This post is relevant for Oracle’s java implementation of 1.6!
Following is the implementation of substring method in String class:
public String substring(int beginIndex, int endIndex) {
if (beginIndex < 0) {
throw new StringIndexOutOfBoundsException(beginIndex);
}
if (endIndex > count) {
throw new StringIndexOutOfBoundsException(endIndex);
}
if (beginIndex > endIndex) {
throw new StringIndexOutOfBoundsException(endIndex - beginIndex);
}
return ((beginIndex == 0) && (endIndex == count)) ? this :
new String(offset + beginIndex, endIndex - beginIndex, value);
}
Now, if you take a look at the highlighted part of the code, it says a new String is created with same char sequence (‘value’), but with different offset!
Let’s take a scenario where we have a huge string, say of 100MB and we take a substring containing last 100 chars of that string.
String s100MB = <100MB String>; //memory occupied is 100MB
String substring = s100MB.subString(s100MB.length() – 100); //does not occupy any additional memory for the chars of substring as it uses the same char array as the parent string
s100MB = null; //‘substring’ still occupies 100MB of memory where as what it requires is only 200k (for 100 chars)
Now, in real world this may not be a very serious issue as GC is not as instantaneous but cases where ‘substring’ hangs around in memory for very long time – this can be unnecessary wastage of memory! (Worse…. think of the original string being 1GB instead of 100MB!)
<![if !supportLists]>- <![endif]>Sarang
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