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

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Q91. Difference between Predicate, Supplier and Consumer ?

Ans. Predicate represents an anonymous function that accepts one argument and produces a result.

Supplier represents an anonymous function that accepts no argument and produces a result.

Consumer represents an anonymous function that accepts an argument and produces no result.

Sample Code for Predicate

Sample Code for Supplier

Sample Code for Consumer

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Q92. 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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Q93. Collections.sort can only be performed on ..

a. Set
b. List
c. Map
d. Any Collection implementation

Core Java

Ans. List

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Q94. Which of the following Java feature promotes access protection or Hiding ?

a. Inheritance
b. Encapsulation
c. Abstraction
d. Composition

Core Java

Ans. Encapsulation

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Q95. Enums cannot be declared ..

a. private
b. public
c. static
d. final

Core Java

Ans. final

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Q96. Difference between parameters and arguments ?

Core Java

Ans. Parameters are the variables that the method is expected to receive along with the method call. Arguments are the values which are passed on while calling the methods.

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Q97. Why Java is not considered pure OOP's language ?

Core Java

Ans. There are 2 reasons for it.

1. Usage of Primitive types - Though Java provides classes for the primitive data types but as the usage of primitives is permissible, its considered unpure OOP's language.

2. Usage of Static members - Static members belong to the class and not objects and hence not considered fit for pure OOP's programming.

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Q98. What are constructors and Destructors in Java ?

Core Java

Ans. Constructors are used for initializing the object state once it is initialized and memory has been reserved for it.

Destructor is used to de-allocate memory allocated by objects.

There are no destructors in Java. Alternatively, Java provides Automatic garbage collection i.e automatically releasing the un-referenced memory.

Sample Code for constructor

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Q99. Why HashTable has been deprecated ?

Core Java

Ans. HashTable has been deprecated. As an alternative, ConcurrentHashMap has been provided. It uses multiple buckets to store data and hence much better performance than HashTable. Moreover, there is already a raw type HashMap. The only difference between the HashTable and HashMap is that Hashtable is synchronized whereas HashMap is not. Most of the synchronized collections have been deprecated and their raw alternative have been presented as preferred.Synchronization has a cost. Using synchronized collection in places where there is no need of it leads to useless utilization of resources. As these collections are rarely used in a static context or shared among threads, Java might have thought it better to just provide the raw collection and let developers implement synchronization if he feels the need to do so. HashMap is now presented as the default and the preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable. Because of this, Java thought it right to deprecate the use of HashTable.'

Synchronization has a cost. Using synchronized collection at a place where there is hardly any need of it would means useless utilization of resources. As these collections are rarely used in static context or shared among threads, Java might have thought it better to just provide the raw collection and let developer implement synchronization if he feels the need to do so.

As HashMap has been presented as default and preferred way of using Map with read optimized hashing, and ConcurrentHashMap has been provided for synchronized access which provides better performance than HashTable, Java thought it right to deprecate the use of HashTable.

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Q100. Difference beween HashSet and TreeSet ?

Core Java

Ans. 1. HashSet doesnt maintain its elements in any specific order and is all random whereas TreeSet maintains elements in natural order 9 order defined by the equals method of TreeSet element type )

2. TreeSet doesnt allow null elements whereas HashMap does.

3. As TreeSet orders elements and is hence insertion is comparatively slower.

4. HashSet performs basic operations like add(), remove(), contains(), size() etc. in a constant size time. A TreeSet performs these operations at the order of log(n) time.

5. HashMap in Java internally backs a HashSet. A NavigableMap backs a TreeSet internally.

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Q101. Can you write a "Hello World" program without using any ";" within it?

Core Java

Ans. Yes, That is possible

class A {
public static void main(String args[]){
if(System.out.printf("Hello World")==null){}
}
}

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Q102. Why two types of memory - stack and heap are required by Java ?

Core Java

Ans. Because of the life cycle requirement for different type of values in java.

variables initialized and used in functions needs to be destructed with the execution of function and hence kept in stack. Same is applicable for the object references initialized within the method. If objects would have been created in stack, they wouldnt have been passed around across methods and hence they are created on heap.

So anything that is required beyond the scope of a method or function is kept on heap which is usually garbage collected by Java.

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Q103. Which elements of a class are ignored during serialization ?

Core Java

Ans. 1. Objects are serialized and not classes and hence Static variables are ignored.

2. Transient is an explicit declaration to ignore the variable during serialization and hence transient instance variables are ignored too.

3. Base class instance variables if the base class hasn't been declared serializable.

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Q104. 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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Q105. 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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Q106. Is this a valid initialization ? Explain.

Collection<Collection> collection = new LinkedList<LinkedList>();

Core Java

Ans. No. It will result in a type mismatch error.

Collection<Collection> collection = new LinkedList<Collection>();

is a valid initialization as collection being reference of "Collection" class can hold object of derived Class "LinkedList" due to runtime Polymorphism. Runtime polymorphism is not applicable to type arguments.

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Q107. 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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Q108. If arrays cannot be resized , Why is this code valid

String[] strArray = new String[2];
strArray = new String[5];

Core Java

Ans. We are not resizing the first array here but assigning the reference strArray to a new Array with size 5.

So after line 2, We have 2 arrays in memory, one with size 2 and other with size 5 with strArray referring to second array with size 5.

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Q109. 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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Q110. What is the difference between List, Set and Map ?

or

What are the different Java Collections Interfaces ?

Core Java

Ans. List - Members are stored in sequence in memory and can be accessed through index.
Set - There is no relevance of sequence and index. Sets doesn't contain duplicates whereas multiset can have duplicates.
Map - Contains Key , Value pairs.

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Q111. 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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Q112. What is a Final Variable ?

Core Java

Ans. Final variable is a variable constant that cannot be changed after initialization.

Sample Code for final variable

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Q113. When are static variables loaded in memory ?

Core Java

Ans. They are loaded at runtime when the respective Class is loaded.

Sample Code for static variable

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Q114. Can we serialize static variables ?

Core Java

Ans. No. Only Object and its members are serialized. Static variables are shared variables and doesn't correspond to a specific object.

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Q115. Can we use null as a key for a map collection ?

Core Java

Ans. Yes , for HashMap. HashMap implements Map interface. HashMap allows one null key and any number of null values.

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Q116. Are constructors inherited? Can a subclass call the parent's class constructor? When?

Core Java

Ans. You cannot inherit a constructor. That is, you cannot create a instance of a subclass using a constructor of one of it's superclasses. One of the main reasons is because you probably don't want to override the superclasses constructor, which would be possible if they were inherited. By giving the developer the ability to override a superclasses constructor you would erode the encapsulation abilities of the language.

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Q117. 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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Q118. Difference between Vector and ArrayList ?

Core Java

Ans. Vectors are synchronized whereas Array lists are not.

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

Q119. 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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Q120. Difference between object instantiation and construction ?

Core Java

Ans. Though It's often confused with each other, Object Creation ( Instantiation ) and Initialization ( Construction ) are different things in Java. Construction follows object creation.

Object Creation is the process to create the object in memory and returning its handler. Java provides New keyword for object creation.

Initialization is the process of setting the initial / default values to the members. Constructor is used for this purpose. If we don't provide any constructor, Java provides one default implementation to set the default values according to the member data types.

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