A Console User Interface for the Stack Machine

Create a package called presentation.

Add a class called StackMachineCUI. This class should have a main method.

A console user interface (CUI) is driven by a perpetual loop of the form:

while(true) {
read next command
if command is quit then break
execute command
print result
}

Scanners

While System.out is barely adequate for console output, System.in is totally inadequate. We can enhance it by wrapping it in a scanner:

Scanner kbd = new Scanner(System.in);

Scanners have many useful methods for extracting tokens from strings and streams.

For example:

String command = kbd.next(); // command = next token in System.in
double operand = kbd.nextDouble();

Commands

Before we can complete our control loop we must decide on the commands we will execute and their syntax and semantics.

Assume sm is some stack machine:

StackMachine sm = new StackMachine();

Here is a list of commands for the stack machine and their interpretations:

quit // terminate program

push NUMBER // sm.push(NUMBER)

pop // sm.pop()

top // display sm.peek()

add // sm.add()

mul // sm.mul()

sub // sm.sub()

div // sm.div()

A Basic Control Loop

Here's a basic control loop. Finish it by replacing the ??? comments:

public static void main(String[] args) {
StackMachine sm = new StackMachine();
Scanner kbd = new Scanner(System.in);
while(true) {
System.out.print("-> ");
String command = kbd.next();
String result = "done";
if (command.equals("quit")) {
break;
}
try {
if (command.equals("push")) {
if (!kbd.hasNextDouble()) {
kbd.nextLine(); // flush
System.out.println("argument to push must be a double");
continue;
}
sm.push(kbd.nextDouble());
} else if (command.equals("pop")) {
// ???
} else if (command.equals("top")) {
result = "" + sm.peek();
} else if (command.equals("add")) {
sm.add();
} else if (command.equals("mul")) {
// ???
} else if (command.equals("sub")) {
// ???
} else if (command.equals("div")) {
// ???
} else {
System.out.println("unrecognized command: " + command);
kbd.nextLine(); // flush
}
} catch (Exception e) {
System.out.println(e.getMessage());
kbd.nextLine(); // flush
continue;
}
System.out.println(result);
}
System.out.println("bye");

}

Here's a sample output:

-> push 3
done
-> push 4
done
-> top
4.0
-> push ten
argument to push must be a double
-> puch
unrecognized command: puch
done
-> add
done
-> top
7.0
-> add
stack needs at least 2 numbers
-> quit
bye