call R with un expression (String)?

Hello all,
I want to call R from java. And I have a expression in Java as a String,
example : (variable 1 + variable 2)* variable 3 and i want R calculate this
expression. How can I do?
ex: 
Java
-int x1,x2;
-float x3;
-String s=(  x1.toString()+x2.toString()   )  *   x3.toString();
R:
calculate expression s???? and return in to Java?

Thanks 
Jin

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