Sunday, October 7, 2012

10 Java String interview Question answers - Advanced

10 Java String interview Question answers
String interview questions in Java is one of Integral part of any Core Java or J2EE interviews. No one can deny importance of String and How much it in any Java application irrespective of whether its core Java desktop application, web application, Enterprise application or Mobile application. String is one of the fundamental of Java programming language and correct understanding of String class is must for any Java programmer. What makes String interview questions in Java even more interesting is the special status of String in terms of features and privileges it has, like + operator is kind of overloaded to perform String concatenation despite the fact that Java does not support operator overloading. There is a separate String pool to store String literal etc. In this article we will some frequently asked question on String in a Java interview which focuses on range of issues like immutability, thread-safety, Security etc.

Java String interview Question

Java String Interview Questions Answers Advanced ExperiencedHere are my list of frequently asked question on String, feel free to add any other interesting question which you faced on String during any Core Java interview :

1) What is String in Java ? Is String is data type?
String in Java is not a primitive data type like int, long or double.  String is a class or in more simple term a user defined type. This is confusing for some one who comes from C background. String is defined in java.lang package and wrappers its content in a character array. String provides equals() method to compare two String and provides various other method to operate on String like toUpperCase() to convert String into upper case, replace() to replace String contents, substring() to get substring, split() to split long String into multiple String.

2) Why String is final in Java
String is final by design in Java, some of the points which makes sense why String is final is Security, optimization and to maintain pool of String in Java. for details on each of this point see Why String is final in Java.

3) What is Difference between String and StringBuffer in Java
This is probably the most common question on String I have seen in Java interviews. Though String and Stringbuffer are two different class they are used in context of concatenating two Strings, Since String is immutable in Java every operation which changes String produces new String, which can be avoided by using Stringbuffer. See String vs StringBuffer  for more details.

4) What is difference in String on C and Java
If you have mentioned C in your resume, then you are likely to face this String interview question. Well C String and Java String are completely different to each other, C String is a null terminated character array while String in Java is an Object. Also String is more feature rich in Java than C.

5) Why char array is better than String for storing password?
This String interview question is debatable and you might not agree with interviewer but this is also a chance to show that how deep and differently you can think of. One of the reason which people give Why you should store password in char array over String is related to immutability, since its not possible to remove erase contents of String but you can erase contents of char array. See Why char array preferred over String for password for complete discussion.

6) How do you compare two String in Java ?
This is another common String interview question which appears on fresher level interviews. There are multiple ways to compare two String like equals() method, equalsIgnoreCase() etc, You can also see 4 ways to compare String in Java for more examples. Main thing which interviewer checks is that whether candidate mentioned equality operator or not "==", comparing String with equality operator is common mistake which works in some case and doesn't work in other. next String interview question is follow-up up of this.

7) Can we compare String using == operator? What is risk?
As discussed in previous String question, You can compare String using equality operator but that is not suggested or advised because equality operator is used to compare primitives and equals() method should be used to compare objects. As we have seen in pitfall of autoboxing in Java that how equality operator can cause subtle issue while comparing primitive to Object, any way String is free from that issue because it doesn't have corresponding primitive type and not participate in autoboxing. Almost all the time comparing String means comparing contents of String i.e. characters and equals() method is used to perform character based comparison. equals() return true if two String points to same object or two String has same contents while == operator returns true if two String object points to same object but return false if two different String object contains same contents. That explains why sometime it works and sometime it doesn't. In short always use equals method in Java to check equality of two String object.

8) How substring method work in Java
This is one of the tricky Java question relate to String and until you are familiar with internals of String class, its difficult to answer. Substring shares same character array as original String which can create memory leak if original String is quite big and not required to retain in memory but unintentionally retained by substring which is very small in size and prevents large array from begin claimed during Garbage collection in Java. See How Substring works in Java for more details.

10)What is String pool in Java
Another tough Java question asked in  String interview. String pool is a special storage area in Java heap, mostly located on PerGen space, to store String literals like "abc". When Java program creates a new String using String literal, JVM checks for that String in pool and if String literal is already present in pool than same object is returned instead of creating a whole new object. String pool check is only performed when you create String as literal, if you create String using new() operator, a new String object will be created even if String with same content is available in pool.

9) What does intern() method do in Java
As discussed in previous String interview question, String object crated by new() operator is by default not added in String pool as opposed to String literal. intern() method allows to put an String object into pool.

11) Does String is thread-safe in Java
If you are familiar with the concept of immutability and thread-safety you can easily answer this String interview question in Java. Since String is immutable, it is thread-safe and it can be shared between multiple thread without external synchronization.

That's all on Java String interview question. In Summary there are lot of specifics about String which needs to be know for any one who has started Java programming and these String question will not just help to perform better on Java Interviews but also opens new door of learning about String. I didn't know many String related concepts until I come across these question which motivated to research and learn more about String in Java.

Other Java String tutorials from Javarevisited Blog

16 comments :

Attila-Mihaly Balazs said...

Please, please don't mention StringBuffer. You should use StringBuilder 99.999% of the time.

Shariq Bharat said...

Hi, Please confirm.. I face this question a lot in my interviews.
String aa = new String("abc");
String bb = "abc";

How many object of string class are created.

Javin @ String to int in Java said...

@Shariq Bharat, Two object, one in each line. Second one will be added into String pool.

Javin @ Must Override Eclipse Error said...

@Attila-Mihaly Balazs , 99.99% agree with you, and one should be using StringBuilder by default.

Sanidhya Sharma said...

question pls
input:"my name is jack"
output:"ym eman si kcaj"

Anonymous said...

To answer Shariq Bharat questions in more detail, Yes, there are two objects are created, but both are created in first line itself. Before creating object using new String(), JVM creates an string literal "abc", that is your first object, now by using content of this object, second object is created by calling constructor of String, this object also has value "abc" but created in heap, rather than in permgen area, where first object was created. In second line no new object is created as bb will get reference of "abc" from String pool from permgen space.

Now, from Java 7 onwards String pool is no longer located in permgen space, they are now part of Java heap space. Which means you can use String literals and intern() method more freely without worrying about OutOfMemoryError in permgen space. If you are still using Java 1.6, then remember that permgen area has less space than heap memory, by default around 64MB, by creating large number of String literals in String pool, you can quickly fill permgen space.

Kapil said...

As always great list of questions from String class. I have always seen questions form String, not just fact based but also task based programming questions e.g.

1) Write a Java program to reverse String in Java without using any API? which means you can not use StringBuffer's reverse() method, neither any of String utility method, all you can have is a character array for reversing contents.

2) Write a Program to check if a String is palindrome or not? For example an String e.g. "madam" is a palindrome but "book" is not palindrome. You also need to solve this question without taking any help from Java String API.

3) Write a Java program to check if two String are Anagram or not? You need to write method e.g. isAnagram(String first, String second) which will return true if second String is anagram of first string. An anagram must contain same number of characters and exactly same characters but on different order e.g. top and pot, or army and mary.

4) Write a method in Java to remove any character from String? For example you need to write mthod remove(String word, char removeThis), this method should return an String without character, which is asked to remove. you can use indexOf, substring and similar methods from String class, but your method must handle corner cases e.g. passing null or empty String, String containing just one character etc.

5) Write a method to Split a comma separated String in Java?

6) Write Java program to print all permutations of a String e.g. passing "ABC" will print all permutations like "BCA", "CBA" etc

Anonymous said...

Hello Javin, Can we use String in Switch case? This question is asked to me at TCS Interview? I said no, because Switch only accept integer values, but interviewer was not happy at all?

Kaushal Kapoor said...

In the JDK 7 release, you can use a String object in the expression of a switch statement

http://docs.oracle.com/javase/7/docs/technotes/guides/language/strings-switch.html

madnorb said...

@Sanidhya A quick implementation, hopefully without bugs :)

public class ReverseWords {
public String run(String input, char delimiter){
if (input == null) return null;
char[] charArray = input.toCharArray();
int wordStartIdx = 0;
for(int i = 0; i from; to--, from++){
char tmp = charArray[from];
charArray[from] = charArray[to];
charArray[to] = tmp;
}
}

}

madnorb said...

I left in a bug :), this is the bugfix:
...
flip(charArray, wordStartIdx, charArray.length-1);
return new String(charArray);
}
...

madnorb said...

My previous String reversal implementation is very basic and limited, the task is a little bit more complicated because of the unicode character representation: http://docs.oracle.com/javase/6/docs/api/java/lang/Character.html#unicode
One have to consider surrogate ranges (a unicode character could be represented by two char-s)

Vikas Grover said...

two object are created to answer of shariq bhagat

for in heap memory and another will in string pool as constant'

and both have different hascode

Vikas Grover said...

my question is

why we create main method , and how it will be created

Karl the Pagan said...

2 months after this posting question #8 became voodoo optimization: http://www.javaadvent.com/2012/12/changes-to-stringsubstring-in-java-7.html

2 years later I'm still seeing this blog post thrown around without the correction. Here is a good overview of the differences: http://jaxenter.com/the-state-of-string-in-java-49450.html

Bhagath Sagar said...

How substring method work in Java ? , Answer for this question need an attention. The new JDK provided the fix for memory leak.

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