80 Java Collections Interview Questions and Answers, Summaries of Computer Science

Download 80 Java Collections Interview Questions and Answers and more Summaries Computer Science in PDF only on Docsity! 80 Java Collections Interview Questions and Answers (2022) Here are Java Collections Interview Questions for fresher as well as experienced candidates to get their dream job. 1) What is framework in Java? A framework is a popular and readymade architecture that contains a set of classes and interfaces. 2) What is the Collection framework in Java? Collection Framework is a grouping of classes and interfaces that is used to store and manage the objects. It provides various classes like Vector, ArrayList, HashSet, Stack, etc. Java Collection framework can also be used for interfaces like Queue, Set, List, etc. 3) Explain Collections Class java.util.Collections is a class consists of static methods that operate on collections. It contains polymorphic algorithms to operate on collections, “wrappers”. This class contains methods for algorithms, like binary sorting, search, shuffling, etc. 4) What is the hashCode()? The hashCode() is a method that returns an integer hash code. EXPLORE MORE Learn Java Programming with Beginners Tutorial08:32 Linux Tutorial for Beginners: Introduction to Linux Operating...01:35 What is Integration Testing Software Testing Tutorial03:04 What is JVM (Java Virtual Machine) with Architecture JAVA...02:24 How to write a TEST CASE Software Testing Tutorial01:08 Seven Testing Principles Software Testing05:01 Linux File Permissions Commands with Examples13:29 How to use Text tool in Photoshop CC Tutorial08:32 What is NoSQL Database Tutorial02:00 Important Linux Commands for Beginners Linux Tutorial15:03 5) Distinguish between ArrayList and Vector in the Java collection framework. ArrayList Vector ArrayList is cannot be synchronized. Vector can be is synchronized. It is not a legacy class. It is a legacy class. It can increase its size by 50% of the size of the array. It can increase its size by doubling the size of the array. ArrayList is not thread-safe. Vector is a thread-safe. 6) What is ArrayList in Java? ArrayList is a data structure that can be stretched to accommodate additional elements within itself and shrink back to a smaller size when elements are removed. It is a very important data structure useful in handling the dynamic behavior of elements. 12) Explain equals() with example Equals() verifies whether the number object is equal to the object, which is passed as an argument or not. The syntax of the equals() method is: public boolean equals(Object o) This method takes two parameters 1) any object, 2) return value. It returns true if the passed argument is not null and is an object of a similar type having the same numeric value. Example: import java.lang.Integer; public class Test { public static void main(String args[]) { Integer p = 5; Integer q = 20; Integer r =5; Short s = 5; System.out.println(p.equals(q)); System.out.println(p.equals(r)); System.out.println(p.equals(s)); } } 13) List out benefits of generic collection The benefits of using the generic collection are:  If the programmers are using generic class, they don’t require typecasting.  It is type-safe and can be checked at the time of compilation.  It provides the stability of the code by detecting bug at the compilation time. 14) Explain the method to convert ArrayList to Array and Array to ArrayList Programmers can convert an Array to ArrayList using asList() method of Arrays class. It is a static method of Arrays class that accept the List object. The syntax of asList() method is: Arrays.asList(item) Java programmers can convert ArrayList to the List object using syntax: List_object.toArray(new String[List_object.size()]) 15) Give example of ArrayList The Example of reverse ArrayList is: import java.util.ArrayList; class Test_ArrayList { public static void main(String[] args) { //Creating a generic ArrayList ArrayList<String> arlTest = new ArrayList<String>(); //Size of arrayList System.out.println("Size of ArrayList at creation: " + arlTest.size()); //Lets add some elements to it arlTest.add("D"); arlTest.add("U"); arlTest.add("K"); arlTest.add("E"); //Recheck the size after adding elements System.out.println("Size of ArrayList after adding elements: " + arlTest.size()); //Display all contents of ArrayList System.out.println("List of all elements: " + arlTest); //Remove some elements from the list arlTest.remove("D"); System.out.println("See contents after removing one element: " + arlTest); //Remove element by index arlTest.remove(2); System.out.println("See contents after removing element by index: " + arlTest); //Check size after removing elements System.out.println("Size of arrayList after removing elements: " + arlTest.size()); System.out.println("List of all elements after removing elements: " + arlTest); //Check if the list contains "K" System.out.println(arlTest.contains("K")); } } 16) Give example to sort an array in dscending order The example of sort an array in decending order is: package com.guru99; public class SelectionSortAlgo { public static void main(String a[]) { int[] myArray = {860,8,200,9}; System.out.println("------Before Sort-----"); printArray(myArray); selection(myArray);//sorting array using selection sort System.out.println("-----After Sort-----"); printArray(myArray); } public static void selection(int[] array) { for (int i = 0; i < array.length - 1; i++) { System.out.println("Sort Pass Number "+(i+1)); int index = i; for (int j = i + 1; j < array.length; j++) { A stack is a special area of computer’s memory that stores temporary variables created by a function. In stack, variables are declared, stored, and initialized during runtime. 20) What is linked list? A linked list is a data structure that can store a collection of items. In other words, linked lists can be utilized to store several objects of the same type. Each unit or element of the list is referred as a node. A node in the Linked list has its data and the address of the next node. It is like a chain. Linked Lists are used to create graphs and trees. 21) Give example of ArrayList The example of ArrayList is: import java.util.ArrayList; class Test_ArrayList { public static void main(String[] args) { //Creating a generic ArrayList ArrayList<String> arlTest = new ArrayList<String>(); //Size of arrayList System.out.println("Size of ArrayList at creation: " + arlTest.size()); //Lets add some elements to it arlTest.add("D"); arlTest.add("U"); arlTest.add("K"); arlTest.add("E"); //Recheck the size after adding elements System.out.println("Size of ArrayList after adding elements: " + arlTest.size()); //Display all contents of ArrayList System.out.println("List of all elements: " + arlTest); //Remove some elements from the list arlTest.remove("D"); System.out.println("See contents after removing one element: " + arlTest); //Remove element by index arlTest.remove(2); System.out.println("See contents after removing element by index: " + arlTest); //Check size after removing elements System.out.println("Size of arrayList after removing elements: " + arlTest.size()); System.out.println("List of all elements after removing elements: " + arlTest); //Check if the list contains "K" System.out.println(arlTest.contains("K")); } } 22) Explain linked list supported by Java Two types of linked list supported by Java are:  Singly Linked list: Singly Linked list is a type of data structure. In a singly linked list, each node in the list stores the contents of the node and a reference or pointer to the next node in the list. It does not store any reference or pointer to the previous node.  Doubly linked lists: Doubly linked lists are a special type of linked list wherein traversal across the data elements can be done in both directions. This is made possible by having two links in every node, one that links to the next node and another one that connects to the previous node. 23) Explain the methods provided by the Queue interface? Methods of Java Queue interface are: Method Description boolean add(object) Inserts specified element into the Queue. It returns true in case it a success. boolean offer(object) This method is used to insert the element into the Queue. Object remove() It retrieves and removes the queue head. Object poll() (): It retrieves and removes queue head or return null in case if it is empty. Object poll() It retrieves and removes queue head or return null in case if it is empty. Object element() Retrieves the data from the Queue, but does not remove its head. Object peek() Retrieves the data from the Queue but does not remove its head, or in case, if the Queue is the Queue is empty, it will retrieve null. 24) Mention the methods provided by Stack class Important methods provided by Stack class are:  push(): Push item into the stack.  empty (): This method finds that whether the stack is empty or not.  pop (): This Java collection framework method removes the object from the stack.  search (): This method searches items in the stack.  peek (): This Java method looks at the stack object without removing it. 25) Define emptySet() in the Java collections framework Method emptySet() that returns the empty immutable set whenever programmers try to remove null elements. The set which is returned by emptySet() is serializable. The syntax of this method is: public static final <T> Set<T> emptySet() 26) Differentiate between Collection and Collections The difference between Collection and Collections are: 34) Define dictionary class The Dictionary class is a Java class that has a capability to store key-value pairs. 35) Define EnumSet java.util.EnumSet is Set implementation that can be used with enum types. EnumSet having all elements must come from one enum type specified explicitly or implicitly. It is not synchronized, and also null keys are not allowed. EnumSet provides methods like EnumSetof(E first, E… rest), complementOf(EnumSet s), and copyOf(Collection c). 36) What are the two ways to remove duplicates from ArrayList? Two ways to remove duplicates from ArrayList are:  HashSet: Developer can use HashSet to remove the duplicate element from the ArrayList. The drawback is it cannot preserve the insertion order.  LinkedHashSet: Developers can also maintain the order of insertion by using LinkedHashSet instead of HashSet. 37) What is IdentityHashMap? IdentityHashMap is a class that implements Serializable, Clonable interfaces, Map, and extends AbstractMap class. It is designed for the case wherein there is a need of reference-equality semantics. 38) What is WeakHashMap? WeakHashMap is an implementation of the Java Map. It is used to store weak references to its keys. Sorting using this Map allows a key-value pair is collected as garbage. Its key is not referenced outside WeakHashMap. 39) What are the methods to make collection thread-safe? The methods to make collection thread safe are:  Collections.synchronizedList(list);  Collections.synchronizedMap(map);  Collections.synchronizedSet(set); 40) Explain UnsupportedOperationException UnsupportedOperationException is an exception whch is thrown on methods that are not supported by actual collection type. For example, Developer is making a read-only list using “Collections.unmodifiableList(list)” and calling call(), add() or remove() method. It should clearly throw UnsupportedOperationException. 41) Name the collection classes that gives random element access to its elements Collection classes that give random element access to its elements are: 1) ArrayList, 2) HashMap, 3) TreeMap, and 4) Hashtable. 42) Explain the difference between Queue and Deque. Queue Deque It is called a single-ended Queue It is called a double-ended Queue Elements in the Queue are added or removed from one end Elements in the Queue are added from either end can be added and removed from the both end It is less versatile. It is more versatile. 43) Mention the implementing List and Set interface Class implementing List interface: 1) ArrayList, 2) Vector, and 3) LinkedList. Class implementing Set interface: 1) HashSet, and 2) TreeSet. 44) Explain the design pattern followed by Iterator The iterator follows the detail of the iterator design pattern. It provides developer to navigate through the objects collections using a common interface without knowing its implementation. 45) What is the peek() of the Queue interface? Peek () is a method of queue interface. It retrieves all the elements but does not remove the queue head. In case if the Queue is empty, then this method will return null. 46) What is CopyOnWriteArrayList? CopyOnWriteArrayList is a variant of ArrayList in which operations like add and set are implemented by creating a copy of the array. It is a thread-safe, and thereby it does not throw ConcurrentModificationException. This ArrayLists permits all the elements, including null. 47) Differentiate between ArrayList and LinkedList void addFirst(Object o) It is used to insert the given element at the beginning. void addLast(Object o) It is used to append the given element to the end. Int size() This method can be used to return the total number of elements in a list. boolean remove(Object o) It can remove the first occurrence of the specified element from this list. int indexOf(Object element) This Java method returns the index with the first occurrence of the mention element in this list, or -1. int lastIndexOf(Object element) It is a Java method that returns the index with the last occurrence of the specified element in this list, or -1. 53) List various classes available in sets Various classes available in sets are: HashSet, TreeSetand, and LinkedHashSet. 54) List methods available in Java Queue interface  boolean add(object)  boolean offer(object)  object remove()  object poll()  object element()  object peek() 55) Differentiate between List and Set. List Set An ordered collection of elements An unordered collection of elements Preserves the insertion order Doesn’t preserves the insertion order Duplicate values are allowed Duplicate values are not allowed Any number of null values can be stored Only one null values can be stored ListIterator can be used to traverse the List in any direction ListIterator cannot be used to traverse a Set Contains a legacy class called vector Doesn’t contains any legacy class 56) Explain for each loop with example For-Each Loop is another form of for loop used to traverse the array. It reduces the code significantly, and there is no use of the index or rather the counter in the loop. Exmple of for each loop: class UsingForEach { public static void main(String[] args) { String[] arrData = {"Alpha", "Beta", "Gamma", "Delta", "Sigma"}; //The conventional approach of using the for loop System.out.println("Using conventional For Loop:"); for(int i=0; i< arrData.length; i++){ System.out.println(arrData[i]); } System.out.println("\nUsing Foreach loop:"); //The optimized method of using the for loop - also called the foreach loop for (String strTemp : arrData){ System.out.println(strTemp); } } } 57) Explain diamond operator Diamond operator enables the compiler to collect the type arguments of generic class. In Java SE, developer can substitute the parameterized constructor with an empty parameter sets (<>) known as diamond operator. 58) Explain randomaccess interface RandomAccess interface is used by List implementations for the indication that they are supporting fast. 59) Name the collection classes that implement random access interface Java.util package has classes that can implement random access interface are: CopyOnWriteArrayList, Stack, ArrayList, and Vector. 60) How to join multiple ArrayLists? The list provides a addall() method multiple ArrayList in Java. For example, consider two lists 1) areaList and 2) secondAreaList. A developer can join them using addall() like: areaList.addAll(secondAreaList); 61) Explain deque Interface Java.util.Deque is Java, an interface that extends Queue interface. It gives support for the insertion and deletion of elements at both the end. This Queue is also called a double-ended queue. 62) Explain Linkedhashmap LinkedHashMap is the implementation of the Map interface. It can also extends the HashMap class. Therefore, like HashMap, LinkedHashMap enables Java developers to allow one null key and more than one null value. 69) How to iterate map? The developer cannot directly iterate map, but, this interface has two methods that gives view set of map. These methods are:  Set<Map.Entry<K, V>>entrySet(): It is a method that returns a set having the entries mention in the map. These entries are generally objected, which has type Map. Entry.  Set<K>keySet(): This Java method returns a set that having the map key. 70) Explain Treemap in Java TreeMap is a class that implements the Map interface LinkedHashMap and HashMap. It can also implements the NavigableMap interface and can extends the AbstractMap class. 71) What is the difference between Hashmap and Hashtable? Hashmap Hashtable It is not synchronized. It is synchronized. HashMap allows one key as a null value. HashTable does not allow null values. Iterator is used to traverse HashMap. Either Iterator or Enumerator is used for traversing a HashTable. It can be used for both HashTable, HashMap and is fail-fast. It can be used with HashTable and is fail-safe. HashMap perform faster than the HashTable. Hashtable is not much faster as compared to HashMap. 72) Explain the internal working of HashSet in Java HashSet in Java internally uses HashMap to store elements. It can also store unique values with no duplicate values. In Java, HashSet developer can have add(E e) method that takes just the element to add as a parameter. It does not accept the key and value pair. 73) Explain Big-O notation with an example The Big-O notation depicts the performance of an algorithm as the number of elements in ArrayList. A developer can use Big-O notation to choose the collection implementation. It is based on performance, time, and memory. For example, ArrayList get(index i) is a method to perform a constant-time operation. It does not depend on the total number of elements available in the list. Therefore, the performance in Big-O notation is O(1). 74) Explain the best practices in Java Collection Framework The best practices in Java Collection Framework are:  Choose the correct type of collection depends on the need.  Avoid rehashing or resizing by estimating the total number of elements to be stored in collection classes.  Write a Java program in terms of interfaces. This will help the developer to change it’s implementation effortlessly in the future.  A developer can use Generics for type-safety.  Use immutable classes given by the Java Development Kit. Avoid implementation of equals() and hashCode() for custom classes.  A programmer should use the Collections utility class for algorithms or to get read-only, synchronized, or empty collections. This will enhance code reusability with low maintainability. 75) Explain various types of queues in Java There are three types of queues in Java:  Priority queue: It is a special type of Queue wherein elements are sorted as per their natural ordering or custom comparator.  Circular Queue: It is a type of Queue in which user operations are performed based on the FIFO method. The last element is connected to the first position in order to make a circle.  Double-ended Queue: A double-ended queue is an abstract data type that generalizes a queue. The elements in this queue can be added or removed from either head or tail. 76) What is the difference between stack and Queue? Stack Queue The working principle of the stack is LIFO. Working principale of queue is FIFO. One end is used to perform the insertion or deletion of elements. One end is used to perform insertion, and another end is used for the deletion of elements. It uses one pointer. It uses two pointers in a simple queue. It does not have any kind of variant. It has variants like priority queue, circular Queue, doubly ended Queue. It is easy to use. It is not easy to use. 77) What is the difference between array and stack? The difference between array and stack is: Array Stack It is a collection of elements that are identified by the index. It is a collection operation that serve as operations push and pop. It has a elements of data types which are same. It has a elements of data types which are different. Elements can be removed or added into the array using random access operation. Elements can be removed or added into a stack using LIFO operation.