In this program, you'll learn to reverse a number using a while loop and a for loop in Java.

```
public class ReverseNumber {
public static void main(String[] args) {
int num = 1234, reversed = 0;
while(num != 0) {
int digit = num % 10;
reversed = reversed * 10 + digit;
num /= 10;
}
System.out.println("Reversed Number: " + reversed);
}
}
```

When you run the program, the output will be:

Reversed Number: 4321

In this program, while loop is used to reverse a number as given in the following steps:

- First, the remainder of
`num`divided by 10 is stored in the variable`digit`. Now,`digit`contains the last digit of`num`, i.e. 4.

`digit`is then added to the variable reversed after multiplying it by 10. Multiplication by 10 adds a new place in the reversed number. One-th place multiplied by 10 gives you tenth place, tenth gives you hundredth and so on. In this case,`reversed`contains 0 * 10 + 4 = 4.

`num`is then divided by 10 so that now it only contains first three digits: 123. - After second iteration,
`digit`equals 3,`reversed`equals 4 * 10 + 3 = 43 and`num`= 12 - After third iteration,
`digit`equals 2,`reversed`equals 43 * 10 + 2 = 432 and`num`= 1 - After fourth iteration,
`digit`equals 1,`reversed`equals 432 * 10 + 1 = 4321 and`num`= 0 - Now
`num`= 0, so the test expression`num != 0`

fails and while loop exits.`reversed`already contains the reversed number 4321.

```
public class ReverseNumber {
public static void main(String[] args) {
int num = 1234567, reversed = 0;
for(;num != 0; num /= 10) {
int digit = num % 10;
reversed = reversed * 10 + digit;
}
System.out.println("Reversed Number: " + reversed);
}
}
```

When you run the program, the output will be:

Reversed Number: 7654321

In the above program, the while loop is replaced by a for loop where:

- no initialization expression is used
- test expression remains the same (
`num != 0`

) - update/increment expression contains
`num /= 10`

.

So, after each iteration, the update expression runs which removes the last digit of `num`.

When the for loop exits, `reversed` will contain the reversed number.

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