Difference between string and StringBuffer or StringBuilder

Ans) Before looking difference between String and StringBuffer or StringBuilder let's see some fundamental properties of String Class in Java.

• String is immutable in Java: String is by design immutable in Java you can check this post for reason. Immutability offers lot of benefit to the String class e.g. his hashcode value can be cached which makes it a faster hashmap key and one of the reason why String is a popular key in HashMap. Because String is final it can be safely shared between multiple threads without any extra synchronization.
• when we represent string in double quotes like "abcd" they are referred as String literal and String literals are created in String pools. When you compare two String literals using equality operator "==" it returns true because they are actually same instance of String. Anyway comparing object with equality operator is bad practice in Java and you should always use equals method to check equality.
• "+" operator is overloaded for String and used to concatenated two string. Internally "+" operation is implemented using either StringBuffer or StringBuilder.
• Strings are backed up by character Array and represented in UTF-16 format. By the way this behavior can cause memory leak in String because same character array is shared between source String and SubString which can prevent source String from being garbage collected. See How SubString works in Java for more details.
• String class overrides equals() and hashcode() method and two Strings are considered to be equal if they contain exactly same character in same order and in same case. If you want ignore case comparison of two strings consider using equalsIgnoreCase() method. See how to correctly override equals method in Java to learn more about best practices on equals method. Another worth noting point is that equals method must be consistent with compareTo() method for String because SortedSet and SortedMap e.g. TreeMap uses compareTo method to compare String in Java.
• toString() method provides String representation of any object and its declared in Object class and its recommended for other class to implement this and provide String representation.

One of its biggest strength Immutability is also biggest problem of Java String if not used correctly. Many a times we create a String and then perform a lot of operation on them e.g. converting string into uppercase, lowercase , getting substring out of it, concatenating with other string etc. Since String is an immutable class every time a new String is created and older one is discarded which creates lots of temporary garbage in heap. If Strings are created using String literal they remain in String pool. To resolve this problem Java provides us two Classes StringBuffer and StringBuilder. StringBuffer is an older class but StringBuilder is relatively new and added in JDK 5.

Differences between String and StringBuffer in Java

Main difference between String and StringBuffer is String is immutable while StringBuffer is mutable means you can modify a StringBuffer object once you created it without creating any new object. This mutable property makes StringBuffer an ideal choice for dealing with Strings in Java. You can convert a StringBuffer into String by its toString() method. String vs StringBuffer or what is difference between StringBuffer and String is one of the popular Java interview questions for either phone interview or first round. Now days they also include StringBuilder and ask String vs StringBuffer vs StringBuilder. So be prepared for that.

Difference between StringBuilder and StringBuffer in Java

StringBuffer is very good with mutable String but it has one disadvantage all its public methods are synchronized which makes it thread-safe but same time slow. In JDK 5 they provided similar class called StringBuilder in Java which is a copy of StringBuffer but without synchronization. Try to use StringBuilder whenever possible it performs better in most of cases than StringBuffer class. You can also use "+" for concatenating two string because "+" operation is internal implemented using either StringBuffer or StringBuilder in Java. If you see StringBuilder vs StringBuffer you will find that they are exactly similar and all API methods applicable to StringBuffer are also applicable to StringBuilder in Java. On the other hand String vs StringBuffer is completely different and there API is also completely different, same is true for StringBuilder vs String.