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

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Q1211. How abstraction works

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Q1212. What is the difference between map and filter methods in streams ?

Core Java

Ans. By using map , you can transform the object values. The map operation allows us to apply a function, that takes input parameter of one type, and returns something else.

Filter is used for filtering the data, it always returns the boolean value. If it returns true, then item is added to list else its filtered out (ignored)

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Q1213. Can we override behavior of a collection class ? How ?

Core Java

Ans. Yes. We can do that.

1. We can create own own implementation class extending the collection class and then override the behavior method.

2. We can override the behavior at the time of instantiation of class as following

List<MyType> list = new ArrayList<MyType>() {
public boolean add(MyType mt) {
super.add(mt);
Collections.sort(list, comparator);
return true;
}
};

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Q1214. How do you achieve String immutability ?

Core Java

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Q1215. How does hashmap works ?

Core Java

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Q1216. Write a program to rotate a matrix.

Core Java

Ans. public class Matrix {

   public static void main(String[] args) {
      int[][] matrix = {
            {1, 2, 3},
            {4, 5, 6},
            {7, 8, 9}
      };
      Matrix.rotateMatrix(matrix);
      for (int i = 0; i < matrix.length; i ){
         for (int j = 0; j < matrix.length; j ){
            System.out.print(matrix[i][j] " ");
         }
         System.out.println();
      }
   }

   public static void rotateMatrix(int[][] matrix){
      int row = matrix.length;
      //first find the transpose of the matrix.
      for (int i = 0; i < row; i ){
         for (int j = i; j < row; j ){
            int temp = matrix[i][j];
            matrix[i][j] = matrix[j][i];
            matrix[j][i] = temp;
         }
      }
      //reverse each row
      for (int i = 0; i< row; i ){
         for(int j = 0; j< row/2; j ){
            int temp = matrix[i][j];
            matrix[i][j] = matrix[i][row - 1 - j];
            matrix[i][row - 1 - j] = temp;
         }
      }

   }

}

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Q1217. What are the pros and cons of Lambda expressions ?

Core Java

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Q1218. Write a code to detect the longest palindromic substring

Core Java

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Q1219. What is headless mode in Java ? Why is it used ?

Core Java

Ans. It’s a mode that can be used for creating Back end application without the front end UI.

The advantage is that it reduces the number of packages that needs to be loaded ( by ignoring ui packages )

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Q1220. What are the distinguished qualities in Java that no other language possess ?

Core Java

Ans. Java is widely adopted as an enterprise standard technology across business domains and technical domains ( Web , Desktop , mobile , data science etc ) and has a vast ecosystem of frameworks and libraries.

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Q1221. What is ServerSocket in Java ?

Core Java

Ans. This class implements server sockets. A server socket waits for requests to come in over the network. It performs some operation based on that request, and then possibly returns a result to the requester.

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Q1222. Can anonymous class implement interface and extend a class

Core Java

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Q1223. How to get collections from a Stream

Core Java

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Q1224. Class and Object methods.

Core Java

Ans. class:- A class describe the contents of an object, an it describe the detail of data field called instance variable and defines the operations called method.

Object:-An object is an element of a class. Object have the behaviors of their class.

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Q1225. What is the use of toString method ?

Core Java

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Q1226. How multiple catch block works ?

Core Java

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Q1227. How does Garbage Collector work? What are the realizations of Garbage Collector.

Core Java

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Q1228. Why should we go for custom defined exceptions even if we have many exceptions provided already using throw keyword

Core Java

Ans. It gives us the liberty to throw the specific exceptions which are required and also we can control the handling of exceptions

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Q1229. What is a comparator ?

Core Java

Ans. Java Comparator compares two Java objects in a “compare(Object 01, Object 02)” format. Using configurable methods, Java Comparator can compare objects to return an integer based on a positive, equal or negative comparison

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Q1230. How to make a collection thread safe?

Core Java

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Q1231. What is string pool and what is the use of intern() function ?

Core Java

Ans. The Java String class intern() method returns the interned string. It returns the canonical representation of string.

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Q1232. What is Throwable ?

Core Java

Ans. Throwable in java is a class that is the superclass of all exceptions and errors which may occurs in java program.It extends obcect class.

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Q1233. Hash map using/without using Java 8

Core Java

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Q1234. Can we assign the reference to this variable ?

Core Java

Ans. No

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Q1235. What are different types of Polymorphism in Java ?

Core Java

Ans. run time polymorphism
compile time polymorphism

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Q1236. Is String mutable or immutable in Java ?

Core Java

Ans. String is Immutable because of String Constant Pool.

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Q1237. Explain Collections in Java

Core Java

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Q1238. Should we do anything to avoid memory leaks in Java ?

Core Java

Ans. Java has a intrinsic way of checking leaks and reclaiming memory called garbage collection. Moreover Java doesn't support pointer arithmetics , so there is very low comparative chance of memory leaks, compared to C,C++

Though there may be tools that may audit code to make sure that we are not leaving much for the garbage collection to work but I have never heard anyone doing anything for memory leaks in Java.

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Q1239. Write a Program to draw a fish ?

Core Java

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Q1240. What is the use of defining equals , compareTo and hashcode methods in a class ? Where are they used ?

Core Java

Ans. equals, compareTo and hashcode are of use when the objects are used within collections.

Equals helps with collections that helps maintaining only unique objects ( like Set )

compare and compareTo helps with collections that helps maintaining objects in order ( TreeSet, TreeMap etc )

hascode helps with collections that facilitates hash searching ( like hashSet, hashMap etc )

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