How the substring() function of String class works..

Consider the following code

String s1 = "Monday";
String s = s1.substring(0,3);
or
s1.substring(0,3).equals("Mon")

substring is clever. It does not make a deep copy of the substring the way most languages do. It just creates a pointer into the original immutable String, i.e. points to the value char[] of the base string, and tracks the starting offset where the substring starts and count of how long the substring is.
The downside of this cleverness is a tiny substring of a giant base String could suppress garbage collection of that big String in memory even if the whole String were no longer needed. (actually its value char[] array is held in RAM; the String object itself could be collected.)
If you know a tiny substring is holding a giant string in RAM, that would otherwise be garbage collected, you can break the bond by using

String s = new String(s1.substring(0,3));