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

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Q61. Write a Program to validate if a particular character occurs after another in a string

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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Q62. 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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Q63. 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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Q64. 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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Q65. 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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Q66. 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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Q67. 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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Q68. 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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Q69. 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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Q70. 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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Q71. 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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Q72. 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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Q73. 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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Q74. 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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Q75. 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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Q76. 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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Q77. 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

^{This question was recently asked at 'datalake solutions'.This question is still unanswered. Can you please provide an answer.}

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Q78. 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

^{This question was recently asked at 'Bloomberg'.This question is still unanswered. Can you please provide an answer.}

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Q79. How to calculate string length without using inbuilt function

Core Java

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Q80. Why string pool concept has been introduced in string ?

Core Java

Ans. Memory Sharing and Optimal memory utilization.

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Q81. What is the use of final String when Strings are immutable ?

Core Java

^{This question was recently asked at 'Cyient'.This question is still unanswered. Can you please provide an answer.}

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Q82. 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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Q83. Why is String a class in Java ?

Core 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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Q84. What will be the output of following code ?

String str1 = "String1";
String str2 = "String1";
System.out.print(str1 == str2);
System.out.print(str1.equals(str2));
str1 = str2;
System.out.print(str1 == str2);

Core Java

a. falsetruetrue