Strange Problems with Strings in Java

I am new to JAVA and found some of its concepts very irritating and no matter how hard I try I can not find suitable explanation for this behavior...of course there are wor around for these problems but still I want to know am I missing something very simple here or JAVA is like this???

1. I have a string array in one of my class A and I want it to be filled through a method of another class B...so I create an object of class B into A and call the method B.xyz and equate it to the string arra but BOOM I can't do it....java throws a nullpointer exception..........I dont know why...

.

public class B{
    public void xyz() {
         String[] mystrings=new String[70];
         for(int i=0;i<5;i++)
             mystrings[i]=value;    
         return mystrings;
    }
}

public class A {
   public void abc() {

      B b=new B();
      String[] StringList;
      StringList=b.xyz();
      System.out.println(StringList.length);
   }
}

I have a similar code fragment now sadly the length of the StrinList becomes 70....if I want to print all the strings of this array I dont have any other way....remember even though the size of mystring is 70 in class B only 5 of its components are properly initialized........SO considering I am in class A and have no way to find out how many times did the for loop in B executed......how do I a开发者_C百科ccurately loop through all the elements of StringList in A.........

PS: There are workarounds to solve this problem but I wanted to know why this happens,i.e, why the length attribute doesn't change according to the components initialized??

If you only need an array of length 5 then only initialize it as that size, e.g.:

public String[] xyz(String value) {
    String[] mystrings = new String[5];
    for (int i = 0; i < mystrings.length; i++) {
        mystrings[i] = value;
    }
    return mystrings;
}

If you want an array that you can expand you should consider using ArrayList instead. E.g.:

public List<String> abc(String value) {
    List<String> list = new ArrayList<String>();
    for (int i = 0; i < 5; i++) {
        list.add(value);
    }
    return list;
}

Then you can get its size, add to it and print the elements like this:

List<String> list = abc("foo");
System.out.println(list.size());
list.add("bar");
for (String value : list) {
    System.out.println(value);
}

Hope that helps.

You declared xyz as a method with return type void in class B. Presumably you want a signature that returns a string array, public String[] xyz()

Also you didn't declare the array correctly in B, the correct declaration is:

String[] myStrings = new String[70];

-- Dan

 String[] mystrings = new String[5]; 

I suggest you look at using List like ArrayList as this wraps arrays to make them easier to use.

String[] mystrings[70];

This creates an array or arrays. There are two []

I suggest you try instead.

String[] mystrings = new String[5];