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.

also note that storage.concat(...) will not change the value of storage. You need to do storage = storage.concat(...). — ILMTitan
– ILMTitan, Commented Jan 31, 2012 at 17:18
As an aside, if your goal is ans and not storage, you can do this easily with bit arithmetic: while(i<3) {ans <<= 1; ans |= this.giveMeBinary();i++;} or whle(i<3) { ans |= this.giveMeBinary()<<i;i++;} — Robᵩ
– Robᵩ, Commented Jan 31, 2012 at 17:19
ahhhhh yes....I see what you mean...it doesn't store the new value into storage. — willkara
– willkara, Commented Jan 31, 2012 at 17:21

npinti · Accepted Answer · 2012-01-31 17:18:01Z

10

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());

answered Jan 31, 2012 at 17:18

npinti

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Comments

Vincent Ramdhanie · Accepted Answer · 2012-01-31 17:16:13Z

1

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

String storage = "";

answered Jan 31, 2012 at 17:16

Vincent Ramdhanie

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1 Comment

Jon Skeet Over a year ago

... and then get an exception in Integer.parseInt because storage still refers to an empty string after the loop?

DaveJohnston · Accepted Answer · 2012-01-31 17:17:57Z

1

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);

answered Jan 31, 2012 at 17:17

DaveJohnston

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Comments

Jon Skeet · Accepted Answer · 2012-01-31 17:29:31Z

1

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?

edited Jan 31, 2012 at 17:29

answered Jan 31, 2012 at 17:16

Jon Skeet

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Comments

Peter Lawrey · Accepted Answer · 2012-01-31 17:31:35Z

1

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();

answered Jan 31, 2012 at 17:31

Peter Lawrey

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Comments

Tomasz Nurkiewicz · Accepted Answer · 2012-01-31 17:16:29Z

0

After you correct NullPointerException, try this:

storage = storage + String.valueOf(binny)

...and then read about StringBuilder.

answered Jan 31, 2012 at 17:16

Tomasz Nurkiewicz

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Comments

juergen d · Accepted Answer · 2012-01-31 17:16:56Z

0

try this:

String storage = "";

answered Jan 31, 2012 at 17:16

juergen d

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Satadru Biswas · Accepted Answer · 2012-01-31 17:17:42Z

0

It throws a NullPointerException because storage is null. Try the following

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());

answered Jan 31, 2012 at 17:17

Satadru Biswas

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Comments

naresh · Accepted Answer · 2012-01-31 17:18:18Z

0

variable storage is not initialized..Following code will throw NPE

storage.concat(....)

answered Jan 31, 2012 at 17:18

naresh

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Comments

driangle · Accepted Answer · 2012-01-31 17:19:10Z

0

The reason you get a NullPointerException is that the "storage" variable is never initialized. So the first call to "storage.concat" will try to perform an operation on a NullPointer.

For doing string concatenation you'll want to use a mutable class like StringBuilder, the String class is immutable (you cannot modify an instance of it).

Here's an example using StringBuilder:

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

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

answered Jan 31, 2012 at 17:19

driangle

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