mapAB challenge options

I passed all tests

Good!

Looking for ways to improve my code

Please post your code so we can look at it.  

and also avoid Null pointer exceptions

You can avoid NullPointerExceptions by checking for null before using a variable.  For example:

import java.util.*;
public class T {
    public static void main(String arg[]){
		String myString = null;
		System.out.println(myString);
		if(myString != null)System.out.println(myString.toUpperCase());
		try{System.out.println(myString.toUpperCase());}
		catch(NullPointerException npe){
			System.out.println("NPE was caught");
		}
		myString = "hello!";
		System.out.println(myString.toUpperCase());
    }
}

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the output is

null
NPE was caught
HELLO!

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what is difference between below 3 lines

The first two lines do the same thing. If you were going to use them a million times, then we could talk about performance.  Maybe one is quicker than the other.
The last one

map.put("ab", "null" + map.get("b"));

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is confusing to me. Why would you want to concatenate a String that contains the word null?  Do you see the difference between

String myString = null;

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and

String myString = "null";

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?
Concatenating null doesn't make sense either.  Maybe, you thinking of when null is returned from one of the get methods  

map.put("ab", map.get("a")+ map.get("b"));

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  the key  ab could have a value something like "nullxxxxx"

gudii9, said that he would close some of his open questions. See
https://www.experts-exchange.com/questions/29049217/mapAB2-errors.html     
I count over 30 open questions at this time.

closing

You didn't post your code for this challenge.

oops here you go. i pass all tests

public Map<String, String> mapAB(Map<String, String> map) {
  if(map.containsKey("a")&&map.containsKey("b")){
    map.put("ab",map.get("a")+map.get("b"));
  }
  return map;
}

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I counted 38 questions open at this time.
Why do you do this?

closing