程序填空题：Int Operator

This program reads a line of prefix integer expression with one leading intergal operator and two integer values, and evaluates the result. A prefix integer expression is like:
```
add 5 3
```

The result of the expression above is: `8`.

Now, fill in the blanks:

```Java
import java.util.Scanner;

enum IntOp {
@@[add()](1) { // for operator add
int eval(int i1, int i2) {
return @@[i1+i2](1);
}
},
@@[ sub()](1) { // for operator sub
int eval(int i1, int i2) {
return @@[i1-i2](1);
}
};

int eval(int i1, int i2) {
return 0;
};
}

public class Main {
public static void main(String[] args) {
Scanner in = new Scanner(System.in);
String op = in.next();
int i1 = in.nextInt();
int i2 = in.nextInt();
System.out.println(IntOp.@@[valueOf(op)](1).eval(i1, i2));
in.close();
}
}
```







答案：
第1空：add()

第2空：i1+i2

第3空： sub()

第4空：i1-i2

第5空：valueOf(op)