Java interview questions

What does System.gc() and Runtime.gc() methods do ?

Answer:

These methods can be used as a hint to the JVM, in order to start a garbage collection. However, this it is up to the Java Virtual Machine (JVM) to start the garbage collection immediately or later in time.

What is the purpose of garbage collection in Java, and when is it used ?

Answer:

The purpose of garbage collection is to identify and discard those objects that are no longer needed by the application, in order for the resources to be reclaimed and reused.

What is the difference between HashSet and TreeSet ?

Answer:

The HashSet is Implemented using a hash table and thus, its elements are not ordered. The add, remove, and contains methods of a HashSet have constant time complexity O(1). On the other hand, a TreeSet is implemented using a tree structure. The elements in a TreeSet are sorted, and thus, the add, remove, and contains methods have time complexity of O(logn).

What’s the difference between Enumeration and Iterator interfaces ?

Answer:

Enumeration is twice as fast as compared to an Iterator and uses very less memory. However, the Iterator is much safer compared to Enumeration, because other threads are not able to modify the collection object that is currently traversed by the iterator. Also, Iteratorsallow the caller to remove elements from the underlying collection, something which is not possible with Enumerations.

What are some of the best practices relating to the Java Collection framework ?

Answer:

• Choosing the right type of the collection to use, based on the application’s needs, is very crucial for its performance. For example if the size of the elements is fixed and know a priori, we shall use an Array, instead of an ArrayList.
• Some collection classes allow us to specify their initial capacity. Thus, if we have an estimation on the number of elements that will be stored, we can use it to avoid rehashing or resizing.
• Always use Generics for type-safety, readability, and robustness. Also, by using Generics you avoid the ClassCastException during runtime.
• Use immutable classes provided by the Java Development Kit (JDK) as a key in a Map, in order to avoid the implementation of the hashCode and equals methods for our custom class.
• Program in terms of interface not implementation.
• Return zero-length collections or arrays as opposed to returning a null in case the underlying collection is actually empty.

What is the tradeoff between using an unordered array versus an ordered array ?

Answer:

The major advantage of an ordered array is that the search times have time complexity of O(log n), compared to that of an unordered array, which is O (n). The disadvantage of an ordered array is that the insertion operation has a time complexity of O(n), because the elements with higher values must be moved to make room for the new element. Instead, the insertion operation for an unordered array takes constant time of O(1).

What do you know about the big-O notation and can you give some examples with respect to different data structures ?

Answer:

The Big-O notation simply describes how well an algorithm scales or performs in the worst case scenario as the number of elements in a data structure increases. The Big-O notation can also be used to describe other behavior such as memory consumption. Since the collection classes are actually data structures, we usually use the Big-O notation to chose the best implementation to use, based on time, memory and performance. Big-O notation can give a good indication about performance for large amounts of data.

What is Java Priority Queue ?

Answer:

The PriorityQueue is an unbounded queue, based on a priority heap and its elements are ordered in their natural order. At the time of its creation, we can provide a Comparator that is responsible for ordering the elements of the PriorityQueue. A PriorityQueue doesn’t allow null values, those objects that doesn’t provide natural ordering, or those objects that don’t have any comparator associated with them. Finally, the Java PriorityQueue is not thread-safe and it requires O(log(n)) time for its enqueing and dequeing operations.

What is Comparable and Comparator interface ? List their differences.

Answer:

Java provides the Comparableinterface, which contains only one method, called compareTo. This method compares two objects, in order to impose an order between them. Specifically, it returns a negative integer, zero, or a positive integer to indicate that the input object is less than, equal or greater than the existing object. Java provides the Comparator interface, which contains two methods, called compare and equals. The first method compares its two input arguments and imposes an order between them. It returns a negative integer, zero, or a positive integer to indicate that the first argument is less than, equal to, or greater than the second. The second method requires an object as a parameter and aims to decide whether the input object is equal to the comparator. The method returns true, only if the specified object is also a comparator and it imposes the same ordering as the comparator.

What is difference between ArrayList and LinkedList ?

Answer:

Both the ArrayList and LinkedList classes implement the List interface, but they differ on the following features:

• An ArrayList is an index based data structure backed by an Array. It provides random access to its elements with a performance equal to O(1). On the other hand, a LinkedList stores its data as list of elements and every element is linked to its previous and next element. In this case, the search operation for an element has execution time equal to O(n).
• The Insertion, addition and removal operations of an element are faster in a LinkedList compared to an ArrayList, because there is no need of resizing an array or updating the index when an element is added in some arbitrary position inside the collection.
• A LinkedList consumes more memory than an ArrayList, because every node in a LinkedList stores two references, one for its previous element and one for its next element.

Check also our article ArrayList vs. LinkedList.

What is difference between Array and ArrayList ? When will you use Array over ArrayList ?

Answer:

The Arrayand ArrayList classes differ on the following features:

• Arrays can contain primitive or objects, while an ArrayList can contain only objects.
• Arrays have fixed size, while an ArrayList is dynamic.
• An ArrayListprovides more methods and features, such as addAll, removeAll, iterator, etc.
• For a list of primitive data types, the collections use autoboxing to reduce the coding effort. However, this approach makes them slower when working on fixed size primitive data types.

What differences exist between HashMap and Hashtable ?

Answer:

Both the HashMap and Hashtable classes implement the Map interface and thus, have very similar characteristics. However, they differ in the following features:

• A HashMap allows the existence of null keys and values, while a Hashtable doesn’t allow neither null keys, nor null values.
• A Hashtable is synchronized, while a HashMap is not. Thus, HashMap is preferred in single-threaded environments, while a Hashtable is suitable for multi-threaded environments.
• A HashMap provides its set of keys and a Java application can iterate over them. Thus, a HashMap is fail-fast. On the other hand, a Hashtable provides an Enumeration of its keys.
• The Hashtable class is considered to be a legacy class.

What is the importance of hashCode() and equals() methods ?

Answer:

In Java, a HashMap uses the hashCodeand equals methods to determine the index of the key-value pair and to detect duplicates. More specifically, the hashCodemethod is used in order to determine where the specified key will be stored. Since different keys may produce the same hash value, the equals method is used, in order to determine whether the specified key actually exists in the collection or not. Therefore, the implementation of both methods is crucial to the accuracy and efficiency of the HashMap.

How HashMap works in Java ?

Answer:

A HashMap in Java stores key-value pairs. The HashMap requires a hash function and uses hashCode and equals methods, in order to put and retrieve elements to and from the collection respectively. When the put method is invoked, the HashMap calculates the hash value of the key and stores the pair in the appropriate index inside the collection. If the key exists, its value is updated with the new value. Some important characteristics of a HashMap are its capacity, its load factor and the threshold resizing.

What is difference between fail-fast and fail-safe ?

Answer:

The Iterator's fail-safe property works with the clone of the underlying collection and thus, it is not affected by any modification in the collection. All the collection classes in java.util package are fail-fast, while the collection classes in java.util.concurrent are fail-safe. Fail-fast iterators throw a ConcurrentModificationException, while fail-safe iterator never throws such an exception.

What differences exist between Iterator and ListIterator ?

Answer:

The differences of these elements are listed below:

• An Iterator can be used to traverse the Set and Listcollections, while the ListIterator can be used to iterate only over Lists.
• The Iterator can traverse a collection only in forward direction, while the ListIterator can traverse a List in both directions.
• The ListIterator implements the Iterator interface and contains extra functionality, such as adding an element, replacing an element, getting the index position for previous and next elements, etc.

What is an Iterator ?

Answer:

The Iterator interface provides a number of methods that are able to iterate over any Collection. Each Java Collection contains the iteratormethod that returns an Iterator instance. Iterators are capable of removing elements from the underlying collection during the iteration.

Why Collection doesn’t extend Cloneable and Serializable interfaces ?

Answer:

The Collection interface specifies groups of objects known as elements. Each concrete implementation of a Collection can choose its own way of how to maintain and order its elements. Some collections allow duplicate keys, while some other collections don’t. The semantics and the implications of either cloning or serialization come into play when dealing with actual implementations. Thus, the concrete implementations of collections should decide how they can be cloned or serialized.

What are the basic interfaces of Java Collections Framework ?

Answer:

Java Collections Framework provides a well designed set of interfaces and classes that support operations on a collections of objects. The most basic interfaces that reside in the Java Collections Framework are:

• Collection, which represents a group of objects known as its elements.
• Set, which is a collection that cannot contain duplicate elements.
• List, which is an ordered collection and can contain duplicate elements.
• Map, which is an object that maps keys to values and cannot contain duplicate keys.

How do you ensure that N threads can access N resources without deadlock ?

Answer:

A very simple way to avoid deadlock while using N threads is to impose an ordering on the locks and force each thread to follow that ordering. Thus, if all threads lock and unlock the mutexes in the same order, no deadlocks can arise.