使用普通旧方法从 java 中的 Arraylist 中删除 null 值

Question

I am trying to remove null and empty value from a Arraylist, but the following code does not work, when i tried to use if(null,=a) if throwing the Nullpointer Exception, but other way works then if(a==null). Program 1 gives error but program 2 works.

Can Someone Please explain the reason

public class RemoveNull {
public static void main(String[] args) {




List<String> arr=new ArrayList<String>();
arr.add("a");
arr.add("a");
arr.add("a");
arr.add("a");
arr.add(null);
arr.add("a");
arr.add("a");
arr.add(null);

List<String> arr1=new ArrayList<String>();
System.out.println(arr);
for(String a:arr)

{
    System.out.println(a);
    if(null!=a || !a.equals(""))
    {
        arr1.add(a);
        //arr.remove(a);
    }
    System.out.println(arr1);
}

System.out.println(arr);

}

results

Exception in thread "main" java.lang.NullPointerException at RemoveNull.main(RemoveNull.java:26)

public class RemoveNull {

    public static void main(String[] args) {


        ArrayList<String> arr=new ArrayList<String>();
        arr.add("a");
        arr.add("a");
        arr.add("a");
        arr.add("a");
        arr.add(null);
        arr.add("a");
        arr.add("a");
        arr.add("");
        arr.add(null);
        arr.add("");
        List<String> arr1=new ArrayList<String>();
        System.out.println(arr);
        for(String a:arr)

        {
            System.out.println(a);
            if(null==a || a.equals(""))
            {

            }
            else
            {
                arr1.add(a);
                //arr.remove(a);
            }
            System.out.println(arr1);
        }
        System.out.println(arr);

    }

}

results

[a, a, a, a, null, a, a, null, null] [a, a, a, a, a, a]

Answer 1

Your problem is here:

if(null!=a || !a.equals(""))

You should write

if(null!=a && !a.equals(""))

Answer 2

You should be using a ListIterator if you want to also possibly remove elements while iterating your list:

ListIterator<String> its = arr.listIterator();
while (itr.hasNext()) {
    String next = itr.next();
    if (next == null || "".equals(next)) {
        itr.remove();
    }
}

Note that you don't need to make a separate updated copy of your list. Instead, you may just iterate the one you already have and remove the null/empty elements.