In java, how would I add a string to a string variable?

Question

I have code that generates a random number form 0-1 3 times and I need it to be added to a variable so it turns into a binary number.So, in theory, this would run three times and possibly give me 101;

String storage = null;
        int i = 0;
        while (i < 3) {
            int binny = this.giveMeBinary();
            storage.concat(String.valueOf(binny));
            i++;
        }

        int ans = Integer.parseInt(storage);

But when I try and run this I get the NullPointerException error for storage. Is there a way to just "add" a string to the variable?

the method giveMeBinary just returns a 0 or a 1.

Solution 1

Your problem is that you are initializing the string to null. Doing something like so should solve your problem:

        String storage = "";
        int i = 0;
        while (i < 3) {
            int binny = this.giveMeBinary();
            storage += (String.valueOf(binny));
            i++;
        }

        int ans = Integer.parseInt(storage);

However, concatenating strings in such a manner is not recommended. What you can do is use a StringBuilder like so:

        StringBuilder storage = new StringBuilder();
        int i = 0;
        while (i < 3) {
            int binny = this.giveMeBinary();
            storage.append(String.valueOf(binny));
            i++;
        }

        int ans = Integer.parseInt(storage.toString());

Solution 2

You get the NullPointerException because you set the variable storage to null. You should start with

String storage = "";

Solution 3

If you want a binary number, i.e. a random number between 0 and 7 or 000 and 111 in binary.

int ans = giveMeBinary() * 4 + giveMeBinary() * 2 + giveMeBinary();

however if you want a decimal number which looks like a binary number.

int ans = giveMeBinary() * 100 + giveMeBinary() * 10 + giveMeBinary();

Solution 4

You have two problems:

• You're never assigning a non-null value to storage, but you're calling the concat method on it. That will always throw a NullPointerException

• You're assuming String.concat will modify the existing string value. It doesn't. Strings are immutable in Java.

I would suggest using a StringBuilder instead, and calling append in the loop:

int i = 0;
StringBuilder builder = new StringBuilder();
while (i < 3) {
    int binny = this.giveMeBinary();
    builder.append(binny);
    i++;
}
int ans = Integer.parseInt(builder.toString());

Quite why you're then parsing a binary number such as "011" as if it were a decimal number is a different matter. What do you actually want the result to be? Do you really want the numbers 0, 1, 10, 11, 100, 101, 110 or 111?

Solution 5

If you want to use String concatenation then just initialise storage to "" (empty string) then use

storage += String.valueOf(binny);

but if you are looping and building Strings you should really use StringBuilder since Strings are immutable

StringBuilder buffer = new StringBuilder();

then

buffer.append(binny);

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