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

Core Java - Interview Questions and Answers for 'String' - 83 question(s) found - Order By Rating

Q1. Why is String a class in 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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Q2. If you have to pull a sub string out of a string, which method would you use - using String methods or Pattern / Matcher ?

Design

Ans. It depends on how complex it is and if in future it would need any sort opf debugging. It's not easy to debug code if it's making heavy use of shortcuts like Lambda , Patterns etc.

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Q3. What is the use of final String when Strings are immutable ?

Core Java

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Q4. Why string pool concept has been introduced in string ?

Core Java

Ans. Memory Sharing and Optimal memory utilization.

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Q5. How to calculate string length without using inbuilt function

Core Java

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Q6. Given a String with letters , numbers and special characters , extract Tokens from it. The rules for Token extraction are ac follows

1. Should for token of all characters till a number or special character is found
2. Should form token of all numbers till a character or special character is found
3. Special character in itself is a token
4. Ignore white spaces

For example - Bob Said "He is here"

should result in

Bob A
Said A
" S
He A
is A
here A
" S

where a is specifying alphabet and S as special character

Design

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Q7. Find different combination of given String and trace the output

Core Java

Ans. private void permutation(String prefix, String sufix)
{
int ln = sufix.length();
if(ln == 0) {
System.out.println(prefix);
} else {
IntStream.range(0, ln).forEach(i->permutation(prefix sufix.charAt(i), sufix.substring(0,i) sufix.substring(i 1, ln)));
}
}

call:permutation("", "abcdef");

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Q8. Suppose we have a string "Java is object oriented language" and we have to reverse every alternate word in string. How would we do that using Java program.

Core Java

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Q9. Why shouldn't we use string concatenation extensively or in a loop ?

Core Java

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 objects are mutable.

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Q10. How is string object immutable if we can concat a string to it ?

Core Java

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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Q11. How can we convert a character or a character array into a String ?

Core Java

Ans. String has an argument constructor that take char array as argument and creates a string.

There is no constructor available with String that takes in a character and creates a String. We can use StringBuilder which has a char argument constructor.

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Q12. How can we convert String into Char array and vice versa ?

Core Java

Ans. There is a method toCharArray() within String class that can be used to convert string to char array.

string.toCharArray();

String class has an argument constructor that takes a char array and create a string

new String(charArray);

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Q13. How can we convert char into a String ?

Core Java

Ans. We can use StringBuilder. StringBuilder accepts char as the argument for it's constructor.

new StringBuilder('').toString();

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Q14. Do we have a method to reverse a string in String class ?

Core Java

Ans. No but we have it in StringBuilder.

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Q15. Is a string an array of characters in Java ?

Core Java

Ans. No, String is a class whose objects are called strings in Java.

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Q16. Is String final in Java ?

Core Java

Ans. Strings are immutable in Java and not final.

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Q17. What is the difference between being final and being immutable ? Are string final or immutable in Java ?

Core Java

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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Q18. Why do we pass an array of strings to main method ?

Core Java

Ans. Array of strings in the main method are the list of arguments or parameters which are sent to the application / program.

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Q19. How does making string as immutable helps with securing information ? How does String Pool pose a security threat ?

Core Java

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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Q20. Are char,String and Char data types in java ?

Core Java

Ans. char is a primitive data type. String is a class. Char is a wrapper class for primitive char.

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Q21. Is String a Data type in Java ?

Core Java

Ans. String is not a primitive type in java. Anything defined within double quotes like "abc" is an object of String class.

Sample Code for String

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Q22. What is the purpose of String Pool ? Don't you think it's a performance overhead to have unnecessary check for string in string pool ?

Core Java

Ans. String Pool makes Java more memory efficient by providing a reusable place for string literals. It might be a little performance inconvenience but results in good amount memory saving.

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Q23. What will be result of following code and why

Integer int1 = 1;
Integer int2 = 1;
String str1 = new String("str");
String str2 = new String("str");
String str3 = "str";
String str4 = "str";

System.out.println(int1 == int2);
System.out.println(str1 == str2);
System.out.println(str3 == str4);

Core Java

Ans. true
false
true

Just like Strings, java maintains an integer constant pool too. So 1 will be maintained in integer constant pool and hence reference int2 will point to same integer in pool.

String pool is only for string literals ( defined by "" ) and not for newly created objects and hence str1 == str2 will return false as they are separate objects in memory.

String pool is used for storing string literals so that they can be reused and str3 and str4 will point to same string literal in string pool.

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Q24. What will be the output of following code

Integer x = 1;
Integer y = 2;
System.out.println(x == y);

What if you change 1 to "1" and Integer to String?

Core Java

Ans. It will print "true" with integers as well as strings. The reason is "Integer constant pool" and "String pool"

String pool maintains pool of string literals. When a string literal is used for the first time, a new string object is created and is added to the pool. Upon it's subsequent usage , the reference for the same object is returned. Similarly java uses integer constant pool.

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Q25. Can StringBuffer and StringBuilder in Java be inherited?

Core Java

Ans. No, they have been declared final and hence cannot be extended.

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Q26. Write a method / program that will determine if the parenthesis are balanced in a given string.

Core Java

Ans. https://www.geeksforgeeks.org/check-for-balanced-parentheses-in-an-expression/

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Q27. Write a Program to validate if a particular character occurs after another in a string

Core Java

Ans. public class Class{
   public static void main(String[] args){
      String str = "xyz123.co@m";

      if(str.indexOf('.') < str.indexOf('@')){
         System.out.println("Not a Valid Email Address");
      }
   }
}

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Q28. Write a Program to display the words of a String in alpahbetical or Dictionary Order

Core Java

Ans. public class BuggyBread {
   public static void main(String args[]) {
       String str = "we are what we repeatedly do excellence then is not an act but a haBit";
       Set<String> wordSet = new TreeSet(); // Using Linked Hash Set as we would like to retrieve words in the insertion order

       for(String word: str.split(" ")){
          wordSet.add(word);
       }

       for(String word: wordSet){
          System.out.print(word);
          System.out.print(" ");
       }
   }
}

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Q29. Write a Program to remove duplicate words from the String

Core Java

Ans.
public class BuggyBread {
public static void main(String args[]) {
String str = "we are what we repeatedly Do excellence, then, is not an act but a haBit";
Set<String> wordSet = new LinkedHashSet(); // Using Linked Hash Set as we would like to retrieve words in the insertion order

for(String word: str.split(" ")){
wordSet.add(word);
}

for(String word: wordSet){
System.out.print(word);
System.out.print(" ");
}
}
}

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Q30. Write a program to print count / no of common words in 2 separate strings

Core Java

Ans. public class BuggyBread {
   public static void main(String args[]) {
       String str1 = "we are what we repeatedly Do excellence, then, is not an act but a haBit";
       String str2 = "we are what we repeatedly Do is";
       String[] str1Words = str1.split(" ");
       String[] str2Words = str2.split(" ");

       Set str1WordsSet = new HashSet();

       for(String word:str1Words){
          str1WordsSet.add(word);
       }

       int commonWordsCount = 0;

       for(String word:str2Words){
          if(str1WordsSet.contains(word)){
             commonWordsCount++;
          }
       }

       System.out.println(commonWordsCount);
   }
}

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