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Interview Questions and Answers for 'Immutable' - 22 question(s) found - Order By Newest

Advanced level question. Frequently asked in High end product companies. Frequently asked in Cognizant ( Based on 2 feedbacks )
  Q1. Why is String immutable in Java ?Core Java
Anonymous
Ans. 1. String Pool - When a string is created and if it exists in the pool, the reference of the existing string will be returned instead of creating a new object. If string is not immutable, changing the string with one reference will lead to the wrong value for the other references.

Example -

String str1 = "String1";
String str2 = "String1"; // It doesn't create a new String and rather reuses the string literal from pool

// Now both str1 and str2 pointing to same string object in pool, changing str1 will change it for str2 too

2. To Cache its Hashcode - If string is not immutable, One can change its hashcode and hence it's not fit to be cached.

3. Security - String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment.

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 Q2. Why Char array is preferred over String for storing password?Core Java
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Ans. String is immutable in java and stored in String pool. Once it's created it stays in the pool until unless garbage collected, so even though we are done with password it's available in memory for longer duration and there is no way to avoid it. It's a security risk because anyone having access to memory dump can find the password as clear text.

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Very frequently asked. Usually asked with questions related to String.
  Q3. What is an Immutable Object ?Core Java
Anonymous
Ans. Object that can't be changed after instantiation.

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Very Frequently asked. Usually asked along with String Class related questions.
 Q4. What is an immutable class ?Core Java
Anonymous
Ans. Class using which only immutable (objects that cannot be changed after initialization) objects can be created.

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 Q5. How to implement an immutable class ?Core Java
Anonymous
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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 Q6. Does Declaring an object "final" makes it immutable ?Core Java
Anonymous
Ans. Only declaring variables as final makes them immutable. Making objects final means that the object handler cannot be used to target some other object but the object is still mutable.

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 Q7. How does making string as immutable helps with securing information ? How does String Pool pose a security threat ?Core Java2017-06-22 11:32:57

Ans. String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Making it mutable might possess threats due to interception by the other code segment or hacker over internet.

Once a String constant is created in Java , it stays in string constant pool until garbage collected and hence stays there much longer than what's needed. Any unauthorized access to string Pool pose a threat of exposing these values.


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 Q8. How is string object immutable if we can concat a string to it ?Core Java2017-08-08 09:50:28

Ans. Because it doesn't make the change in the existing string but would create a new string by concatenating the new string to previous string. So Original string won't get changed but a new string will be created. That is why when we say

str1.concat("Hello");

It means nothing because we haven't specified the reference to the new string and we have no way to access the new concatenated string. Accessing str1 with the above code will still give the original string.

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 Q9. Why String is popular HashMap key in Java?Core Java
Admin
info@buggybread.com
Ans. Since String is immutable, its hashcode is cached at the time of creation and it doesnt need to be calculated again. This makes it a great candidate for key in a Map and its processing is fast than other HashMap key objects. This is why String is mostly used Object as HashMap keys.

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 Q10. What are stateless objects ? How are they different from immutable objects ? Which of these two is thread safe ?
Admin
info@buggybread.com
Ans. Stateless objects are the objects without instance fields (instance variables). The class may have compile time constants i.e static final fields.

Immutable objects are the objects which have state but the state cannot be changed after initialization.

Both are Thread safe.

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 Q11. What are advantages of stateless and / or immutable objects ?
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Ans. They can be shared across multiple threads as they are thread safe.

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 Q12. Which of the following class creates mutable objects ?

a. Boolean
b. File
c. String
d. StringBuffer
Anonymous
Ans. StringBuffer

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 Q13. What are the benefits of creating immutable objects ?Core Java2017-01-22 16:01:54

Ans. 1. Security and Safety - They can be shared across multiple threads as they are thread safe. Moreover, it protects then from bad state due to interception by the other code segment. One such problem due to mutability and access by alternate code segment could be the change of hash code and then the impact on its search with hash collections.

2. Reuse - In some cases they can be reused as only one copy would exist and hence it can be relied upon. For example - String Pool

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 Q14. Make the following class immutable

public class Employee {

   private long employeeId;

   private String employeeName;

   public long getEmployeeId() {
      return employeeId;
   }

public void setEmployeeId(long employeeId) {
      this.employeeId = employeeId;
   }

   public String getEmployeeName() {
      return employeeName;
   }

public void setEmployeeName(String employeeName) {
      this.employeeName = employeeName;
   }
}
Core Java2017-05-15 10:09:35

Ans. public class Employee {

   private long employeeId;

   private String employeeName;

   public Employee(long employeeId,String employeeName){
      this.employeeId = employeeId;
      this.employeeName = employeeName;
   }

   public long getEmployeeId() {
      return employeeId;
   }

   public long getEmployeeName() {
      return employeeName;
   }
}

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 Q15. What are the examples of immutable objects in Java ?Core Java2017-05-29 08:35:00

Ans. Following Classes in Java SE creates immutable objects

String class
Wrapper Classes like Integer, Float etc.
StackTraceElement
Most Enum classes
File Class
Locale Class

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 Q16. What is the difference between being final and being immutable ? Are string final or immutable in Java ?Core Java2017-07-22 15:57:55

Ans. Being final in the sense of objects or object reference means that the reference cannot be reassigned to a different object whereas being immutable means that the object contents cannot be changed.

Strings in Java are immutable.

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 Q17. Does Wrapper classes produces immutable objects ? Core Java2017-08-08 09:46:33

Ans. Yes

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 Q18. Why shouldn't we use string concatenation extensively or in a loop ?Core Java2017-08-08 09:52:53

Ans. Because String being an immutable object creates a new object upon each concatenation cycle. If there is any such need , we should use String Builder whose object is immutable.

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 Q19. Is immutability an advantage with Wrapper classes over primitive types ? Core Java2017-08-24 21:43:19

Ans. There is no concept of de referencing with primitive types and hence they are implicitly immutable. Having wrapper classes as mutable offers disadvantages compared to primitive types. Wrapper classes being immutable offer similar advantage as primitive types.It actually overshadows the disadvantage wrapper class could have if they are immutable.

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 Q20. Which immutable classes have you worked with ? Can you name some immutable Collections ? How to make an immutable collection ?Core Java2017-08-28 17:11:22

 This question is still unanswered. Can you please provide an answer.


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 Q21. What are the benefits of immutability or Immutable classes ?Core Java2017-08-28 19:36:18

 This question is still unanswered. Can you please provide an answer.


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 Q22. Which of the following class creates immutable objects ?Core Java
a. String
b. StringBuffer
c. StringBuilder
d. None of these create immutable objects.

Ans.a. String



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