NullPointerException

[maxspyderweb]

Hi to everyone, I want to start telling you sorry about my english..

Here is the java class

package utility;


public class Decodificatore {

	public static NBit[] decode(String bits, int n, int freq){
		NBit.len = n;
		int nnbit =bits.length()/n;
		int subnnbit = nnbit/freq;
		System.out.println(nnbit+""+subnnbit);
		
		NBit[] decoded = new NBit[subnnbit];
		for(int i=0; i<subnnbit;i++){
			NBit[] semidec = new NBit[freq];
				for(int j=0; j<freq;j++){
					String s = bits.substring(i*freq+j, i*freq+j+n);
					semidec[j].set(s);
					
				}
			decoded[i]= ConfrBitFreq.Confronta(semidec);
		}
		return  decoded;
	}
}

the error is

Exception in thread "main" java.lang.NullPointerException
	at utility.Decodificatore.decode(Decodificatore.java:17)
	at Input.main(Input.java:13)

and the NBit class is

package utility;

import java.util.BitSet;

public class NBit {
	
	public static int len = 0;
	
	public BitSet nBit = new BitSet(len);
	
	public void set(String s)
	{
		for(int i=0; i < len ; i++)
		{
			String sts = s.substring(i,i+1);
			if(sts == "0"){
				nBit.set(i, i+1, false);
			} else nBit.set(i,i+1,true);
		}
	}
	
	public static boolean equal(NBit a, NBit b){
		boolean risultato = false;
		for(int i=0; i< len; i++){
			if(a.nBit.get(i)==b.nBit.get(i)){
				risultato = true;
			} else {risultato = false; continue;}
		}
		return risultato;
	}


	public static String NBitoString(NBit[] bites)
	{
		String finale = "";
		for(int j=0; j < bites.length;j++){
		
			String s="";
		
		for(int i=0; i<len;i++){
			if(bites[j].nBit.get(i)){
				s=s+"1";
			} else s=s+"0";
		}
		finale = finale+s;
		}
		return finale;
	}

	
}

sorry I'm totally new to java, as you could see this is quite chaotic but I think the problem is something very simple, please help me! xD

[eRaaaa]

		NBit[] semidec = new NBit[freq];
				for(int j=0; j<freq;j++){
					String s = bits.substring(i*freq+j, i*freq+j+n);
					semidec[j].set(s);

With NBit[] semidec = new NBit[freq]; you are creating an array with the size of freq, but each item in the array (0-freq-1) is initialized with null! So on the last line you call null.set(s) --> NPE
So you have to create NBit objects first.

[maxspyderweb]

thank you but how can I inizialize an array of objects
I have instancied the object first and then I've overwritten all the null with the method set(String) but yeah semidec[i] is null ok I've added a constructor in the class NBit

	public NBit(){

		nBit.set(0, len, false);
	}

but there are no call like new NBit() so it doesn't work.. I dunno what do to..

[Tolls]

Before doing this:

semidec[j].set(s);

You want to create a new NBit() and assign that to semidec[i].

[maxspyderweb]

ok done, I've done a for cycle that creates for every NBit[] xxx = new NBit();

I think this is not an efficient method, I would like to create a Type that rappresent an n number of bits (nbits) I've created an object any suggestions?

[Tolls]

Since you're looping around the array anyway why is it inefficient?