the order of the map. Doubly-linked list implementation of the List and Deque interfaces.

Map Interface

For example, if addition is being performed by one thread, other operation cannot be performed until the first one is over. The Collections Framework in Java follows a certain hierarchy. All the classes and interfaces in the Collections Framework come under the java.util package.

Why do we need collections in Java?

They are used to standardize the way in which objects are handled in the class. Collection in Java organizes data to make storing, accessing, and editing of data easier. – An Interface is a system where related or unrelated entities can interact. Comparing ObjectSpace Inc.’s JGL and Sun’s Collections Framework turns out to be like comparing apples and kiwi fruits. Provided Sun doesn’t break its promise in this area, I’ll be happy to invest my resources in adopting Collections in earnest. Doug Lea later developed a concurrency package, comprising new Collection-related classes.

Why Collection Framework ?

The first argument of these methods is the collection which the algorithm manipulates. The next() method is used to obtain a reference to the next object in the collection. The hasNext() Why do we need collections in Java? method is used to test whether the collection has another object that should be processed. Not every method in Collection is supported by every concrete class that extends it.

What is collection process work?

A Collections Specialist is a professional who is responsible for resolving overdue bills and collecting payments from those who owe. They must ensure that they keep their employer's assets safe while also making sure those responsible for unpaid bills are contacted immediately.

Collections and arrays are similar in that they both hold references to objects and they can be managed as a group. However, unlike arrays, collections do not need to be assigned a certain capacity when instantiated. Collections can also grow and shrink in size automatically when objects are added or removed. Collections cannot hold basic data type elements such as int, long, or double; instead, they hold Wrapper Classes such as Integer, Long, or Double. The List interface extends Collection to define an ordered collection with duplicates allowed. The List interface adds position-oriented operations, as well as a new list iterator that enables the user to traverse the list bi-directionally. ArrayList, LinkedList, and vector are classes implementing List interface.

3- Interfaces Iterator and RandomAccess – Data Access Methods

The Java Collections Framework is a unified architecture for representing and manipulating collections. It contains interfaces, their implementation classes, and algorithms to process the data stored in a collection. The Collection interface is extended by other interfaces like List, Set, and Queue. A collection is an object that can hold references to other objects.

  • As we know every project has three layers Model, View, and Controller.
  • Your passion for java is making way for lots to understand these vast/complex java concepts more easily.
  • The ArrayList class maintains the insertion order and is non-synchronized.
  • TheCollection Framework in Javais a collection of interfaces and classes to store, process and transfer the data efficiently.
  • The name deque is short for “double-ended queue” and is usually pronounced “deck”.
  • An object is said to be a collection object if it holds or stores a group of other objects.
  • Because the hash calculation is fast and can have a large number of buckets, there are few elements to search and retrieval is very fast.

The java.util.Queue interface defines the queue data structure, which stores elements in the order in which they are inserted. New additions go to the end of the line, and elements are removed from the front. This interface is implemented by java.util.LinkedList, java.util.ArrayDeque, and java.util.PriorityQueue. LinkedList, of course, also implements the List interface and can also be used as one.


It reduces the complexity of the application (i.e. array code is reduced). That means Reduces programming effort by providing useful data structures and algorithms so we don’t have to write them by our self. ArrayList and LinkedList provide the array and linked list implementations of the List interface that was described earlier. ArrayList is satisfactory for most purposes, but you should use LinkedList when you plan to do a lot of insertions or deletions at various points in the list. The type E in these methods refers to the parameterized element type of the List class. We’ll look at concrete implementations of these shortly.

How do you manage collections?

  1. Update your A/R management software.
  2. Reevaluate your lockbox placement.
  3. Review your policy regarding billing dates and procedures.
  4. Review the technology your company is utilizing to interact with customers.
  5. Discuss your terms with your buyer at the time of sale.

Both LinkedList and ArrayDeque also implement the java.util.Deque interface, giving it more flexibility. Linked lists are useful when you are doing a lot of insertions or deletions on a collection. Java 7 adds skip list implementations of the NavigableMap and NavigableSetinterfaces in the java.util.concurrent package for concurrent programming. Set is implemented by java.util.HashSet, java.util.LinkedHashSet, and java.util.TreeSet. More specifically, it uses a java.util.HashMap to store the hashes and elements and to prevent duplicates. Java.util.LinkedHashSet extends this by creating a doubly linked list that links all of the elements by their insertion order.

Collections Framework Class Diagram

➲ The limitation of a variable is that it can store only one value at a time. Arrays are not recommended to use with respect to memory whereas collections are recommended to use with respect to memory. In this blog we will understand basics of JAVA Collections framework. We bring 10+ years of global software delivery experience to every partnership.

Why do we need collections in Java?

Due to this reason java.util package classes are collectively called the collection framework. The Map interface is also part of the java collection framework but it does not inherit the collection interface. The map interface is preferred when values are stored in the form of keys and value pairs. It uses a dynamic array to store the duplicate element of different data types. The ArrayList class maintains the insertion order and is non-synchronized. The elements stored in the ArrayList class can be randomly accessed.

What is an Array Class in Java and How to Implement it?

In this way, clients can look but not modify, while you maintain full access. Note that the ordering maintained by a set must be consistent with equals if it is to correctly implement the Set interface. Queues typically, but do not necessarily, order elements in a FIFO (first-in-first-out) manner. Among the exceptions are priority queues, which order elements according to a supplied comparator or the elements’ natural ordering.

The sort algorithm reorders a List so that its elements are in ascending order according to an ordering relationship. The simple form takes a List and sorts it according to its elements’ natural ordering. The second form of the sort takes a Comparator in addition to a List and sorts the elements with the Comparator.

We all had it during our childhood where we used to store our coins. This piggy bank is called as Collection and the coins are nothing but objects. Technically, a collection is an object or a container that stores a group of other objects. It outputs true if the collection has an element specified. Sets are unordered collections that can be iterated and contain each element at most once.

Why do we need collections in Java?

You can now add names easily to the HashSet by calling the hashset.add method. Lastly, instantiate the Iterator interface to import all its methods and decide how to output the names. The difference between the above method of adding elements to the list in the array and the former is that you can add or remove elements during run time. Array ListThis class helps when dealing with dynamic arrays. A dynamic array is an array whose size is unpredictable since it keeps adding up. Other arrays usually have a fixed size where you have to state the number of elements to be stored.

Before the Collections Framework, it was hard for programmers to write algorithms that worked for different kinds of collections. Java came with some Collection classes, like Vector, Stack, Hashtable, and Array, but they had their disadvantages.

  • If the size of the array is five and the user store only four values then memory is wasted.
  • It ensures that thekey/valuepairs are arranged in ascending order according to the key.
  • You can create a LinkedList using an existing Collection, and the list will contain all the elements in the original collection in the same order.
  • Whit this sometimes memory may be wasted or sometimes memory may not be sufficient.
  • Java Collections Framework is one of the core parts of the Java programming language.

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