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Rarely asked as it was introduced with Java 8.

Q3241. What are the errors identified by DocLint ?

Ans. Bad syntax, such as unescaped characters or unmatched parentheses

Bad HTML, such as invalid or missing tags or attributes

Bad references, such as referencing a non-existent type with @see

Accessibility errors, such as a missing summary or caption from a table

Missing info, such as an undocumented parameter

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Q3242. What are some differences between Spring caching and Hibernate caching?

Hibernate

Ans. Spring caching makes use of its intercepting capabilities to add caching to method calls. Therefore, the entire object is cached and reused. Hibernate, on the other hand, has more domain-specific knowledge of the Entity being cached and can handle the objects more appropriately.

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Q3243. What is the use of runnable interface if we can always create a new thread using Thread class ?

Core Java

Ans. It's a choice to be made whether to use first approach ( Thread class ) or second approach ( runnable interface ) by the programmer. The second facility has been given for cases where your class is already extending some parent class and hence cannot extend another class ( for Thread ) as Java doesn't allow multiple inheritance.

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Q3244. Why do we need Thread class even in case we execute thread using runnable interface ?

Core Java

Ans. Thread class holds the definition of start method ( This is the method that starts execution of new thread and then calls run method within the scope of new thread ). Interfaces don't hold any definition and so does runnable. So it makes it necessary the usage of Thread class , whatever implementation you choose.

When your class extends the thread class, it carries the definition of start method from parent Thread class onto itself and hence new yourClass.start() helps starting a new thread and then executing run method in that new thread scope.

When you implement runnable interface , you are just making it sure to the JVM that you have implemented the required method ( run() ) which the Thread start method will look for upon executing start method.

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Q3245. Why Java uses Unicode System ?

Core Java

Ans. Before Unicode there were many language / country standards like ASCII , ISO , KOI and BIG 5. In order to accommodate multiple language letters and to overcome problems with using different multiple standards , a new language standard was developed i.e unicode system.

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Q3246. What is the difference between out.println(a+b); and out.println (a+" " +b); in Java? with a=2 and b=1

Core Java

Ans. First will give the ouput as 3 and the second will give output as 2 1

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Q3247. Which sorting algorithm is used by Collections.sort() in Java ?

Core Java

Ans. The sorting algorithm is a modified mergesort. This algorithm offers guaranteed n log(n) performance.

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Q3248. What doesn Http keep alive header does ?

Java EE

Ans. Http is connection less by default. header informs hosts to keep the connection alive so that the same connection can be reused for multiple communication.

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Q3249. What is the difference between Data Type and Data Structure ?

Core Java

Ans. Data type: a set of values together with operations on that type
Data structure: a physical implementation of a data type

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Q3250. Is it correct to say that Interfaces are abstract data types ?

Ans. No.

Data Type holds data whereas Interface doesn't hold anything. Interface is a contract about how to communicate with the underlying Class.

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Q3251. Do you see Class as a Data Type or Data Structure ?

Ans. Class can be better seen as Data Type. This could be implemented as a Data Structure too in some cases.

One thing worth understanding here is that Data type and Data structure are conceptual things. Class could be implementation of either of these.

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Q3252. How many elements will be there in TreeSet after the last line and Why ?

TreeSet set = new TreeSet();
set .add(new String("abc"));
set .add(new String("abc"));

Core Java

Ans. One.

As we haven't specified the type of TreeSet, it being evaluated with the first element insertion. Once it's identified that it's of type String and as no comparator has been defined, the comparison is done using the String compareTo method. String compareTo method compares the elements by the content / value.

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Q3253. Which of the following data type you will prefer to store price - BigDecimal or Double ?

Ans. BigDecimal if memory and Performance is not critical concern, otherwise double.

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Q3254. Which takes more memory - float or Float ?

Core Java

Ans. float is a native data type whereas Float is a class. A Float object will always take more memory than float variable as there are metadata overheads with the objects.

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Q3255. Difference between HashMap and WeakHashMap ?

Ans. WeakHashMap uses weak reference for keys, which means if the key object doesn't have any reference then both key/value mapping will become eligible for garbage collection.

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Q3256. Difference between Stack and Heap memory ?

Core Java

Ans. Stack memory areas is used to hold method and local variables while objects are always allocated memory in the heap. The heap memory is shared between multiple threads whereas Stack memory isn't.

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Q3257. Which tree is used to implement TreeMap in Java?

Ans. Red Black Tree

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Q3258. What is collision in hash collections ?

Core Java

Ans. Collision is the situation when two different elements have the same Hash Code.

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Q3259. When are static and instance methods resolved ? During compile time or Runtime ?

Ans. Static methods are resolved during compile time and hence they cannot participate in runtime polymorphism. Instance methods are resolved during runtime.

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Q3260. What will the following code result ?

List> intList = new ArrayList>();

intList.add(Optional.empty());
intList.add(Optional.of(2));
intList.add(Optional.of(3));
intList.add(Optional.of(4));

System.out.println(intList.get(null));

Ans. Compile time error at last line as the get method expect argument of type native int.

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Q3261. What will the following code result ? Will it compile ?

List> intList = new ArrayList>();

intList.add(Optional.empty());
intList.add(Optional.of(2));
intList.add(Optional.of(3));
intList.add(Optional.of(4));

System.out.println(intList.get((Integer)null));

Ans. Yes but the last line will throw NullPointerException upon execution.

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Q3262. What will be the output ?Integer a = 10, b =10;Integer c = 10, d = 1000;System.out.println(a == b);System.out.println(c ==d);

Core Java

Ans. truefalse

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Q3263. How to determine if the linked list has a cycle in it ?

Ans. http://stackoverflow.com/questions/494830/how-to-determine-if-a-linked-list-has-a-cycle-using-only-two-memory-locations

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Q3264. Can we have multiple classes in single file ?

Ans. Yes, but there can be only 1 public class in a single file.

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Q3265. What is a BlockingQueue?

Data Structures

Ans. BlockingQueue is a Queue that supports operations that wait for the queue to become non-empty when retrieving and removing an element, and wait for space to become available in the queue when adding an element.

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Q3266. What is OutOfMemoryError in Java?

Core Java

Ans. OutOfMemoryError in Java is a subclass of VirtualMachineError and is thrown by JVM when it runs out of heap memory.

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Q3267. How to access a Web Element if there are many elements with the same XPath ?

Ans. By specifying the Index like

//button[@class='button'])[2]

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Q3268. What is Apache Kafka ?

BigData

Ans. Kafka is a distributed, partitioned, replicated commit log service. It provides the functionality of a messaging system through messages being written to logs.

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Q3269. What is a broker in Apache Kafka ?

Ans. Kafka is run as a cluster comprised of one or more servers each of which is called a broker

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Q3270. What is a Topic in Apache Kafka ?

Ans. A topic is a category or feed name to which messages are published

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