For simple RC-circuit, the solution can be obtained analytically without numerical calculation.
For example: V(t)=Vc(t=0)* e[sup]-t/(RC) [/sup] for charging a capacitor.
You can get parameters(R,C,E=Vc)  and draw it with java code (and you can find formula from any textbook).

However, if you need to solve it numerically. You need to learn numerical methods.
Runge-Kutta's 4th-order method will give you much better result than Euler's method.