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

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Q971. What is ModuleLayer ?

Core Java

Ans. A Module layer is created from a graph of modules and is a function that maps modules to their class loaders.

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Q972. Difference between named and unnamed modules ?

Core Java

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Q973. What is the use of Requires, Exports, Opens and provides within Module Descriptor ?

Core Java

Ans. Requires specifies the dependencies for the module, i.e the modules required for this module to function.

Exports specifies the packages exported by the module.

Opens specifies the packages opened by the module.

Provides specifies the services provided by the module

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Q974. Does Java 9 allows same package within different modules ?

Core Java

Ans. Yes it allows if the modules are mapped to different class loaders.

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Q975. What are the qualified and unqualified packages ?

Core Java

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Q976. How does modularity in Java 9 facilitates better encapsulation ?

Core Java

Ans. By providing dependencies , services and exports explicitly

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Q977. How does Java 9 provides backward compatibility with libraries that haven't published themselves as modules ?

Core Java

Ans. Though Automatic modules. Any JAR on the module path without module descriptor ends up as automatic module

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Q978. What is the default setting for Automatic modules regarding exporting packages and reading modules ?

Core Java

Ans. As one purpose of automatic module is to provide backward compatibility with the module less dependencies, automatic modules exports all packages and include all other modules.

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Q979. What is JShell?

Core Java

Ans. JShell is a feature in java 9 using which we can code and test without compiling using javac and see the result of calculations directly.

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Q980. Can we have private methods in interfaces ?

Core Java

Ans. Yes, With Java 9

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Q981. What could be the reason for allowing private methods in java 9 ?

Core Java

Ans. Java 8 allowed for method implementation using default methods in interfaces. As those default methods could contain complex logic and might need organizing the logic into multiple methods, they have allowed for private methods.

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Q982. What is the difference between implementing interfaces and inheritance in Java ?

Core Java

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Q983. What is a weak reference ?

Core Java

Ans. A weak reference is a reference that gets removed on the next garbage collection cycle.

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Q984. Write a program that count the number of access to a method?

Core Java

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Q985. Can you explain following exception

java.lang.ArithmeticException: Non-terminating decimal expansion; no exact representable decimal result.

Core Java

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Q986. What are we doing here

list.stream().collect(Collectors.groupingBy(ClassA::getElement1,
Collectors.averagingDouble(ClassA::getelement2)));

Core Java

Ans. We are averaging elements of a collection by element1 grouped by element1

* list is the reference of collection
* element1 is the member element returned by getElement1 method of ClassA
* element2 is the member element returned by getElement2 method of ClassA

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Q987. What will the following code return

list.stream().collect(Collectors.groupingBy(ClassA::getElement1,
Collectors.averagingDouble(ClassA::getElement2)));

Core Java

Ans. Map<DataType1,DataType2> where DataType1 is the data type of element1 ( returned by getelement1 method ) and DataType2 is the data type of element2 ( returned by getElement2 method )

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Q988. What is the advantage of BidDecimal over Double ?

Core Java

Ans. BigDecimal provides more precision as compared to double.

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Q989. What is the difference between following rounding modes

RoundingMode.CEILING
RoundingMode.DOWN
RoundingMode.FLOOR
RoundingMode.HALF_DOWN
RoundingMode.HALF_UP
RoundingMode.HALF_EVEN

Which is the one you use often and Why ?

Core Java

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Q990. Have you worked on creating any cron job ?

Core Java

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Q991. Have you used any framework to capture command line arguments or options ?

Core Java

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Q992. Write code to get count of every 5 letter word in a string using lambda expression

Core Java

Ans. String str = "I had been saying that he had been there";

Map<String,Long> countWords = Arrays.asList(str.split(" ")).stream().filter(p->p.length() = 5).collect(Collectors.groupingBy(p->p,Collectors.counting()));

System.out.println(countWords);

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Q993. Can you write code so that we can keep track of Keys that have their values changed along with the number of times they were changed.

Core Java

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Q994. What are the different types of methods in Java ?

Core Java

Ans. Static Methods
Instance Methods
Accessor / Mutator or getter setters

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Q995. Are getters setter / accessor mutator methods instance methods or static methods ? why ?

Core Java

Ans. They are usually instance methods. They are usually applied to DAOs / DTOs / POJO that are used to transport objects and used to access object elements. As they are tied to objects and are not common to all objects of a class, hence they are created as non static.

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Q996. Design a class that find all files with SSN.

Core Java

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Q997. Describe inheritance and polymorphism and their importance for OOP.

Core Java

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Q998. What is the difference between instantiation and initialization ?

Core Java

Ans. Instantiation is the process of reserving memory for the object

For ex -

List list = new ArrayList();

Initialization is the process of setting the default value for the object or default object construction.

List list = new ArrayList(set);

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Q999. Why do we need to close the Scanner ?

Core Java

Ans. For releasing input stream reference (System.in), you should close scanner.

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Q1000. Can you explain filter and collectors in java 8 ?

Core Java

Ans. Filters are used to filter object in stream on certail criteria. And collector is used to collect outcome of the stream. Collector is a terminal operation.
List numbers = List.of(1,2,3,4,5,6,7);
List evenNumbers = numbers.stream().filter(n -> n%2==0).collect(Collectors.toList());
The above code will filter out odd numbers and return the list of even number.

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