How can I start a new android activity using class name in a string?

Here is a code by which you can start activity using the name of the activity

String activityToStart = "com.example.MainActivity";
try {
    Class<?> c = Class.forName(activityToStart);
    Intent intent = new Intent(this, c);
    startActivity(intent);
} catch (ClassNotFoundException ignored) {
}

EDIT

Here class name will be full name of the class with the package name.
For example if your package name will be x.y.z and if you have Activity name called A then the full name (activityToStart) of the Activity A will be x.y.z.A.