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Core Java - Interview Questions and Answers for 'String class' - 22 question(s) found - Order By Newest

Advanced level question. Frequently asked in High end product companies. Frequently asked in Cognizant and Deloitte ( Based on 2 feedback )
  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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   Like         Discuss         Correct / Improve     java   oops   string   string class   immutable  immutability   advanced     Asked in 37 Companies      expert        frequent

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Very frequently asked in different variations. Frequently asked in Deloitte ( 2 feedback ) , HCL Tech ( 3 feedback ) and Coginizant (CTS)
  Q2. Explain the scenerios to choose between String , StringBuilder and StringBuffer ?

or

What is the difference between String , StringBuilder and StringBuffer ?
Core Java
Anonymous
Ans. If the Object value will not change, use String Class because a String object is immutable.

If the Object value can change and will only be modified from a single thread, use StringBuilder because StringBuilder is unsynchronized(means faster).

If the Object value may change, and can be modified by multiple threads, use a StringBuffer because StringBuffer is thread safe(synchronized).

  Sample Code for String

  Sample Code for StringBuffer

  Sample Code for StringBuilder

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   Like         Discuss         Correct / Improve     java   string class   string   stringbuilder   stringbuffer     Asked in 27 Companies      basic        frequent

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 Q3. What are different ways to create String Object? Explain.Core Java
Admin
info@buggybread.com
Ans. There are total six ways

1. literals

When we create a String using double quotes, JVM looks in the String pool to find if any other String is stored with same value. If found, it just returns the reference to that String object else it creates a new String object with given value and stores it in the String pool.

2. new keyword

When we use new operator, JVM creates the String object but dont store it into the String Pool. We can use intern() method to store the String object into String pool or return the reference if there is already a String with equal value present in the pool.

3. string buffer
4. string builder
5. System.out.println
6. char to string




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   Like         Discuss         Correct / Improve     java   string class   string   jvm   memory management   string pool     Asked in 4 Companies      basic        frequent

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 Q4. Why Char array is preferred over String for storing password?Core Java
Admin
info@buggybread.com
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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   Like         Discuss         Correct / Improve     java   string class   string   immutable  immutability   string pool   garbage collection   advanced     Asked in 4 Companies      Expert


  Q5. What is a String Pool ?Core Java
Anonymous
Ans. String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and returning its reference.

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   Like         Discuss         Correct / Improve     java   oops   string   string class   string pool   heap memory     Asked in 31 Companies      intermediate        frequent

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 Q6. What will this code print ?

String a = new String ("TEST");
String b = new String ("TEST");
if(a == b) {
System.out.println ("TRUE");
} else {
System.out.println ("FALSE");
}
Core Java
Anonymous
Ans. FALSE. == operator compares object references, a and b are references to two different objects, hence the FALSE. .equals method is used to compare string object content.

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   Like         Discuss         Correct / Improve     string   string class   java   ==   object references   coding     Asked in 2 Companies      basic        frequent

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Very Frequently asked. Usually asked along with String Class related questions.
  Q7. 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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Very frequently asked. Usually difference between String,StringBuffer and StringBuilder is asked in different variations.
  Q8. Difference between StringBuffer and StringBuilder ?Core Java
Anonymous
Ans. StringBuffer is synchronized whereas StringBuilder is not.

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  Q9. Write a method to check if input String is Palindrome?Core Java
Admin
info@buggybread.com
Ans. private static boolean isPalindrome(String str) {

if (str == null)
return false;

StringBuilder strBuilder = new StringBuilder(str);

strBuilder.reverse();

return strBuilder.toString().equals(str);

}

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   Like         Discuss         Correct / Improve     java   string   stringbuilder   stringbuilder   string class   code   palindrome     Asked in 38 Companies      Basic        frequent


 Q10. Write a method that will remove given character from the String?Core Java
Admin
info@buggybread.com
Ans.
private static String removeChar(String str, char c) {    
if (str == null)
return null;

return str.replaceAll(Character.toString(c), "");
}


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   Like         Discuss         Correct / Improve     java   string   string class   code   write code


 Q11. 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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   Like         Discuss         Correct / Improve     java   string class   string   immutable  immutability   hashmap   immutable  immutability   hashcode   hash code   advanced     Asked in 2 Companies      expert        frequent


 Q12. What does String intern() method do?Core Java
Admin
info@buggybread.com
Ans. intern() method keeps the string in an internal cache that is usually not garbage collected.

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   Like         Discuss         Correct / Improve     java   string class   string   intern method   garbage collection   advanced     Asked in 2 Companies      expert


 Q13. How substring() method of String class create memory leaks?Core Java
Admin
info@buggybread.com
Ans. substring method would build a new String object keeping a reference to the whole char array, to avoid copying it. Hence you can inadvertently keep a reference to a very big character array with just a one character string.

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   Like         Discuss         Correct / Improve     java   string class   string   substring   memory leaks   jvm   memory management   advanced   architecture   technical architect   technical lead      expert


Very frequently asked in HCL Tech ( Based of 4 inputs )
  Q14. Write a program to reverse a string iteratively and recursively ?Core Java
Admin
info@buggybread.com
Ans. Using String method -

new StringBuffer(str).reverse().toString();

Iterative -

public static String getReverseString(String str){
StringBuffer strBuffer = new StringBuffer(str.length);
for(int counter=str.length -1 ; counter>=0;counter--){
strBuffer.append(str.charAt(counter));
}
return strBuffer;
}

Recursive -

public static String getReverseString(String str){
if(str.length <= 1){
return str;
}
return (getReverseString(str.subString(1)) + str.charAt(0);
}

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   Like         Discuss         Correct / Improve     java   string   reverse   stringbuffer   string class   code     Asked in 6 Companies        frequent


 Q15. Which String class methods are used to make string upper case or lower case?Core Java
Admin
info@buggybread.com
Ans. toUpperCase and toLowerCase

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   Like         Discuss         Correct / Improve     java   string class   string


 Q16. How to convert String to byte array and vice versa?Core Java
Admin
info@buggybread.com
Ans. We can use String getBytes() method to convert String to byte array and we can use String constructor new String(byte[] arr) to convert byte array to String.

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   Like         Discuss         Correct / Improve     java   string class   string


 Q17. What will be the output of the following code ?

String s1 = "Buggy Bread";
String s2 = "Buggy Bread";
if(s1 == s2)
   System.out.println("equal 1");
String n1 = new String("Buggy Bread");
String n2 = new String("Buggy Bread");
if(n1 == n2)
   System.out.println("equal 2");
Core Java
Admin
info@buggybread.com
Ans. equal 1

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   Like         Discuss         Correct / Improve     java   string class   string   string pool   code   coding      basic        frequent


 Q18. Which methods of the Pattern class have equivalent methods in the String class? Core Java
Admin
info@buggybread.com
Ans. split() and macthes()

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   Like         Discuss         Correct / Improve     java   io   string class   string   pattern


 Q19. How does the String Class stores the string in memory ?Core Java2016-06-14 14:33:03

Ans. It stores the string as a character array with 2 bytes for each character.

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   Like         Discuss         Correct / Improve     String  String Class


 Q20. How is String class unique ?Core Java2016-11-30 15:46:13

Ans. String class is immutable as well as final. Because of these properties , String objects offer many benefits

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 doesnt 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 its 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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   Like         Discuss         Correct / Improve     immutable  immutability  immutability  String  string class     Asked in 1 Companies


 Q21. Which method of String class is used to convert Boolean to String ?Core Java2016-12-17 11:00:44

Ans. toString() is an overloaded method of String class that is used to convert many data types to String, Boolean being one of them.

toString(Boolean bool)

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   Like         Discuss         Correct / Improve     String  String class  Boolean     Asked in 1 Companies      Basic


 Q22. Why is String a class in Java ?Core Java2019-02-06 11:14:14

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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   Like         Discuss         Correct / Improve     string class




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