Checking if a number is Armstrong number or not
In addition to the points made by justjofe, I also have a few suggestions.

You might consider making the method armstrong(int) return a boolean instead of a String, and then using the returned boolean to construct the output of your program. That way, you separate the logic of determining whether a number is an Armstrong number from the program's interface. In this case, a better name for the method might be "isArmstrong" instead of "armstrong":

public static void main(String[] args) {
    Scanner in = new Scanner(System.in);

    // ...

    if (isArmstrong(in.nextInt())) {
        System.out.println("armstrong number");
    } else {
        System.out.println("not armstrong number");
    }
}

private static boolean isArmstrong(int number) {
    // ...
    return sum == oldNumber;
}


And now about the code that checks a number for being an Armstrong number itself. I think assigning a new value to the method parameter number is confusing, because it forces you to introduce a local variable that serves no other purpose than to store the original value of the method parameter. So why not do it the other way around, using the variable declared inside the method for the loop and leaving the method parameter number assigned to the original value?

private static boolean isArmstrong(int number) {
    double sum = 0.0;
    int n = number;

    while (n &gt; 0) {
        int temp = n % 10;
        n = n / 10;
        sum = sum + Math.pow(temp, 3);
    }

    return sum == number;
}


After this change, the while loop combined with the declaration of n (or whatever you want to call it) actually lends itself to being expressed as a for loop instead:

private static boolean isArmstrong(int number) {
    double sum = 0.0;

    for (int n = number; n &gt; 0; n = n / 10) {
        int temp = n % 10;
        sum = sum + Math.pow(temp, 3);
    }

    return sum == number;
}


I would now even go so far as to skip the introduction of the temp variable and just invoke Math.pow(n % 10, 3), since you only need to evaluate n % 10 once per loop iteration, but that is purely a matter of taste.

Finally, some comments on points mentioned by justjofe:


First, about closing the scanner: Be aware that calling close() on the Scanner will also call System.in.close() which might not be desirable. There are several threads concerning this at stackoverflow, and one answer says this:


  Generally speaking, the code that creates a resource is also responsible for closing it. System.in was not instantiated by by your code, but by the VM. So in this case it's safe to not close the Scanner […]


However, this doesn't address the issue of garbage collection with regards to the Scanner object raised by justjofe, and I don't know enough to give specific advice here, so I'll just make you aware of this issue.
Second, a small heads-up on compound assignment operators such as += and /=: While it might appear that


sum = sum + Math.pow(temp, 3); and
sum += Math.pow(temp, 3);


are equivalent, this is actually not the case. If you make sum an int instead of a double, the second statement will compile, however, the first one will not. The reason for this is that the second statement is actually equivalent to


sum = (int) (sum + Math.pow(temp, 3));


(as stated here in the Java Language Specification). Since Math.pow(double, double) returns a double, the result needs to be cast to an int before being assigned to an int variable. The compound assignment operator does this for you, but when using an ordinary =, you have to do the casting yourself.