Interview Questions and Answers for 'Cla' | Search Interview Question

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Q31. What are wrapper classes ?

Core Java

Ans. They are wrappers to primitive data types. They allow us to access primitives as objects.

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Q32. How to implement an immutable class ?

Core Java

Ans. We can make a class immutable by

1. Making all methods and variables as private.

2. Setting variables within constructor.

Public Class ImmutableClass{

private int member;
ImmutableClass(int var){
member=var;
}
}

and then we can initialize the object of the class as

ImmutableClass immutableObject = new ImmutableClass(5);

Now all members being private , you cant change the state of the object.

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Q33. What are the contents of Hibernate configuration file ( hibernate.cfg.xml ) ?

Hibernate

Ans. HBM Files ( Mapping )
DB Connection ( DB Connection String , User Name , Password , Pool Size )
SQL Dialect ( SQL variant to be generated )
Show SQL ( Show / No show SQL on Console )
Auto Commit ( True / False )

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Q34. 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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Q35. What is the difference between these two method declarations ?

private static void method(String[] arg)

and

private static void method(String... arg)

Core Java

Ans. First expects the argument as a string array whereas second expects variable number of string arguments or a string array.

So we can call both by providing string array as an argument but second can be called with 0 to n string arguments which cannot be done for first.

for example - We can call second method with any of following

method();
method("Hello");
method("Hello","World");
method(new String[4]);

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Q36. What is the difference between inner class and sub class ?

Core Java

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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Q37. What are the benefits of using Wrapper classes over primitive types ?

Core Java

Ans. With Collection classes , we cannot use primitive types. Moreover for any class using generic types, we cannot use primitive types.

They add more functionality by means of additional methods.

As their reference can be null , they offer consistent check for uninitialized state.

They facilitate caching and reuse by means of constant Pools.

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Q38. What are the different types of inner classes ?

Core Java

Ans. Simple Inner Class,
Local Inner Class,
Anonymous Inner Class,
Static Nested Inner Class.

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Q39. Difference between Checked and Unchecked exceptions ?

Core Java

Ans. Checked exceptions are the exceptions for which compiler throws an errors if they are not checked whereas unchecked exceptions are caught during run time only and hence can't be checked.

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Q40. Difference between Abstract and Concrete Class ?

Core Java

Ans. Abstract classes are only meant to be sub classed and not meant to be instantiated whereas concrete classes are meant to be instantiated.

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Very Frequently asked. Usually asked along with String Class related questions.

Q41. What is an immutable class ?

Core Java

Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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Q42. Explain Autoboxing ?

Core Java

Ans. Autoboxing is the automatic conversion that the Java compiler makes between the primitive types and their corresponding object wrapper classes

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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations.

Q43. Difference between StringBuffer and StringBuilder ?

Core Java

Ans. StringBuffer is synchronized whereas StringBuilder is not.

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Q44. When do you get ClassCastException?

or

What is ClassCastException ?

Ans. As we only downcast class in the hierarchy, The ClassCastException is thrown to indicate that code has attempted to cast an object to a subclass of which it is not an instance.

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Q45. Difference Between this() and super() ?

Core Java

Ans. 1.this is a reference to the current object in which this keyword is used whereas super is a reference used to access members specific to the parent Class.

2.this is primarily used for accessing member variables if local variables have same name, for constructor chaining and for passing itself to some method whereas super is primarily used to initialize base class members within derived class constructor.

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Q46. What are the Disadvantages of using Collection Classes over Arrays ?

Core Java

Ans. Collections can only hold objects, It can't hold primitive data types.

Collections have performance overheads as they deal with objects and offer dynamic memory expansion. This dynamic expansion could be a bigger overhead if the collection class needs consecutive memory location like Vectors.

Collections doesn't allow modification while traversal as it may lead to concurrentModificationException.

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Q47. In which cases , moving methods to utility class could be useful ?

Core Java

Ans. It could be worthy to move a method to util class if the method needs to be shared, doesn't require polymorphic behavior and need not be overridden in special cases.

Don't belong to one group through is-a relationship ( You can share through parent class method )

Don't implement a specific interface ( java 8 default methods )

Doesn't involve complex computing as you will be loosing the benefit of object state with just static method.

Doesn't require polymorphic behavior as static methods don't participate in runtime polymorphism.

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Q48. If an Abstract class has only abstract methods, What's the difference between such a class and an interface ?

Core Java

Ans. Such a class still can have member elements which can be inherited and hence facilitate code reuse. Moreover Abstract class can have non final static elements whereas interfaces are only allowed to have static final elements.

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Q49. Can we overload method as following ?

void method(int... x){};
void method(int[] x){};

Core Java

Ans. No. Because java internally treats var args as arrays and hence both method declarations will generate the same byte code and hence would result in ambiguity while determining call binding.

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Q50. What is the use of Object class ?

Core Java

Ans. 1. Default definition for some of the methods , like equals, hashcode etc that gets carried to all objects even if you don't define anything. But that default definition is kind of like assigning 0 to integer, it just provide a safe state , nothing much for comparison.

2. Places where you have no idea about what object you may receive and just want to perform out of 8 basic methods of object class, something like printing their string representation ( defined by their toString method ), or equality ( using their equals method )

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Q51. Differences between class not found exception and noclassdef found error ?

Core Java

Ans. ClassNotFoundException is an exception that occurs when we try to load a class at run time using Class.forName() or loadClass() methods and mentioned classes are not found in the classpath.

NoClassDefFoundError is an error that occurs when a particular class is present at compile time, but was missing at run time.

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Q52. Which access specifier can be used with Class ?

Core Java

Ans. For top level class we can only use "public" and "default". We can use private with inner class.

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Q53. How to display and set the Class path in Unix ?

Ans. To display the current CLASSPATH variable, use these commands in UNIX (Bourne shell):

% echo $CLASSPATH

To delete the current contents of the CLASSPATH variable,

In UNIX: % unset CLASSPATH; export CLASSPATH

To set the CLASSPATH variable,

In UNIX: % CLASSPATH=/home/george/java/classes; export CLASSPATH

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Q54. What is a Final Class ?

Ans. A Class that cannot be sub classed.

Sample Code for final class

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Q55. Difference between boolean and Boolean ?

Core Java

Ans. boolean is a primitive type whereas Boolean is a class.

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Q56. What are Wrapper Classes ? What are Primitive Wrapper Classes ?

Core Java

Ans. A wrapper class is any class which "wraps" or "encapsulates" the functionality of another class or component. A Wrapper Class that wraps or encapsulates the primitive data type is called Primitive Wrapper Class.

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Q57. What Design pattern Wrapper Classes implement ?

Design

Ans. Adapter.

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Q58. What is a class loader ? What are the different class loaders used by JVM ?

Core Java

Ans. Part of JVM which is used to load classes and interfaces.

Bootstrap , Extension and System are the class loaders used by JVM.

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Q59. Difference between nested and inner classes ?

Ans. Inner class is a type of nested class.

Inner classes are non static nested classes i.e the one that are associated with the instance of outer class.

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Q60. What is a nested interface ?

Ans. Any interface declared inside a class or an interface. It is static by default.

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