List of bugs fixed in the Java SE Development Kit 6u121 release. All objects in java inherit a default implementation of hashCode() function defined in Object class. This function produce hash code by typically converting the. Suppose we have a HashMap<String, Integer> in Java. How do I update (increment) the integer-value of the string-key for each existence of the string I find? This class implements a hash table, which maps keys to values. Any non-null object can be used as a key or as a value. To successfully store and retrieve objects from. The compiler may have to allocate a temporary variable to hold the value of i - 1, which means the postfix version might be slower. How to get the memory.

Top Java Collection Interview Questions. Without argument, java collections is one of the most important area where you will be tested in any position whether junior or senior. The scope is so much wide, that its almost impossible to cover all the questions.

Yet based on my previous interviews, I am attempting to put as many as possible GOOD java collection interview questions you must know. Windows Xp Tablet Pc Edition 2005 Sp 2 Msdn Dvd Movies. I am aiming both beginners and senior level questions, so bear with me if you found some questions too basic because they might be useful for some junior developers.

Questions in this post. General questions. What is the Java Collections API? List down its advantages? Explain Collections hierarchy? Why Collection interface does not extend Cloneable and Serializable interface?

Java Collection Framework Interview Question with Answer for experienced Java developers, programmers. Java Map Interface. A map contains values on the basis of key i.e. Each key and value pair is known as an entry. Map contains only unique keys.

Why Map interface does not extend Collection interface? What are main classes implementing List interface? How to convert an array of String to Array. List? 7) How to reverse the list? What are main classes implementing Set interface? How Hash. Set store elements?

Can a null element added to a Tree. Set or Hash. Set? What are main classes implementing Map interface?

What are Identity. Hash. Map and Weak. Hash. Map? 1. 3) Explain Concurrent.

Hash. Map? How it works? How hashmap works? How to design a good key for hashmap? What are different Collection views provided by Map interface? When to use Hash. Map or Tree. Map?

List down its advantages? By definition, a collection is an object that represents a group of objects.

Like in set theory, a set is group of elements. Easy enough !! Prior to JDK 1. JDK has some utility classes such as Vector and Hash. Table, but there was no concept of Collection framework.

Later from JDK 1. JDK felt the need of having a consistent support for reusable data structures. Finally, the collections framework was designed and developed primarily by Joshua Bloch, and was introduced in JDK 1. Its most noticeable advantages can be listed as: Reduced programming effort due to ready to use code. Increased performance because of high- performance implementations of data structures and algorithms. Provides interoperability between unrelated APIs by establishing a common language to pass collections back and forth. Easy to learn APIs by learning only some top level interfaces and supported operations.

Explain Collection’s hierarchy? Java Collection Hierarchy. As shown in above image, collection framework has one interface at top i. It is extended by Set, List and Queue interfaces.

Then there are loads of other classes in these 3 branches which we will learn in following questions. Remember the signature of Collection interface. It will help you in many question.

We will see the reason in 4th question in this question bank. Why Collection interface does not extend Cloneable and Serializable interface? Well, simplest answer is “there is no need to do it“. Extending an interface simply means that you are creating a subtype of interface, in other words a more specialized behavior and Collection interface is not expected to do what Cloneable and Serializable interfaces do. Another reason is that not everybody will have a reason to have Cloneable collection because if it has very large data, then every unnecessary clone operation will consume a big memory.

Beginners might use it without knowing the consequences. Another reason is that Cloneable and Serializable are very specialized behavior and so should be implemented only when required. For example, many concrete classes in collection implement these interfaces.

So if you want this feature. Collection has a method add(Object o). Map can not have such method because it need key- value pair. There are other reasons also such as Map supports key. Set, value. Set etc. Collection classes does not have such views.

Due to such big differences, Collection interface was not used in Map interface, and it was build in separate hierarchy. List interface related. Why we use List interface? What are main classes implementing List interface? A java list is a “ordered” collection of elements. This ordering is a zero based index.

It does not care about duplicates. Apart from methods defined in Collection interface, it does have its own methods also which are largely to manipulate the collection based on index location of element.

These methods can be grouped as search, get, iteration and range view. All above operations support index locations. The main classes implementing List interface are: Stack, Vector, Array. List and Linked. List. Read more about them in java documentation. How to convert an array of String to arraylist? This is more of a programmatic question which is seen at beginner level.

The intent is to check the knowledge of applicant in Collection utility classes. For now, lets learn that there are two utility classes in Collection framework which are mostly seen in interviews i. Collections and Arrays. Collections class provides some static functions to perform specific operations on collection types. And Arrays provide utility functions to be performed on array types. Use it reverse() method to reverse the list. What are main classes implementing Set interface?

It models the mathematical set in set theory. Set interface is like List interface but with some differences.

First, it is not ordered collection. So no ordering is preserved while adding or removing elements. The main feature it does provide is “uniqueness of elements“.

It does not support duplicate elements. Set also adds a stronger contract on the behavior of the equals and hash. Code operations, allowing Set instances to be compared meaningfully even if their implementation types differ. Two Set instances are equal if they contain the same elements. Based on above reasons, it does not have operations based on indexes of elements like List. It only has methods which are inherited by Collection interface.

Main classes implementing Set interface are : Enum. Set, Hash. Set, Linked. Hash. Set, Tree. Set.

Read more on related java documentation. How Hash. Set store elements? You must know that Hash. Map store key- value pairs, with one condition i. Hash. Set uses Map’s this feature to ensure uniqueness of elements.

In Hash. Set class, a map declaration is as below. Hash. Map< E,Object> map. This is added as value for each key.

Object PRESENT = new Object(). So when you store a element in Hash. Set, it stores the element as key in map and “PRESENT” object as value. This post will help you in answering all the Hash. Map related questions very easily. Can a null element added to a Tree.

Set or Hash. Set? As you see, There is no null check in add() method in previous question. And Hash. Map also allows one null key, so one “null” is allowed in Hash. Set. Tree. Set uses the same concept as Hash. Set for internal logic, but uses Navigable.

Map for storing the elements. So essentially, Tree. Set also does not support null keys. It will throw Null. Pointer. Exception if you try to add null element in Tree. Set. Map interface related. Why we use Map interface?

What are main classes implementing Map interface? Map interface is a special type of collection which is used to store key- value pairs. It does not extend Collection interface for this reason.

This interface provides methods to add, remove, search or iterate over various views of Map. Main classes implementing Map interface are: Hash. Map, Hashtable, Enum. Map, Identity. Hash. Map, Linked. Hash. Adobe Reader Free Download For Windows 8 Filehippo Firefox there. Map and Properties. What are Identity.

Hash. Map and Weak. Hash. Map? Identity. Hash. Map is similar to Hash. Map except that it uses reference equality when comparing elements. Identity. Hash. Map class is not a widely used Map implementation.

While this class implements the Map interface, it intentionally violates Map’s general contract, which mandates the use of the equals() method when comparing objects. Identity. Hash. Map is designed for use only in the rare cases wherein reference- equality semantics are required. Weak. Hash. Map is an implementation of the Map interface that stores only weak references to its keys.