How substring() works in java?

substring() is a function to get substring from string. A String class is immutable in java.
A String can be implemented as an object with three fields ( normal string class have even more, but consider 3 for now) -- a character array, an offset into that array, and a length.
Consider the following code

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

substring() creates a new String and returns it back. But 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. So only length and offset are different per string but the character array is shared with the original string class. This can be shown in figure:

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));