Android - string.equals(null) returns null pointer even when string was initialized

Question

I am doing this database operation in my application where I pull a certain column value to check if it exists. I have no problems executing the query and assigning the value to a string (I can even output this value in the Log). My problem arises when I try to check if my string is equal to null, ironically, I'm getting a NullPointer.

Here is my function:

public boolean isDone(String id){
    boolean isDone = false;
    String timeOut = new String();
    timeOut = "A";

    String query = "SELECT TimeOut FROM " + TABLE_ROUTE_TABLE + " WHERE _id = '" + id + "'";
    Cursor cursor = mDB.rawQuery(query, null);

    if(cursor.moveToFirst()){
        timeOut = cursor.getString(cursor.getColumnIndex("TimeOut"));
        Log.d("Hi", "timeOut = " + timeOut);
    }

    if(timeOut.equals(null)){
        isDone = false;
    }
    else{
        isDone = true;
    }

    return isDone;
}

I am getting the NPE at if(timeOut.equals(null)). I don't understand why I am getting an NPE when I put the string as part of a boolean condition when I am able to use it and display its value (which may sometimes be null).

I have a feeling I'm overlooking something basic. Any ideas?

why dont you just String timeOut=null; ........ if(cursor..) timeOut=something; and check for if(timeOut==null) { } ? technically, creating an empty string to intialize it would be overhead anyway, since the object would not be used but created, as far as I know — damian
– damian, Commented Sep 3, 2013 at 13:45

Mauren · Accepted Answer · 2013-09-03 13:46:06Z

2

If your timeOut variable is null, this test will always cause a NPE, because null instances have no methods. You should be testing with == operator, e.g. timeOut == null.

Why it is null, however, I can't understand, since you say it has value when printed.

answered Sep 3, 2013 at 13:46

Mauren

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1 Comment

Razgriz Over a year ago

Thanks. timeOut == null worked. Will accept this as the answer since this got in first.

Wakoo · Accepted Answer · 2013-09-03 13:48:45Z

1

The issue is the equals compare 2 string together, since one of them is null, you get the NPE.

You can fix it by just testing

timeOut == null

answered Sep 3, 2013 at 13:48

Wakoo

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Comments

Piyush · Accepted Answer · 2013-09-03 13:49:29Z

1

Yes... null instance have no methods..

SO you should initialize your variable timeOut==null;

answered Sep 3, 2013 at 13:49

Piyush

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Andres · Accepted Answer · 2013-09-03 13:51:26Z

1

The null checking should be done on this way:

 if(timeout == null)

Because you want to know if the object even exists, that means, if the pointer is null or not. The content of a string cannot be null but the pointer to it can be null.

answered Sep 3, 2013 at 13:51

Andres

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