Paytm Interview Experience for Senior Software Engineer | 4 Years Experienced

Technical Round(1 hr):

1. Asked about my current company projects and techstack used
2. what is thread? Synchronized keyword? Diff. ways of creating threads in java, which one is better? difference b/w multithreading & multiprogramming
3. What is txn in DBMS? ACID properties
4. Internal implementation of map. How put of HashMap works? https://www.javatpoint.com/working-of-hashmap-in-java
5. Reverse singly linked list. Also write recursive code (https://www.geeksforgeeks.org/recursively-reversing-a-linked-list-a-simple-implementation/)
6. https://www.geeksforgeeks.org/detect-and-remove-loop-in-a-linked-list/
7. https://www.geeksforgeeks.org/sort-an-array-of-0s-1s-and-2s/
8. https://www.geeksforgeeks.org/design-a-stack-that-supports-getmin-in-o1-time-and-o1-extra-space/
9. Write code for singleton class and make it thread safe.

Technical Round(1 hr):

1. What is inheritance? Give an example
2. Function overloading and overriding
3. Static func. related question
   Java

   class MyBase{
   public static void func(){
    System.out.println("Base static called");
   }
   }
   
   class MyDerived extends MyBase{
   public void func(){                // is this allowed?
    System.out.println("Derived static called");
   }
   }
4. SOLID principles
5. HashMap and HashSet in java. How they work?
6. How Garbage Collector works? When it is called and how can we call it explicitly? (https://www.tutorialspoint.com/how-can-we-call-garbage-collection-gc-explicitly-in-java)
7. https://www.geeksforgeeks.org/minimum-cost-to-reach-from-the-top-left-to-the-bottom-right-corner-of-a-matrix/
8. Which design patterns are used by you? Explain observer pattern.  Also, Tell scenario where Singleton pattern was used in your project.
9. https://www.geeksforgeeks.org/maximum-size-rectangle-binary-sub-matrix-1s/
10. Difference b/w abstract class and interface. Why both are required? Can abstract class have constructors?
11. How can we make a variable immutable in java? Is it possible to modify final variable value?

Technical Round(1 hr):

1. Adv. and disadv. of indexing
2. A db Table having  100 Million rows, A B C are 3 columns

   A -> 1000 Unique values   B -> 1 Million Unique Values   C -> 10 Million Unique Values

   query made oftenly on db-> “select * from T where A = ? and B = ? and C = ?”

   How we should create indexes in the table? Can’t we create composite indexes on A,B?

3. Isolation levels in SQL.
4. Public key, private key, cryptography.
5. Diff b/w MongoDB and SQL
6. HAshMap put() and get() complexity
7. Time complexity of following code:
   Java

   int fib(int n) {
      if (n == 0 || n == 1) {
          return n;
      }
      return fib(n-1) + fib(n-2);
   }
8. https://www.geeksforgeeks.org/maximize-sum-of-k-elements-in-array-by-taking-only-corner-elements/
9. What are features of spring and spring boot?
10. Do you know about Redis?
11. what is HTTP and HTTPS?