What is the difference between String.subString() and String.subSequence()

Using str.subSequence(begin, end) returns a CharSequence which is a read-only form of the string represented as a sequence of chars. For example:

String string = "Hello";
CharSequence subSequence = string.subSequence(0, 5);

It's read only in the sense that you can't change the chars within the CharSequence without instantiating a new instance of a CharSequence.

If you have to use str.subSequence(begin, end), you can cast the result to a String:

String string = "Hello";
String subSequence = (String) string.subSequence(0, 5);

and use all the normal String operators like subSequence += " World";

To understand this, the very first thing you need to know is that what is the difference between substring and subsequence

substring is a continuous part or subpart of a string

whereas

subsequence is the part of a string or sequence, that might be continuous or not but the order of the elements is maintained

For example, let's say we have the following strings:

str_a="hello there"
str_b="hello"
str_c="ello th"
str_d="hllo"
str_e="ho hre"
str_f="there hello"

str_b is a substring of str_a, str_c is also a substring of str_a but str_d is not a substring of str_a as this substring is not continuous.

Now all substrings are subsequences as the order is maintained.

str_d is a subsequence of str_a, str_e is also a subsequence of str_a however str_f is not a subsequence of str_a as in this case the order is not maintained.

Now for java, there is no appropriate clarification regarding these methods in javadoc.

Subsequence

Subsequence is a generalisation of substring, suffix, and prefix. Finding the longest string which is a subsequence of two or more strings is known as the longest common subsequence problem.

Example: The string "anna" is a subsequence of the string "banana":

banana
 || ||
 an na

Substring

A substring of a string is a prefix of a suffix of the string, and equivalently a suffix of a prefix. If one string is a substring of another, it is also a subsequence, which is a more general concept.

Example: The string "ana" is a substring (and subsequence) of banana at two different offsets:

banana
 |||||
 ana||
   |||
   ana

Read more here.

But as far as Java is concerned, there isn't any difference in their use as stated clearly in the javadoc. Also as it's stated in there, the method subSequence has only been implemented in class String so as to keep it compliant with the CharSequence interface. And this method's name is indeed just a misnomer.