Cannot understand the output...

[sonir shah]

public class ForSwitch
{
public static void main(String args[])
{
char i;
LOOP:   for (i=0;i<5;i++) 
{
switch(i++) 
{
case '0': System.out.println("A");
case 1: System.out.println("B"); break LOOP;
case 2: System.out.println("C"); break;
case 3: System.out.println("D"); break;
case 4: System.out.println("E");
case 'E' : System.out.println("F");
}
}
}
}

I tried running the program , and I got the output:
C
E
F
Can any one explain me how do we get this output.I am a bit confused..
Sonir

[Rob Ross]

This one is tricky.
The first time through the loop, i is zero. When the switch variable is evaluated, it is still zero (immediately after being evaluated, it gets incremented to 1 by the pre-increment operator).
The first case statement is NOT comparing the numeric value zero, but the character zero. This is a big difference. The evaluated value of i is numeric 0, and does not match any of the case statements, therefore no code gets executed in the first pass.
The second loop iteration starts, and i gets incremented...but remember it got bumped up by one in the last switch statement, so the second loop iteration, i has the value 2. Now the switch statement is executed for the second time. The value is evaluated as 2, and then i gets incremented to 3. The case where i=2 gets executed, and the string "C" is printed. Then execution breaks out of the switch statement and the next loop iteration begins.
The third time through the loop, i is evaluated as 4, then incremented by one, and the 4 case is executed, printing "E". Since there is no break after "E" is printed, the next statement also executes, printing F.
The loop test now fails, because i is equal to 5, so the loop exits.
IF the first case had been zero "0" instead of the character 0 ('0') nothing would have printed out, because the entire loop would have been broken out of by the labeled break in the first case statement. I missed those single quotes myself the first time I looked at this!
Rob

[Jennifer Wallace]

I was trying to differentiate between the char '0' and char 0.

char c='0';
char i=0;
System.out.println(Integer.toHexString(c));//30
System.out.println(Integer.toHexString(i));//0
if (c!=i)
System.out.println(c+" is not equal to "+i);
// 0 is not equal to 0
why is that here in a regular System.out.println statement the output doesn't differentiate between two different values (0x30 and 0x00)?
[ January 22, 2002: Message edited by: Jennifer Wallace ]
[ January 22, 2002: Message edited by: Jennifer Wallace ]

[Seany Iris]

IF the first case had been zero "0" instead of the character 0 ('0') nothing would have printed out, because the entire loop would have been broken out of by the labeled break in the first case statement. I missed those single quotes myself the first time I looked at this!

I don't think so.
If the first case is number "0",It will print out:A B.
Because first the value of i is "0",the first case will be executed,and prints out "A",then i incremented by one,and the second case is meeted,prints out "B",then the entire loop would have been broken out.

[mark stone]

say case 1 was selected then what would have happened. I am asking beceause there is a label in Break there.
what would happen next if case 1 were to be selected ?

[eric sun]

Notice here, if the first case number is "0" , there will be error with imcompatible types. Here need byte, short, char, int type.
1:LOOP: for (i=0;i<5;i++) {
2: switch(i++) {
3: case '0': System.out.println("A");
4: case 1: System.out.println("B"); break LOOP;
5: case 2: System.out.println("C"); break;
6: case 3: System.out.println("D"); break;
7: case 4: System.out.println("E");
8: case 'E' : System.out.println("F");
9:}
10:}
Now, let explain why the answer is C E F step by step.
Loop 1 :
1: i=0
2: swith(i=0) but (0)i++ = 1 => i
3-9: no case equal i ( 0 )
10: act the i++ in for() (1)i++=2 => i
Loop 2 :
1: i=2
2: swith(i=2) but (2)i++ = 3 => i
5: case 2: System.out.println("C"); break;
10: act the i++ in for() (3)i++=4 => i
Loop 3 :
1: i=4
2: swith(i=4) but (4)i++ = 5 => i
7: case 4: System.out.println("E");
8: case 'E' : System.out.println("F");
10: act the i++ in for() (4)i++=5 => i
end

[Rob Ross]

Originally posted by Seany Iris:

I don't think so.
If the first case is number "0",It will print out:A B.
Because first the value of i is "0",the first case will be executed,and prints out "A",then i incremented by one,and the second case is meeted,prints out "B",then the entire loop would have been broken out.

Yup. you're right!
It will print A B before it breaks out.
Also, Eric, what did you mean by:

Notice here, if the first case number is "0" , there will be error with imcompatible types. Here need byte, short, char, int type.

The case label '0' is a char literal, and it's a perfectly valid case argument. You continue with your explaination ok and never mention this again. If you try compiling this code you'll see it works fine.
Rob