Why i'm getting a null string when i try to add the splitted string?

I have splited a string and from the recieved string i am trying to form a word everything is comming fine but everytime i get null string in the first index why?

Here is my code:

    String assetClasses = "Gold:Stocks:Fixed Income:Commodity:Interest Rates";
    String[] splits = assetClasses.toString().split(":");

    //System.out.println("splits.size: " + splits.length);
    String ab = null;
for(int i=0; i<splits.length;i++){
    System.out.println(splits[i]);
    ab+=splits[i];
}
System.out.println(ab);

The output:

Gold Stocks Fixed Income Commodity Interest Rates

nullGoldStocksFixed IncomeCommodityInterest Rates