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

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Q661. Upon compiling the following class, What .class files will get created

public class OuterClass{
class InnerClass{
}
}

Core Java

Ans. OuterClass.class AND OuterClass$InnerClass.class

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Q662. Write a Program to validate an email address

Core Java

Ans. public class Class{
public static void main(String[] args){
String str = "xyz@123.com";

if(!str.contains("@") && !str.contains(".") && (str.indexOf('.') < str.indexOf('@'))){
System.out.println("Not a Valid Email Address");
}
}
}

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Q663. Write a Program to validate if a particular character occurs after another in a string

Core Java

Ans. public class Class{
   public static void main(String[] args){
      String str = "xyz123.co@m";

      if(str.indexOf('.') < str.indexOf('@')){
         System.out.println("Not a Valid Email Address");
      }
   }
}

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Q664. Write a Program to find the missing integer in an array of consecutive numbers?

Core Java

^{This question was recently asked at 'Knight Capital'.This question is still unanswered. Can you please provide an answer.}

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Q665. How to find the median number in an array of integers ?

Core Java

Ans.

Arrays.sort(numArray);
double median;
if (numArray.length % 2 == 0)
median = ((double)numArray[numArray.length/2] (double)numArray[numArray.length/2 - 1])/2;
else
median = (double) numArray[numArray.length-1/2];

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Q666. Write a Java program to print 10 random numbers between 1 to 100?

Core Java

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Q667. Why don't Java objects have destructors ?

Core Java

Ans. Java manages memory using built-in garbage collector

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Q668. What is the difference between && and & in Java ?

Core Java

Ans. && is a Logical whereas & is a bitwise operator

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Q669. Why is it important to close files and streams ?

Core Java

Ans. Optimizing Resource utilization. When you open a file using any programming language , that is actually a request to operating system to access the file. When such a request is made , resources are allocated by the OS. When you gracefully close those files, those resources are set free.

In Java if you don’t close the streams and files, Garbage collection identifies the open files which have lost the reference and hence will close them.

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Q670. Write a method / program that will determine if the parenthesis are balanced in a given string.

Core Java

Ans. https://www.geeksforgeeks.org/check-for-balanced-parentheses-in-an-expression/

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Q671. What are the core OOPs concepts ?

Core Java

Ans. Abstraction, Encapsulation, Polymorphism , Composition and Inheritance

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Q672. How are the concept of Association related to Composition and Inheritance ?

Core Java

Ans. Composition and Inheritance are the different types of Associations for Classes.

Composition is a has-a association between classes.
Inheritance is a is-a association between classes.

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Q673. What is an execution engine within JVM ?

Core Java

Ans. Execution engine contains interpreter and JIT compiler, which covert byte code into machine code.

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Q674. What are some of the Java Naming conventions ?

Core Java

Ans. 1. Class name should be a noun and should start with capital case character.

2. Interface Name should be an adjective and should start with capital case character.

3. Method and Variable names should follow lower camel case notation.

4. package name should be all lower case.

5. constant variables (static final) should be all capital case.

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Q675. write program to do matrix multiplication

Core Java

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Q676. How do threads share information ?

Core Java

Ans. Through common memory area.

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Q677. What is Substitutability in Java ?

Core Java

Ans. Substitutability means that the type of the variable does not have to exactly match with the type of the value assigned to that variable. For example - Object reference of Parent object can hold derived object.

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Q678. What is the precedence of operators in Java ?

Core Java

Ans. http://introcs.cs.princeton.edu/java/11precedence/

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Q679. Can we override abstract methods ?

Core Java

Ans. Abstract methods are the methods that need to be overridden in the derived class ( either as implementing method or again abstract method ) so it's not only allowed but its required to override abstract method in the derived class.

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Q680. Can we override an abstract method with abstract method in the derived class ?

Core Java

Ans. Yes, but in that case the derived class itself should be abstract. We cannot have an object of a class without definition for the method which has been declared abstract in the parent.

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Q681. Given array of integers, find first two numbers that adds up to 10.

Core Java

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Q682. Write a program to reverse words of a sentence.

Core Java

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Q683. how to get most significant bit from a byte?

Core Java

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Q684. What are the types of Executor Interfaces ?

Core Java

Ans. Executor - Simple Interface that supports launching new tasks.

ExecutorService - Subinterface of Executor, which adds features that help manage the lifecycle, both of the individual tasks and of the Executor itself.

ScheduledExecutorService - Subinterface of ExecutorService, supports future and-or periodic execution of tasks.

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Q685. If two Interfaces have same Method, how to handle that Method in a Class implementing these two Interfaces?

Core Java

Ans. It doesn't matter how you define that method in implementing class. there will be only one definition provided and that method can be accessed using reference of either of the interfaces.

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Q686. What is a mutator in Java?

Core Java

Ans. Mutator is another name for setter methods, i.e the method allows for mutating the property of an object and eventually the state of the object.

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Q687. Why Java doesn't provide a default constructor if we define an argument constructor ?

Core Java

Ans. Java provides a default constructor ( no argument ) if no constructor is provided by the programmer. Java makes a check during pre compilation if there is any constructor provided by the programmer. If any constructor is provided, Java understands that programmer has provided all the mechanism for initialization and construction of the object and may not even intend to create objects by calling a no argument constructor. For example - One may like to have objects created with self defined state otherwise restrict its creation.

We make the constructor private if we want to make a class singleton as we would like to restrict creation of new objects using new operator and default constructor. Similar could be the situation wherein we would like to construct objects in a particular manner only.

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Q688. What are Util Classes ? What are its advantages ?

Core Java

Ans. Classes having only static methods in java is called util classes. As they don't have specific object state, they are basically intend to have shared code and state through static elements and methods. They save resources by sharing code.

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Q689. What is your favorite type of data? Why ?

Core Java

Ans. [Open Ended Answer]

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Q690. Can StringBuffer and StringBuilder in Java be inherited?

Core Java

Ans. No, they have been declared final and hence cannot be extended.

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