Interview Questions and Answers for 'Java' | Search Interview Question

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Q621. How is HashSet maintained in memory by Java ?

Ans. HashSet is maintained as HashMap by Java with values of the HashSet as Keys of the HashMap and value of the HashMap as the constant PRESENT.

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Q622. What is Closeable ?

Core Java

Ans. A Closeable is an interface which is a source or destination of data that can be closed.

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Not frequently asked as it was introduced with Java 8.

Q623. What is the @FunctionalInterface annotation ?

Core Java

Ans. This is an informative annotation that specify that the interface is a functional interface. A Function Interface has only one abstract method and many default methods. Compiler generates an error if the interface specified with the annotation doesn't abide by the specifications for functional interface.

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Q624. Name some Java REST services frameworks ?

Rest

Ans. Jersey , Restlet , RestX, Spring , RestEasy ,Restfulie, Play Framework

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Q625. Is this valid in Java ?

Long x = new Long ("42");

Core Java

Ans. Yes. Long wrapper class has overloaded constructor which takes String as input and then translate it to the long value and stored it as long.

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Q626. What is classpath in a project?

Core Java

Ans. ClassPath is the path where Java looks for class files to resolve the dependencies. For example - If you are using a class "xyz" in your code and have specified the respective import, Where should Java look for the definition of xyz. Java determines using the class path settings.

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Q627. Why is a constant defined as a static final in Java?

Core Java

Ans. Final makes sure that the value doesn't change after initialization and static makes sure that there is only one copy that can be shared across objects.

Making it non static will unnecessarily create a different copy per object wherein the same value will kept for all copies ( as its final and cannot be changed ).

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Q628. What is the difference between public class and class ?

Core Java

Ans. Class without any access specifier has the default scope i.e it can be accessed by any class within same package.

Class declared public can be accessed from anywhere.

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Q629. Will this code compile ?

public class BuggyBread1{
abstract public void test();
}

Core Java

Ans. No. It will give the compilation error saying "The abstract method test in type BuggyBread1 can only be defined by an abstract class".

We need to declare the class abstract for it to have any abstract method.

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Q630. Will this Code compile ?

abstract public class BuggyBread1{
abstract public void test(){};
}

Core Java

Ans. No. This will give a compilation error saying "Abstract methods do not specify a body".

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Q631. Give an Example for Builder Pattern ?

Core Java

Ans. String str = new StringBuilder().append("Buggy").append("Bread").toString();

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Q632. Is this Polymorphism ?

Map<String, List<String>> inventoryManagerCountMap = new HashMap<String, ArrayList<String>>();

Core Java

Ans. No. This will result in compilation error as Polymorphism cannot be performed on Object types.

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Rarely asked as it was introduced with Java 8.

Q633. Difference between DoubleSummaryStatistics , IntSummaryStatistics and LongSummaryStatistics ?

Core Java

Ans. They all does the same task i.e to compute statistical information on the stream of data. They differ by the way they store the statistical information as they expect a different data type of the values being used.

IntSummaryStatistics and LongSummaryStatistics expect non floating point values and hence stores the statistical information like min,max and sum as non floating values ( int or long ) whereas DoubleSummaryStatistics stores these information as floating value.

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Q634. Have you ever used StringTokenizer ?

Core Java

Ans. No, Alternatively I have used String.split("\\s") for achieving the same function.

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Q635. What is ConcurrentLinkedDeque ?

Core Java

Ans. It's a Collections concrete class that provides implementation of a Double Ended queue that allows concurrent insertion, removal, and access operations that can be executed safely across multiple threads.

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Q636. What is CopyOnWriteArrayList ?

Ans. Its a type of ArrayList in which all Write operations , i.e add and set are performed by creating a new copy. This array never changes during the lifetime of the iterator, so it never throws ConcurrentModificationException

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Q637. What are the advantages and disadvantages of CopyOnWriteArrayList ?

Core Java

Ans. This collections class has been implemented in such a manner that it can never throw ConcurrentModificationException. As it performs update and write operations by creating a new copy of ArrayList, It's slower compared to ArrayList.

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Q638. Name few Concurrent Collection Classes ?

Ans. ConcurrentHashMap
ConcurrentLinkedDeque
ConcurrentLinkedQueue
ConcurrentMap
ConcurrentNavigableMap
ConcurrentSkipListMap
ConcurrentSkipListSet

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Q639. Name few Collections Map implementations ?

Ans. AbstractMap
ConcurrentHashMap
ConcurrentSkipListMap
EnumMap
HashMap
IdentityHashMap
LinkedHashMap
SystemFlavorMap
TreeMap
WeakHashMap

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Q640. What is ArrayIndexOutOfBoundException ?

Core Java

Ans. Exception thrown by the application is we try to access an element using an index which is not within the range of array i.e lower than 0 or greater than the size of the array.

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Q641. how can we generate JavaDoc documentation for our code ?

Eclipse

Ans. In Eclipse, Go to File -> Export -> Java -> JavaDoc

Select the projects , other properties and output directory for which JavaDoc and Click Finish.

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Q642. Can we initialize member variables within static block ?

Ans. Static block is like static method that gets executed upon class loading. The way static method allows accessing member variables ( using object references only ), the same static block can also access and initialize member variables.

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Q643. Name few interfaces that extends Collection Interface ?

Ans. http://www.buggybread.com/2015/02/java-collections-interfaces-that.html

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Q644. Name few classes that extends Number Class ?

Ans. http://www.buggybread.com/2015/02/java-util-classes-that-inherit-number.html

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Q645. Name few classes that extend Thread class ?

Core Java

Ans. http://www.buggybread.com/2015/02/java-threads-classes-that-inherit.html

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Q646. Name few classes that extends Collections class ?

Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-inherit.html

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Q647. Name few Date and Time Classes you have used in your projects ?

Ans. http://www.buggybread.com/2015/02/java-util-date-time-classes-and.html

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Q648. Name few Calendar related classes and Interfaces ?

Ans. http://www.buggybread.com/2015/02/java-util-calendar-classes-and.html

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Q649. Name few abstract classes of Java collections framework ?

Ans. http://www.buggybread.com/2015/02/java-collections-abstract-classes.html

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Q650. Name few classes that implement Cloneable interface ?

Ans. http://www.buggybread.com/2015/02/java-collections-classes-that-implement_58.html

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