Checking for null element in an Array

Question

I want to check whether an Array element is null.

I have initialized an array of String which has a size of 2. I looped through the array and check whether an array element is null. If it's null, I will add a String "a" to that position only.

My codes below will produce the following output:

1=a
2=a

Code:

public class CheckArrayElementIsNull {
    public static void main(String[] args) {
        String[] arr = new String[2];       
        for(int i = 0; i < arr.length; i++) {
            if(arr[i] == null) {
                arr[i] = "a";
            }
            System.out.println(i + "=" + arr[i]);   
            if(arr[i] == null) {
                System.out.println(i + "= null");
            }
        }
    }
}

I tried to add a break after my if condition but is not printing out anything.

 public class CheckArrayElementIsNull {
    public static void main(String[] args) {
        String[] arr = new String[2];       
        for(int i = 0; i < arr.length; i++) {
            if(arr[i] == null) {
                arr[i] = "a"; 
                break;
            }
            System.out.println(i + "=" + arr[i]);   
            if(arr[i] == null) {
                System.out.println(i + "= null");
            }
        }
    }
}

I expected this output:

1=a
2=null

You should check and print the null value before you assign to it, otherwise that element won't be null when you check it the second time. — shree.pat18
– shree.pat18, Commented Aug 26, 2014 at 8:50

Dragondraikk · Accepted Answer · 2014-08-26 08:54:56Z

2

You have a few problems in your loop.

    for(int i = 1; i < arr.length - 1; i++) { //<---- This does not iterate over the entire array, leaving out the first and last elements
        if(arr[i] == null) {
            arr[i] = "a"; 
            break; //<---- This terminates the loop entirely, if you want to stop all instructions past this one try using continue instead
        }
        System.out.println(i + "=" + arr[i]);   
        if(arr[i] == null) { //This code is unreachable as arr[i] is initialized if it was detected as null before
            System.out.println(i + "= null");
        }else{
            System.out.println(i + "=" + arr[i]); 
        }
    }

Instead you should try

 for(int i = 0; i < arr.length; i++) {
    if(arr[i] == null) {
        arr[i] = "a";
        System.out.println(i + "= null");
        break;
    }
    System.out.println(i + "=" + arr[i]); 
}

edited Aug 26, 2014 at 8:54

answered Aug 26, 2014 at 8:49

Dragondraikk

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6 Comments

Stefan Over a year ago

That will still assign all values in the array to a.

Dragondraikk Over a year ago

As far as I understand the question that is the desired effect: setting all null-values to "a" instead

mlethys Over a year ago

@Stefan not all values, only those with null

tz reur Over a year ago

setting the first null value found to a in the array.

Dragondraikk Over a year ago

@tz reur Oh, only the first? Fixed that :)

|

Mustafa sabir · Accepted Answer · 2014-08-26 09:19:02Z

0

It is because break will break the iteration of the for loop, and exit out of loop immediately. You should change your design a bit. From what I understand you are trying to achieve (Assigning "a" to the first null element encountered and stop, print all elements in the array) following should work:-

public class CheckArrayElementIsNull {

  public static void main(String[] args) {
     String[] arr = new String[3];       
     boolean flag=true; //use a flag that will check if null is encountered
     for(int i = 0; i < arr.length; i++) {
       if(arr[i] == null && flag) { // first time flag will be true
         arr[i] = "a"; 
         flag=false; //set flag to false , to avoid further assignment of 'a' to null values
        }
        System.out.println(i + "="+ arr[i]);    //print array
        }
     }
  }

edited Aug 26, 2014 at 9:19

answered Aug 26, 2014 at 8:56

Mustafa sabir

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2 Comments

Dragondraikk Over a year ago

Wouldn'T this cause a NullPointerException in System.out.println if the array contains more than one null element?

Mustafa sabir Over a year ago

@Dragondraikk No it wont , it will simply print null, this is because i am passing an object of String type (which has value null) , so the compiler knows which toString() method to call. NPE would take place when I would write System.out.println(null), because in this case compiler cannot deduce which toString() method to call.

Stefan · Accepted Answer · 2014-08-26 08:54:28Z

0

Try assigning the value at the index you expect not to be null before you enter your for loop:

String[] arr = new String[3];
arr[1] = "a";
for (int i = 0; i < arr.length; i++)
{
  if (arr[i] == null)
  {
    System.out.println(i + "=null");
  }
  else
  {
    System.out.println(i + "=" + arr[i]);
  }
}

answered Aug 26, 2014 at 8:54

Stefan

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