How SubString method works in Java - Memory Leak Fixed in JDK 1.7

Substring method from String classis one of most used method inJava, and it's also part of an interestingString
interview question e.g. How substring works in Java or sometime asked as how does substring creates memoryleak in Java. In order to answer these questions, you knowledge of implementation
details is required. Recently one ofmy friend was drilled onsubstring method in Java during a Java
interview, he was using substring() method from long time, and of course all of us has used this, but what surprises him was interviewer's obsession on Java substring, and deep
dive till the implementation level. Though String is a special class in Java, and subject of many interview questions e.g.Why char array
is better than String for storing password
. In this case it was, substring method, which took center stage. Most of us rather just usesubstring(..), and than forgot. Not every Java programmer go into code, and see how exactly it's working. To get a feel of how his interview was let's start
.

Update: This issue was actually a bug http://bugs.sun.com/view_bug.do?bug_id=6294060, which is fixed in substring implementation of Java 7. Now, Instead of sharing original character array, substring method creates a copy
of it. In short, substring method only retains as much data, as it needed. Thanks to Yves Gillet for pointing this. As some of my readers pointed out,java.lang.String
class has also grown into some change in Java 1.7 version and offset and count variable which is used to track positions are removed from String. This may save some bytes with each String instance, but not sharing original array makes substring perform
linearly, as compared to constant time previously. Anyway, it's worth to remove any string related memory leak in Java. Having said that, if you have not yet upgraded your Server to Java 7 and still working on Java 1.6 updates, this is one thing, which is
worth knowing.

Question starts with normal chit chat, and Interviewer ask, "Have you used substring method in Java", and my friend proudly said Yes, lot many times, which brings a smile on interviewer's face. He says well, that’s
good. Next question was Can you explain what does substring do? My friend got an opportunity toshow off his talent, and how much he knows about Java API; He said substring
method is used to get parts of String in Java. It’s defined in java.lang.String class, and it's anoverloaded method.
One version of substring method takes just beginIndex, and returns part of String started from beginIndex till end, while other takes two parameters, beginIndex and endIndex, and returns part of String starting from beginIndex to endIndex-1. He also stressed
that every time you call substring() method in Java, it will return a new String becauseString is immutable in Java.




Next question was, what will happen if beginIndex is equal to length in substring(int beginIndex), no it won't throw IndexOutOfBoundExceptioninstead it will return

empty String. Same is the case when beginIndex and endIndex is equal, in case of second method. It will only throw StringIndexBoundException when beginIndex is negative, larger than endIndex or larger than length of String.

So far so good, my friend was happy and interview seems going good, until Interviewee asked him, Do you know how substring works in Java? Most of Java developers fail here, because they don't know how exactly
substring method works, until they have not seen
the code of java.lang.String. If you look substring method inside String class, you will figure out that it calls String (int offset, int count, char value [])constructor
to create new String object. What is interesting here is, value[], which is the same character array used to represent original string. Sowhat's wrong with this?

In case If you have still not figured it out, If the original string is very long, and has array of size 1GB, no matter how small a substring is, it will hold 1GB array. This will also stop original string to begarbage
collected, in case if doesn't have any live reference. This is clear case of memory leak in Java, where memory is retained even if it's not required. That's howsubstring method creates
memory leak.

How SubString in Java works

Obviously next question from interviewer would be,how do you deal with this problem? Though you can not go, and change Java substring method, you can still make some work around, in case you are creating substring
of significant longer String. Simplesolution is to trim the string, and keep size of character array according to length of substring. Luckily java.lang.String has constructor
to do this, as shown in below example.

// comma separated stock symbols from NYSEString listOfStockSymbolsOnNYSE = getStockSymbolsForNYSE(); //calling String(string) constructorString apple = new String(

listOfStockSymbolsOnNYSE.substring(appleStartIndex, appleEndIndex)

);

If you look code on java.lang.String class, you will see that thisconstructor trim the array,
if it’s bigger than String itself.

public String(String original) {        ...        if (<
4000
/span>originalValue.length > size) {            // The array representing the String is bigger than the new            // String itself.  Perhaps this constructor is being called            // in order to trim the baggage, so make a copy of the array.            int off = original.offset;            v = Arrays.copyOfRange(originalValue, off, off+size);        } else {            // The array representing the String is the same            // size as the String, so no point in making a copy.            v = originalValue;        }    ... }

Another way to solve this problem is to call intern() method on substring, which will than fetch anexisting
string from pool or add it if necessary. Since the String in the pool is a real string it only take space as much it requires. It’s also worth noting that sub-strings are not internalized,
when you call intern() method on original String. Most developer successfullyanswers first three questions, which is related to usage of substring, but they get stuck on last two, How
substring creates memory leak or How substring works. It's not completely there fault, because what you know is that every time
substring() returns new String which is not exactly true, since it’s backed by samecharacter array.

This was the only interview question, which bothers my friend little otherwise, its standard service level company Java interview in India. By the way, he got the call a day after ,even though he struggled little bit onHow
SubString method works in Java, and that was the reason he shared this interview experience with me.

Read more: http://javarevisited.blogspot.com/2011/10/how-substring-in-java-works.html#ixzz2ki0sQ1Dx