Variable a = new Variable("a", VarType.INT);
Variable b = new Variable("b", VarType.INT);
Variable c = new Variable("c", VarType.INT);

Expression e1 = new Operator("=", a, new Variable(VarType.INT, 3.0));
Expression e2 = new Operator("=", b, new Variable(VarType.INT, 2.0));
Expression e3 = new Operator("+", a, b);
Expression e4 = new Operator("=", c, e3);

try {
    e1.calculate();
    e2.calculate();
    e4.calculate();

    System.out.println(c);
} catch (Exception e) {
    e.printStackTrace();
}