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Rarely asked as it was introduced with Java 8.

Q1951. What is DocLint in Java 8 ?

Ans. DocLint Provide a means to detect errors in Javadoc comments early in the development cycle and in a way that is easily linked back to the source code.

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Rarely asked as it was introduced with Java 8.

Q1952. What are the errors identified by DocLint ?

Core Java

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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Q1953. 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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Q1954. 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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Q1955. 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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Q1956. 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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Q1957. 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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Q1958. 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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Q1959. 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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Q1960. 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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Q1961. 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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Q1962. 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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Q1963. Which tree is used to implement TreeMap in Java?

Ans. Red Black Tree

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Q1964. 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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Q1965. 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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Q1966. 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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Q1967. 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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Q1968. 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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Q1969. 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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Q1970. 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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Q1971. 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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Q1972. 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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Q1973. 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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Q1974. Do you know of any facility to convert between Scala and Java Util Collections ?

Scala

Ans. We can use JavaConversions which is a part of Scala Library.

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Q1975. Which interfaces are implemented by BatchUpdateException?

Ans. [Iterable]

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Q1976. What is the package name for CertStoreParameters class?

Ans. java.lang

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Q1977. Which is the Parent Class of CertStoreParameters class?

Ans. Clonable

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Q1978. Which access specifiers can be used with top level class ? a. public or default b. public or private c. public or protected d. protected or default

Core Java

Ans. public or default

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Q1979. Which of the following can be marked static ?

a. Methods , Variables and Initialization Blocks.
b. Methods , Variables , Initialization Blocks and Outer Classes and nested Classes.
c. Methods , Variables , Initialization Blocks and Outer Classes.
d. Methods , Variables , Initialization Blocks and nested Classes

Core Java

Ans. Methods , Variables , Initialization Blocks and nested Classes

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Q1980. Which of the following cannot be marked static ?

a. Constructors , Classes ( Outer ) , Classes ( nested ), Interfaces , Local variables , Inner Class methods and instance variables.
b. Constructors , Classes ( Outer ) , Interfaces , Local variables , Class variables , Class Methods , Inner Class methods and instance variables.
c. Constructors , Classes ( Outer ) , Interfaces , Local variables , Inner Class methods and instance variables.
d. Constructors , Classes ( Outer ) , Classes (Nested), Interfaces , Local variables , Inner Class methods and instance variables

Ans. Constructors , Classes ( Outer ) , Interfaces , Local variables , Inner Class methods and instance variables.

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