Introduction

A Java Collection Interview Questions blog discusses the questions with proper explanations. Before introducing Collection Framework (or JDK 1.2), the standard techniques for aggregating Java objects (or collections) were Arrays, Vectors, or Hash Tables. There was no standard interface for all of these collections. As a result, while the primary goal of all the collections is the same, their implementation was specified independently, and there was no correlation between them. Furthermore, users need help remembering each collection class’s methods, syntax, and constructors.

What is JAVA Collection?

In Java, a collection is a framework that provides an architecture for storing and manipulating a collection of objects. JDK 1.2 created a new “Collection Framework” framework containing all collection classes and interfaces. 

Collections in Java are capable of doing any data operations, such as searching, sorting, insertion, manipulation, and deletion.

Java Collections are the one-stop solutions for all the data manipulation jobs such as storing data, searching, sorting, insertion, deletion, and updating data. Java collection responds as a single object, and a Java Collection Framework provides various Interfaces and Classes.

JAVA Collection Interview Questions

These JAVA Collection interview questions can be classified majorly into the following three categories:

• JAVA Collection Interview Question for Freshers
• JAVA Collection Interview Question for experienced

Java Collection Interview Question for Freshers

What are the advantages of the Collection framework?

During JAVA Collection Interview Questions, the interviewer may ask you to explain the advantages of the JAVA Collection framework. The following are the advantages of the Collection framework:-

Consistent API: The API has a core set of interfaces like Collection, Set, List, or Map, and all the classes (ArrayList, LinkedList, Vector, and so on) that implement these interfaces have some standard set of methods.

Cuts programming effort: Instead of worrying about the Collection’s design, a programmer may concentrate on how best to use it in his program. As a result, the fundamental principle of Object-oriented programming (i.e. abstraction) has been applied successfully.

It improves the program speed and quality by offering high-performance implementations of functional data structures and algorithms. The programmer does not have to worry about the optimum implementation of a particular data structure in this scenario. They can use the best implementation to improve their program’s performance significantly.

What do you Understand by Collection Framework in Java?

The Java Collections framework provides an architecture to store and manage a group of objects. It permits the developers to access prepackaged data structures and algorithms to manipulate data. The collection framework includes the following:

1. Interfaces
2. Classes
3. Algorithm

What is a priority queue in Java?

When the objects are meant to be processed in order of priority, a PriorityQueue is used. A Queue is known to follow the First-In-First-Out method. However, there are occasions when the queue components must be handled in order of priority, where the PriorityQueue comes into play. The priority heap is the foundation of the PriorityQueue. The members of the priority queue are ordered according to natural ordering or by a Comparator provided at queue construction time. You must prepare this as it will be asked during JAVA Collection Interview Questions.

What is the use of the List Interface?

The List interface in Java is an ordered collection of elements. It maintains the insertion order and allows duplicate values to be stored within. This interface contains various methods which enable the smooth manipulation of elements based on the element index. The main classes implementing the List interface of the Collection framework are ArrayList, LinkedList, Stack, and Vector.

What is ArrayList in Java?

During JAVA Collection Interview Question, the interviewer may ask you to explain the ArrayList in JAVA. ArrayList implements List Interface where the elements can be dynamically added or removed from the list. ArrayList in the Collection framework provides positional access and insertion of features. It is an ordered collection that permits duplicate values. The size of an ArrayList can be increased dynamically if the number of elements is more than the initial size. 

Why does Collection not extend the Cloneable and Serializable interfaces?

The Collection interface in Java specifies a group of objects called elements. The maintainability and ordering of elements depend entirely on the concrete implementations provided by each Collection. Thus, there is no use in extending the Cloneable and Serializable interfaces. You must prepare this as it will be asked during JAVA Collection Interview Questions.

What is the need for overriding the equals() method in Java?

The initial implementation of the equals method helps check whether two objects are the same. But if you want to compare the objects based on the property, you must override this method.

How are the Collection objects sorted in Java?

You must prepare this as it will be asked during JAVA Collection Interview Questions. Sorting in Java Collections is implemented via Comparable and Comparator interfaces. When the Collections. The sort() method is used, and the elements get sorted based on the natural order specified in the compareTo() method. On the other hand, when Collections. sort(Comparator) method is used; it sorts the objects based on compare() method of the Comparator interface.

Why is the Collection used in Java?

During JAVA Collection Interview Questions, the interviewer may ask you to explain why the java collection is used. The Java Collections Framework provides the following benefits: Reduces programming effort: By providing functional data structures and algorithms, it frees you to concentrate on the essential parts of your program rather than on the low-level “plumbing” required to make it work.

Differentiate between ArrayList and Vector.

ArrayList	Vector
Non-synchronized in nature	Synchronized in nature
It is not a legacy class	Is a legacy class
Increase s size by 1/2 of the ArrayList	Increases size by double of the ArrayList
It is not thread-safe	It is thread-safe

Java Collection Interview Question for Experienced

What do you understand about BlockingQueue in Java?

BlockingQueue interfaces with concurrent Utility classes such as ConcurrentHashMap, Counting Semaphore, CopyOnWriteArrayList, etc. In addition to queueing, the BlockingQueue interface enables flow control by adding blocking if either BlockingQueue is full or empty. 

A thread attempting to enqueue an element in a full queue will be blocked until another thread clears the queue, either by dequeuing one or more elements or clearing the queue entirely. BlockingQueue’s methods are all atomic and use internal locks or other forms of concurrency management. This question might be asked during the JAVA Collection interview Question.

What is the use of the Properties Class in Java? What is the Advantage of the Properties File?

The key and value pair are both strings in the properties object. The java.util.Properties class is a Hashtable subclass.

It can be used to calculate the value of a property based on its key. Java Collection Interview Question, The Properties class has methods for reading and writing data to and from the properties file. It can also be used to obtain a system’s attributes.

Advantage of the Properties file: 

If the information in a properties file is modified, no recompilation is required: You don’t need to recompile the java class if any information in the properties file changes. It is used to keep track of information that needs to be updated frequently.

Why does HashMap allow null, whereas HashTable does not allow null?

The objects used as keys must implement the hashCode and similar methods to save and retrieve objects from a HashTable. Null cannot implement these methods because it is not an object. HashMap is a more advanced and improved variant of Hashtable.HashMap was invented after HashTable to overcome its shortcomings of HashTable.You must prepare this as it will be asked during JAVA Collection Interview Questions.

Why do we need a synchronized ArrayList when we have Vectors in Java?

 This question might be asked during the JAVA Collection interview Question. We need synchronized ArrayLists even though we have Vectors:

• Vector is slightly slower than ArrayList because it is synchronized and thread-safe.
• Vector’s functionality is that it synchronizes each action. A programmer’s preference is to synchronize an entire sequence of actions. Individual operations are less safe and take longer to synchronize.
• Vectors are considered outdated in Java and have been unofficially deprecated. Vector also synchronizes on a per-operation basis, which is rarely done. Most java programmers prefer to use ArrayList since they will almost certainly synchronize the ArrayList explicitly if they need to do so

Why does not the Map interface Extend the Collection Interface or vice-versa?

If Map extends the Collection Interface, “Key-value pairs” can be the only element type for this type of Collection. Java Collection Interview Question,  However, this provides a minimal (and not useful) Map abstraction. You can only inquire about what value a specific key corresponds to and delete an entry if you know what value it corresponds to.

The three “Collection view procedures” on Maps represent the fact that Maps can be viewed as Collections (of keys, values, or pairs) (keySet, entrySet, and values). While it is theoretically feasible to see a List as a Map mapping indices to items, this has the unfortunate side effect of changing the Key associated with every element in the List before the deleted member. This is why Collection can not be made to extend the Map Interface either. You should prepare for this question during the JAVA Collection interview question.

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What do you understand by BlockingQueue?

During JAVA Collection Interview Questions, the interviewer may ask you to explain your point of view about BlockingQueue. The BlockingQueue interface belongs to the java.util.concurrent package. This interface enhances flow control by activating blocking if a thread tries to dequeue an empty or enqueue an already full queue. While working with the BlockingQueue interface in Java, remember that it does not accept a null value. Java Collection Interview Question, If you try to do that, it will instantly throw a NullPointerException. The below figure represents the working of the BlockingQueue interface in Java.

What is a Priority Queue in Java?

A priority queue in Java is an abstract data type similar to a regular queue or stack data structure but has a unique feature called priority associated with each element. In this queue, a high-priority element is served before a low-priority element, Java Collection Interview Question, irrespective of its insertion order. The PriorityQueue is based on the priority heap. The elements of the priority queue are ordered according to the natural ordering or by a Comparator provided at queue construction time, depending on which constructor is used.

Can you add a null element into a TreeSet or HashSet?

You must prepare this as it will be asked during JAVA Collection Interview Questions. Only one null element can be added in HashSet, but in TreeSet, it can’t be added as it uses NavigableMap to store the elements. This is because NavigableMap is a subtype of SortedMap that doesn’t allow null keys. So, in case you try to add null elements to a TreeSet, it will throw a NullPointerException.

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Why does Map not Extend the Collection Interface?

You should prepare for this question during the JAVA Collection interview question. The Map interface in Java follows a key/value pair structure. In contrast, the Collection interface is a collection of objects stored in a structured manner with a specified access mechanism. Java Collection Interview Question, The major reason Map doesn’t extend the Collection interface is that the add(E e) method of the Collection interface doesn’t support the key-value pair like the Map interface’s put(K, V) method. It might not extend the Collection interface, but it still is an integral part of the Java Collections framework.

Differentiate between HashSet and HashMap.

Conclusion

So this brings us to the end of the JAVA Colle
ction Interview Questions blog. This set of Java Collection Interface Questions will help you ace your job interview. The Collection in Java is a framework that provides an architecture to store and manipulate a group of objects. Java Collections can achieve all the operations you perform on data, such as searching, sorting, insertion, manipulation, and deletion.

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