In order to compile Calculadora.lex you need to download jflex http://www.jflex.de/download.html, copy it in the directory and type in the shell:
jflex Calculadora.lex
After that a file called Yytoken.java will be created. Finally type javac *.java and execute the program with java Calculadora file.in > file.out where file.in is your test and file.out the output.
Below are some examples:
exp.in
2 + 2
3 * 5
12 * 400
exp.out
4
15
4800
exp.in
21 + 12
11 * 22
5 - 1
123
exp.out
33
242
4
123
exp.in
10 + 20 * 30
10 * 20 + 30
30 * 20 +10
1 * 2 + 3 * 4 + 5 * 6 + 7 * 8
exp.out
610
230
610
100
exp.in
1+-2--3*-4
-11+22-0+4*-1
2+32/2/2/2
- - - - 3
64/-2/2/2/-2*-2*-2
exp.out
-13
7
6
3
16
exp.in
(1+-2)-(-3*-4)
2 * ((-11+22-0)+(4*-1))
2 + (((32/2)/2)/2)
- (- - - 3 * (2 + 5))
(64/-2/2/2/-2)-(2*-2*-2)
((((((1+2)*3)+4)*((5+6)*7)+8)*9))
exp.out
-13
14
6
21
-4
9081