java - how the subString() function of string class works

Question

please see the following code.

String s = "Monday";
if(s.subString(0,3).equals("Mon"){}

String s2 = new String(s.subString(0,3));
String s3 = s.subString(0,3);  

I know that line 2 will still point to "Monday" and have a new String object with the offset and count set to 0,3.

The line 4 will create a new String "Mon" in string pool and point to it.

But not sure what about line 5 whether it will behave like line 2 or line 4.

If i am wrong for line 2 or 4 also please correct..

Answer 1

As pointed out by Pete Kirkham, this is implementation specific. My answer is only correct for the Sun JRE, and only prior to Java 7 update 6.

You're right about a normal substring call just creating a new string referring to the same character array as the original string. That's what happens on line 5 too. The fact that the new string object reference happens to be assigned to a variable doesn't change the behaviour of the method.

Just to be clear, you say that in line 2 the new string will still point to "Monday" - the char array reference inside the string will be to the same char array as one used for "Monday". But "Monday" is a string in itself, not a char array. In other words, by the time line 2 has finished (and ignoring GC) there are two string objects, both referring to the same char array. One has a count of 6 and the other has a count of 3; both have an offset of 0.

You're wrong about line 4 using a "string pool" though - there's no pooling going on there. However, it is different to the other lines. When you call the String(String) constructor, the new string takes a copy of the character data of the original, so it's completely separate. This can be very useful if you only need a string which contains a small part of a very large original string; it allows the original large char array to be garbage collected (assuming nothing else needs it) while you hold onto the copy of the small portion. A good example of this in my own experience is reading lines from a line. By default, BufferedLineReader will read lines using an 80-character buffer, so every string returned will use a char array of at least 80 characters. If you're reading lots of very short lines (single words) the difference in terms of memory consumption just through the use of the odd-looking

line = new String(line);

can be very significant.

Does that help?