String

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The reasons why string is immutable are, 
 
1) Imagine StringPool facility without making string immutable , its not possible at all because in case of string pool one string object/literal e.g. "Test" has referenced by many reference variables , so if any one of them change the value others will be automatically gets affected i.e. lets say 
 
String A = "Test" 
String B = "Test" 
 
Now String B called "Test".toUpperCase() which change the same object into "TEST" , so A will also be "TEST" which is not desirable. 
 
2)String has been widely used as parameter for many Java classes e.g. for opening network connection, you can pass hostname and port number as string , you can pass database URL as string for opening database connection, you can open any file in Java by passing name of file as argument to File I/O classes. 
 
In case, if String is not immutable, this would lead serious security threat , I mean some one can access to any file for which he has authorization, and then can change the file name either deliberately or accidentally and gain access of those file. Because of immutability, you don't need to worry about those kind of threats. This reason also gel with, Why String is final in Java, by making java.lang.String final, Java designer ensured that no one overrides any behavior of String class. 
 
3)Since String is immutable it can safely shared between many threads ,which is very important for multithreaded programming and to avoid any synchronization issues in Java, Immutability also makes String instance thread-safe in Java, means you don't need to synchronize String operation externally. Another important point to note about String is memory leak caused by SubString, which is not a thread related issues but something to be aware of. 
 
4) Another reason of Why String is immutable in Java is to allow String to cache its hashcode , being immutable String in Java caches its hashcode, and do not calculate every time we call hashcode method of String, which makes it very fast as hashmap key to be used in hashmap in Java.  This one is also suggested by  Jaroslav Sedlacek in comments below. In short because String is immutable, no one can change its contents once created which guarantees hashCode of String to be same on multiple invocation. 
 
5) Another good reason of Why String is immutable in Java suggested by Dan Bergh Johnsson on comments is: The absolutely most important reason that String is immutable is that it is used by the class loading mechanism, and thus have profound and fundamental security aspects. Had String been mutable, a request to load "java.io.Writer" could have been changed to load "mil.vogoon.DiskErasingWriter" 
 
Security and String pool being primary reason of making String immutable, I believe there could be some more very convincing reasons as well, Please post those reasons as comments and I will include those on this post. By the way, above reason holds good to answer, another Java interview questions "Why String is final in Java".  Also to be immutable you have to be final, so that your subclass doesn't break immutability.  what do you guys think ? 
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String is immutable or final in Java

The reasons why string is immutable are, 1) Imagine StringPool facility without making string immutable , its not possible at all because...

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Here 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. 
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10 Java String interview Question & Answers

Here are my list of frequently asked question on String, feel free to add any other interesting question which you faced on String during a...

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