10 Things Every Java Programmer Should Know about String

10 Things Every Java Programmer Should Know about String
String in Java is very special class and most frequently used class as well. There are lot many things to learn about String in Java than any other class, and having a good knowledge of different String functionalities makes you to use it properly.
Given heavy use of Java String in almost any kind of project, it become even more important to know subtle detail about String. Though I have shared lot of String related article already here in Javarevisited, this is an effort to bring some of String feature together. In this tutorial we will see some important points about Java String, which is worth remembering. You can also refer my earlier post 10 advanced Java String questions to know more about String.

Though I tried to cover lot of things, there are definitely few things, which I might have missed; please let me know if you have any question or doubt on java.lang.String functionality and I will try to address them here.

1) Strings are not null terminated in Java.
Unlike C and C++, String in Java doesn't terminate with null character. Instead String are Object in Java and backed by character array. You can get the character array used to represent String in Java by calling toCharArray() method of java.lang.String class of JDK.

2) Strings are immutable and final in Java
Strings are immutable in Java it means once created you cannot modify content of String. If you modify it by using toLowerCase(), toUpperCase() or any other method, It always result in new String. Since String is final there is no way anyone can extend String or override any of String functionality. Now if you are puzzled why String is immutable or final in Java. checkout the link.
3) Strings are maintained in String Pool
As I Said earlier String is special class in Java and all String literal e.g. "abc" (anything which is inside double quotes are String literal in Java) are maintained in a separate String pool, special memory location inside Java memory, more precisely inside PermGen Space. Any time you create a new String object using String literal, JVM first checks String pool and if an object with similar content available, than it returns that and doesn't create a new object. JVM doesn't perform String pool check if you create object using new operator.

4) Use Equals methods for comparing String in Java
String class overrides equals method and provides a content equality, which is based on characters, case and order. So if you want to compare two String object, to check whether they are same or not, always use equals() method instead of equality operator. Like in earlier example if we use equals method to compare objects, they will be equal to each other because they all contains same contents.
5) Use indexOf() and lastIndexOf() or matches(String regex) method to search inside String
String class in Java provides convenient method to see if a character or sub-string or a pattern exists in current String object. You can use indexOf() which will return position of character or String, if that exist in current String object or -1 if character doesn't exists in String. lastIndexOf is similar but it searches from end. String.match(String regex) is even more powerful, which allows you to search for a regular expression pattern inside String.
6) Use SubString to get part of String in Java
Java String provides another useful method called substring(), which can be used to get parts of String. basically you specify start and end index and substring() method returns character from that range. Index starts from 0 and goes till String.length()-1. By the way String.length() returns you number of characters in String, including white spaces like tab, space. One point which is worth remembering here is that substring is also backed up by character array, which is used by original String. This can be dangerous if original string object is very large and substring is very small, because even a small fraction can hold reference of complete array and prevents it from being garbage collected even if there is no other reference for that particular String.

7) "+" is overloaded for String concatenation
Java doesn't support Operator overloading but String is special and + operator can be used to concatenate two Strings. It can even used to convert int, char, long or double to convert into String by simply concatenating with empty string "". internally + is implemented using StringBuffer prior to Java 5 and StringBuilder from Java 5 onwards. This also brings point of using StringBuffer or StringBuilder for manipulating String.
8) Use trim() to remove white spaces from String
String in Java provides trim() method to remove white space from both end of String. If trim() removes white spaces it returns a new String otherwise it returns same String. Along with trim() String also provides replace() and replaceAll() method for replacing characters from String. replaceAll method even support regular expression.

9) Use split() for splitting String using Regular expression
String in Java is feature rich. it has methods like split(regex) which can take any String in form of regular expression and split the String based on that. particularly useful if you dealing with comma separated file (CSV) and wanted to have individual part in a String array.

10) Don't store sensitive data in String
String pose security threat if used for storing sensitive data like passwords, SSN or any other sensitive information. Since String is immutable in Java there is no way you can erase contents of String and since they are kept in String pool (in case of String literal) they stay longer on Java heap ,which exposes risk of being seen by anyone who has access to Java memory, like reading from memory dump.