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

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Q481. What is the difference between being final and being immutable ? Are string final or immutable in Java ?

Ans. Being final in the sense of objects or object reference means that the reference cannot be reassigned to a different object whereas being immutable means that the object contents cannot be changed.

Strings in Java are immutable.

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Q482. Explain Java 8 concurrency mechanism.

Core Java

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Q483. Will the following code compile ?

final HashMap hm = new HashMap();
hm.put("Ravi", 221);

Core Java

Ans. Yes, it will compile without error. final in this context means that the reference hm cannot be assigned to a new hash map but didn't restrict from changing the state of collection already held by hm.

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Q484. What will be the output of following code

class TestMain {
public static void main(String[] args) {
String s1 = "ravi";
String s2 = new String("ravi");
Integer i = 10;
Integer a1 = new Integer("10");
System.out.println(s1 == s1);
System.out.println(i == a1);
}
}

Core Java

Ans. false
false

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Q485. Write code to find second largest number in an array of integers.

Core Java

Ans. int arr[]={1,3,5,6,4,8,9,2,10};
Arrays.sort();
System.out.println(arr[arr.length-1]);

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Q486. Explain Inheritance and Polymorphism.

Core Java

Ans. Getting parent class object properties into child class is called Inheritance.

The process of representing one form into multiple forms is called polymorphism.

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Q487. Write a Program to find age by birth date ?

Core Java

Ans. LocalDate today = LocalDate.now();
LocalDate birthday = LocalDate.of(1960, Month.JANUARY, 1);
Period p = Period.between(birthday, today); //Now access the values as below
System.out.println(period.getDays());
System.out.println(period.getMonths());
System.out.println(period.getYears());

Sample Code for LocalDate

Sample Code for Period

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Q488. How to calculate lcm of two numbers ?

Core Java

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Q489. In Java, would you like to see an explicit destructor?

Core Java

Ans. Yes , a gc method to explicitly call garbage collection immediately on the object. It will provide better flexibility over application that require optimal usage of memory.

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Q490. In Java, what is the closest thing to pointers?

Core Java

Ans. References.

Unlike pointers in C , Java references are strongly typed and we cannot do pointer arithmetics with them.

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Q491. Why do we pass an array of strings to main method ?

Core Java

Ans. Array of strings in the main method are the list of arguments or parameters which are sent to the application / program.

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Q492. What is the use of upcasting in Java ?

Core Java

Ans. Interfacing

Car extends Vehicle

Car car = new Car();

// now you need to call an API changeTires() which can change tires for all Vehicle Types and accepts argument of type Vehicle and hence you will have to upcast it

changeTires((Vehicle)car);

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Q493. Why doesn't java support operator overloading ?

Core Java

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Q494. Can an interface implements another interface ?

Core Java

Ans. No. No interface provides any implementation and hence the declaration isn't viable.

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Q495. Can an interface be defined inside a class just like we can have inner classes inside a class ?

Core Java

Ans. Yes , we can have an interface that is part of a class.

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Q496. Can a source file contain more than one class declaration ?

Core Java

Ans. Yes, but only one of them can be declared public.

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Q497. How do we define constant variables in Java ?

Core Java

Ans. By using static and final modifiers.

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Q498. Which java package is loaded by default and hence doesn't require an explicit import statement ?

Core Java

Ans. java.lang

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Q499. Can a class be declared "protected" ?

Core Java

Ans. No, only methods can be declared protected.

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Q500. How does making string as immutable helps with securing information ? How does String Pool pose a security threat ?

Core Java

Ans. String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment or hacker over internet.

Once a String constant is created in Java , it stays in string constant pool until garbage collected and hence stays there much longer than what's needed. Any unauthorized access to string Pool pose a threat of exposing these values.

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Q501. What is the problem we could face if we be very liberal with importing classes even if we are not using them ? For example

import java.util.*;
import java.io.*;

Core Java

Ans. It makes application little heavy as we are importing classes that aren't required.

It creates class name conflicts as similar name classes might be available across different packages. In case of such conflicts, we will have to specify the package name with the class name at the time of it's usage.

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Q502. What does import java.util.* means in java ?

Core Java

Ans. It means import all the classes and interfaces within java.util package and make them available to use within the current class or interface. This is shorthand wild card annotation for importing all classes within a particular package. This won't import the classes within the sub packages of java.util.

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Q503. What is the purpose of re throwing the exception when we can provide the handling at the first catch itself ?

Core Java

Ans. Sometimes the calls move across layers of classes and functions and hence each layer needs to perform some function like cleaning if something goes wrong deep inside.

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Q504. Is JDK required on each machine where a Java application needs to be run ?

Core Java

Ans. No, to run application, machine needs just a JRE (Java Runtime Environment)

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Q505. Can we compile and execute a Java class without main method ?

Core Java

Ans. No without main method can not be executed it will throw an error illegal start of type. Main method is the entry point of application.

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Q506. Can we call a constructor for an object twice ?

Core Java

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Q507. Are char,String and Char data types in java ?

Core Java

Ans. char is a primitive data type. String is a class. Char is a wrapper class for primitive char.

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Q508. Is String a Data type in Java ?

Core Java

Ans. String is not a primitive type in java. Anything defined within double quotes like "abc" is an object of String class.

Sample Code for String

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Q509. What is an infinite loop ?

Core Java

Ans. Infinite loop is a programming condition wherein the control goes into an infinite loop because the loop termination condition can never be met. For example -

for(int x=1;x>0;x++){
}

in this loop, with each increment the condition x > 0 will remain true till infinity.

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Q510. What are the different types of loops in Java ?

Core Java

Ans. for, while and do while

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