Insert string in beginning of another string

Sure, use StringBuilder.insert():

_sb.insert(0, _s);

Other answers explain how to insert a string at the beginning of another String or StringBuilder (or StringBuffer).

However, strictly speaking, you cannot insert a string into the beginning of another one. Strings in Java are immutable¹.

When you write:

String s = "Jam";
s = "Hello " + s;

you are actually causing a new String object to be created that is the concatenation of "Hello " and "Jam". You are not actually inserting characters into an existing String object at all.

---

^{1 - It is technically possible to use reflection to break abstraction on String objects and mutate them ... even though they are immutable by design. But it is a really bad idea to do this. Unless you know that a String object was created explicitly via new String(...) it could be shared, or it could share internal state with other String objects. Finally, the JVM spec clearly states that the behavior of code that uses reflection to change a final is undefined. Mutation of String objects is dangerous.}

The first case is done using the insert() method:

_sb.insert(0, "Hello ");

The latter case can be done using the overloaded + operator on Strings. This uses a StringBuilder behind the scenes:

String s2 = "Hello " + _s;