What does retainAll () do in Java?

The retainAll() method of ArrayList is used to remove all the array list’s elements that are not contained in the specified collection or retains all matching elements in the current ArrayList instance that match all elements from the Collection list passed as a parameter to the method.

What does set retainAll do?

retainAll. Retains only the elements in this set that are contained in the specified collection (optional operation). In other words, removes from this set all of its elements that are not contained in the specified collection.

How does Addall work in Java?

The addAll() is a method of Java Collections class which adds all of the specified elements to the specified collection. The elements to be added may be specified individually or as an array.

elementsElements are the elements to be inserted into c.Required

What is the use of retainAll collection C method from collection interface?

retainAll — removes from the target Collection all its elements that are not also contained in the specified Collection . That is, it retains only those elements in the target Collection that are also contained in the specified Collection .

What is LinkedList Java?

Linked List is a part of the Collection framework present in java. util package. This class is an implementation of the LinkedList data structure which is a linear data structure where the elements are not stored in contiguous locations and every element is a separate object with a data part and address part.

What is set Java?

A Set is a Collection that cannot contain duplicate elements. It models the mathematical set abstraction. … Two Set instances are equal if they contain the same elements. The Java platform contains three general-purpose Set implementations: HashSet , TreeSet , and LinkedHashSet .

What is difference between collection and collections?

The List, Set, and Queue are the main sub-interfaces of the collection interface.

Collection vs Collections in Java with Example.
It is an interface.It is a utility class.
It is used to represent a group of individual objects as a single unit.It defines several utility methods that are used to operate on collection.
Sep 2, 2020

What is collections API in Java?

The Java Collections API provide Java developers with a set of classes and interfaces that makes it easier to work with collections of objects, e.g. lists, maps, stacks etc. Rather than having to write your own collection classes, Java provides these ready-to-use collection classes for you.

Which is faster and uses less memory?

Answer is DATAREADER as it fetches only one row at a time whereas Dataset stores it as a table as whole.so it need much more network resourses.

What is map in Java?

A Map is an object that maps keys to values. A map cannot contain duplicate keys: Each key can map to at most one value. It models the mathematical function abstraction. … The Java platform contains three general-purpose Map implementations: HashMap , TreeMap , and LinkedHashMap .

Which collection is faster in Java?

There is no fastest or best collection. If you need fast access to elements using index, ArrayList is your answer. If you need fast access to elements using a key, use HashMap . If you need fast add and removal of elements, use LinkedList (but it has a very poor index access performance).

Is HashMap part of collection?

Class 1: HashMap

HashMap is a part of Java’s collection since Java 1.2. It provides the basic implementation of the Map interface of Java. It stores the data in (Key, Value) pairs.

Can maps have null keys?

It is thread-safe and can be shared with many threads. HashMap allows one null key and multiple null values whereas Hashtable doesn’t allow any null key or value.

Can Map have duplicate keys?

Duplicate keys are not allowed in a Map. Basically, Map Interface has two implementation classes HashMap and TreeMap the main difference is TreeMap maintains an order of the objects but HashMap will not. HashMap allows null values and null keys. Both HashSet and HashMap are not synchronized.

Is Java Map a collection?

Characteristics of a Map:

Because a Map is not a true collection, its characteristics and behaviors are different than the other collections like List or Set. A Map cannot contain duplicate keys and each key can map to at most one value.

Can we iterate HashMap?

There is a numerous number of ways to iterate over HashMap of which 5 are listed as below: Iterate through a HashMap EntrySet using Iterators. Iterate through HashMap KeySet using Iterator. Iterate HashMap using for-each loop.

Can HashMap be null?

HashMap: HashMap implements all of the Map operations and allows null values and one null key. HashMap does not maintain an order of its key-value elements. Therefore, consider to use a HashMap when order does not matter and nulls are acceptable.

What is a Multimap Java?

A Multimap is a new collection type that is found in Google’s Guava library for Java. A Multimap can store more than one value against a key. Both the keys and the values are stored in a collection, and considered to be alternates for Map<K, List<V>> or Map<K, Set<V>> (standard JDK Collections Framework).

How many ways can you iterate a Map in Java?

There are generally five ways of iterating over a Map in Java.

What is iterator in Java?

An Iterator is an object that can be used to loop through collections, like ArrayList and HashSet. It is called an “iterator” because “iterating” is the technical term for looping. To use an Iterator, you must import it from the java. util package.

How do you iterate through a set?

Iterating over Set using Iterator
  1. Obtain the iterator by calling the iterator() method.
  2. You can use while or for loop along with hasNext(), which returns true if there are more elements in the Set.
  3. Call the next() method to obtain the next elements from Set.