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

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Q91. Difference between ArrayList and Vector ?

Core Java

Ans. Vector is Synchronized one where as Arraylist is not Synchronized.

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Q92. How can we run a java program through command line ?

Core Java

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Q93. What is the difference between final and static in java ?

Core Java

Ans. final keyword is used to restrict the user from modifying the variable, extending the class and overriding a method

static keyword is used for memory management which can be used for variable, class, method where in it belongs to the class not to the instance of object

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Q94. Does garbage collection call dispose ?

Core Java

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Q95. Does garbage collection happens in String pool ?

Core Java

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Q96. Does garbage collection affects performance of the application ?

Core Java

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Q97. Difference between composition and aggregation in java ?

Core Java

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Q98. Why is String a class in Java ?

Core Java

Ans. 1. Creating a class helps in specifying operations on the Strings like length , sub string , concatenation etc.

2. It acts as a Wrapper class for "Array of Characters" and hence facilitates it's usage in collections, assignment to null.

3. Immutability of String objects facilitates in reuse , security and caching.

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Q99. Difference between equals and equalsignorecase in Java ?

Core Java

Ans. "equals" method is the method of object class that needs to be overridden to check object equality. This is not specific to any class like String.

equalsignorecase is the method of String class that provides a definition that ignores the case of characters during comparison.

The only difference between them in String class is that the equals() methods considers the case while equalsIgnoreCase() methods ignores the case during comparison.

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Q100. Difference between equals and hashcode in java ?

Core Java

Ans. "equals" is the method of object class which is supposed to be overridden to check object equality. x.equals(y) means the references x and y are holding objects that are equal with the equality defined by the definition of equals method.

Hashcode is used for bucketing in Hash implementations like HashMap, HashTable, HashSet etc. The value received from hashcode() is used as bucket number for storing elements. This bucket number is the address of the element inside the set/map. when you do contains() then it will take the hashcode of the element, then look for the bucket where hashcode points to and if more than 1 element is found in the same bucket (multiple objects can have the same hashcode) then it uses the equals() method to evaluate if object are equal, and then decide if contain() is true or false, or decide if element could be added in the set or not.

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Q101. Difference between equals and compareTo in Java ?

Core Java

Ans. "equals" is the method of object class which is supposed to be overridden to check object equality. x.equals(y) means the references x and y are holding objects that are equal.

The compareTo() method is used for comparing two objects in Java. It is usually defined for the classes whose objects needs to be ordered through Comparable interface or need to be part of an ordered collection like TreeSet or TreeMap.

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Q102. Is ConcurrentHashMap synchronized ?

Core Java

Ans. Yes, ConcurrentHashMap is synchronized.

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Q103. Is HashMap Synchronized ?

Core Java

Ans. No HashMap isn't synchronized. ConcurrentHashMap and Hashtable are synchronized.

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Q104. Does abstract class have public constructor?

Core Java

Ans. Yes, an abstract class can have a constructor in Java.

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Q105. Explain CloneNotSupportedException ?

Core Java

Ans. If we do not mention, Cloneable Interface to the Class which we want to Clone then we get this exception, only if we try to clone an object

Like:
public class TestClone{
@Override
   protected Object clone() throws CloneNotSupportedException {
      return super.clone();
   }
}

In Main, You try to do:
TestClone clone = new TestClone();

      TestClone clone2 = (TestClone) clone.clone();

You will get CloneNotSupportedException.

Just add -> public class TestClone implements Cloneable {

and things are fixed.

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Q106. How many heaps will be there with 3 threads ?

Core Java

Ans. All threads share a common heap.

Each thread has a private stack, which it can quickly add and remove items from. This makes stack based memory fast, but if you use too much stack memory, as occurs in infinite recursion, you will get a stack overflow.

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Q107. How to write an iterator for an iterator of iterators?

Core Java

Ans. CustomArrayList myarrayList = new CustomArrayList();
      myarrayList.add("Value 1");
      myarrayList.add("Value 2");
      myarrayList.add("Value 3");

      for (String string : myarrayList) {
         System.out.println(string);
      }

package sample.utils;

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;

public class CustomArrayList implements Iterable {

   private ArrayList mylist = new ArrayList();

   public void add(T t) {
      this.mylist.add(t);
   }

   @Override
   public Iterator iterator() {
      return new CustomIterator(mylist);
   }

   class CustomIterator implements Iterator {

      private int indexPosition = 0;
      private List internalList;

      public CustomIterator(List internalList) {
         this.internalList = internalList;
      }

      @Override
      public boolean hasNext() {
         if (internalList.size() >= indexPosition 1) {
            return true;
         }
         return false;
      }

      @Override
      public E next() {
         E val = internalList.get(indexPosition);
         indexPosition ;
         return val;
      }

   }

}

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Q108. Can we have a final argument in a method ? What is the use of that if we do so ?

Core Java

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Q109. why is "".equals(str); safer than str.equals("")?

Core Java

Ans. str.equals("") this statement will throw NullPointerException if str is null where as "".equals(str) works fine even if str is null

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Q110. Write a method that would solve two problems at the same time ?

Core Java

Ans. We can return value for one problem solving or computation and set the value for other in some member element.

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Q111. Can a constructor be declared static ? Why ?

Core Java

Ans. No.

When we declare a method static, it means that "this belongs to class as whole and not particular instance". The whole purpose of constructor is to initialize a object and hence there is no sense having static constructor.

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Q112. Can constructors be final ?

Core Java

Ans. No. They are never inherited and therefore are not subject to hiding or overriding.

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Q113. Have you ever worked on continuations in Java ?

Core Java

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Q114. What are inline functions ? Do we have inline functions in Java ?

Core Java

Ans. Inline functions , just like C++ Macros is an optimized technique used by compiler to reduce the execution time. If the function is working on pre identified values ( which aren't resolved at runtime ), the function can execute the method and evaluate the outcome at compile time only instead of making a function call at runtime.

In Java, the optimizations are usually done at the runtime or JVM level. At runtime, the JVM perform some analysis to determine which methods to inline. Java compiler would never inline any method and there is no way in java for the developer to explicitly define inlining of methods as it's take intrinsically care of during runtime only.

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Q115. What is the default size of list ? How it is growable ?

Core Java

Ans. Default size is 10.
Regard how size will increase.

Inside add() method there is one method called ensureCapacity() which will check list has enough size available.

This ensureCapacity() method if found size of list need to increase then it will call grow() method which will create new list of extra size and copy existing list data to new one and return.

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Q116. Can a method return more than one value at a time in Java ?

Core Java

Ans. object / native type returned can only be one but an object can comprise of a array , collection or a group of different value types.

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Q117. Difference between Base Class, Derived Class , Abstract Class and Concrete Class ?

Core Java

Ans. Base Class is a relative term used to signify the Parent class in Parent - Child or Base - derived relationship.

Derived Class is a relative term used to signify the Child class in Parent - Child or Base - derived relationship.

Abstract Class is a class that is not allowed to be instantiated and hence can serve only a base class for it's usage.

Concrete Class is the class which is supposed to be instantiated and hence provide definition for all implementing interface methods and extending abstract methods.

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Q118. Can an abstract class be a derived class ?

Core Java

Ans. Yes, an abstract class can extend another class.

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Q119. What is the relationship between Base Class and Abstract Class ?

Core Java

Ans. Abstract Class is a class that's only allowed to be a Base class for it's usage. It can never be instantiated on it's own.

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Q120. What is a base class ? Is Abstract class and Base class the same thing ?

Core Java

Ans. Base Class is a relative term. For example - Class Vehicle is a Base class of Class Car. Even though Base class is derived in this case , there is no restriction on instantiation of Vehicle class.

Abstract Class is a class that is meant only to be a Base Class and not allowed to be instantiated.

Abstract Class is a Base Class that isn't allowed to be instantiated and is only meant for carrying definition to derived class and help participate in runtime polymorphism.

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