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Q541. What is the use of upcasting in 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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Q542. Why doesn't java support operator overloading ?

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Q543. Can an interface implements another interface ?

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

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Q544. Can an interface be defined inside a class just like we can have inner classes inside a class ?

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

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Q545. Can a source file contain more than one class declaration ?

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

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Q546. How do we define constant variables in Java ?

Ans. By using static and final modifiers.

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Q547. Which java package is loaded by default and hence doesn't require an explicit import statement ?

Ans. java.lang

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Q548. Can a class be declared "protected" ?

Ans. No, only methods can be declared protected.

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Q549. How does making string as immutable helps with securing information ? How does String Pool pose a security threat ?

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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Q550. 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.*;

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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Q551. What does import java.util.* means in 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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Q552. What is the purpose of re throwing the exception when we can provide the handling at the first catch itself ?

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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Q553. Is JDK required on each machine where a Java application needs to be run ?

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

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Q554. Can we compile and execute a Java class without main method ?

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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Q555. Can we call a constructor for an object twice ?

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Q556. Are char,String and Char data types in java ?

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

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Q557. Is String a Data type in 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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Q558. What is an infinite loop ?

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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Q559. What are the different types of loops in Java ?

Ans. for, while and do while

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Q560. Is this code legal in java i.e nested class within interface implementing the parent interface ?

public interface MyInterface {
public class MyClass implements MyInterface {
}
}

Ans. Yes that's legal in java

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Q561. Can an inner class be subclass of it's parent class ?

Ans. Yes that can be done

public class OuterClass {
public class InnerClass extends OuterClass {
}
}

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Q562. What is the difference between inner class and sub class ?

Ans. Inner Class is a class that is nested within another class whereas sub class is a class that extends or inherit another class.

Inner class can only be accessed using reference of outer class ( Class name , if inner class is static or object reference, if inner class is non static ) whereas Sub class is accessed directly.

In terms of memory, inner class is stored just like another class having it's own body whereas sub class carries the body of parent class as well as it's own fields in memory.

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Q563. Explain OOPs

or

Explain OOPs Principles

or

Explain OOPs Concepts

or

Explain OOPs features

or

Tell me something about OOPs

Ans. OOPs or Object Oriented Programming is a Programming model which is organized around Objects instead of processes. Instead of a process calling series of processes, this model stresses on communication between objects. Objects that all self sustained, provide security by encapsulating it's members and providing abstracted interfaces over the functions it performs. OOP's facilitate the following features

1. Inheritance for Code Reuse
2. Abstraction for modularity, maintenance and agility
3. Encapsulation for security and protection
4. Polymorphism for flexibility and interfacing

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Usually asked in relation to casting and ClassCastException.

Q564. What is instanceOf operator ? Explain it's use ?

Ans. The operator instanceOf is used to verify if the specified object is the instance of specified class or interface.

Syntax if(x instanceOf ABC)

where x is an object reference and ABC could be a class name or interface name. The above statement will be true if x holds an object that is an instance of ABC or any of the child class of ABC or if x holds an object that implements ABC.

instanceOf operator is used to verify in case of downcasting. For ex -

DerivedClass extends BaseClass

x is the reference of BaseClass but holds DerivedClass object ( Polymorphism )

There is an operation that is defined in Derived Class, let's say derivedClassMethod()

We cannot call derivedClassMethod() directly using x as x is reference of BaseClass and not DerivedClass and hence can only access methods that are defined in BaseClass and overridden in derived class.

Though we can cast it to DerivedClass as following
((DerivedClass)x).derivedClassMethod();

But it may throw ClassCastException in case x doesn't hold an instance of DerivedClass at that point.

So before casting it to DerivedClass we may like to make sure that it is an instance of DerivedClass and hence won't throw ClassCastException.

So we make a check for it

if(x instanceOf DerivedClass) {
((DerivedClass)x).derivedClassMethod();
}

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Very frequently asked. Usually among very first few questions.

Q565. Define encapsulation in Java ?

Ans. Encapsulation is a feature of OOP's that binds the data and it's associated methods together as a single unit and facilitate protection and data hiding by providing minimal interface to outside. For example - member variables are declared private and are accessed through public methods. Moreover we have private methods that can only be used internally and hence providing minimal interface to outside class through use of public methods.

Sample Code for encapsulation

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Q566. Write a Program for insertion sort.

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Q567. Write a method, that compare two numbers and return the bigger one ?

Ans. int compare(int a, int b) {
a>b? return a: return b;
}

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Q568. What is the quickest way to find count of duplicate elements in an arraylist, without using iteration or loops ?

Ans. We can copy the elements to a Set and then find the difference of count between ArrayList and Set. As Set don't allow duplicates , they will be removed in the set.

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Q569. What problem we could have with ArrayList which aren't possible with Vectors ?

Ans. ArrayLists aren't synchronized and hence doesn't allow synchronized access. As multiple threads can access an arraylist in parallel, it may result in an inconsistent state.

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Q570. What is meant by "Vector is synchronized and ArrayList isn't " ?

Ans. It means that only 1 thread can access have access to Vector at a time and no parallel access is allowed whereas Array List allows parallel access by multiple threads.

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