在 Java 中将 long 分配给 Long

Question

I am currently going through one of the Android Tutorials . I found an expression like:

Bundle bundle = getIntent().getExtras();
Long longVariable = bundle.getLong(someId);
if( longVariable != null )
{
  //doSomething
}

After looking at Bundle.getLong in the API I saw that it returns a long (primitive). Now, I didn't write Java for quite a while, only C# in the meanwhile, but how can the object Long longVariable variable ever be null?

Answer 1

If getLong() really has return type long it can't be, ie l will never be null. Probably the programmer didn't look up the return type, but inferred from the method name a Long would be returned.

Answer 2

What if you pass in the wrong id? Then it is not found, but do you get null, or an exception is raised? Documentation states that:

public long getLong (String key)
Returns the value associated with the given key, 
or 0L if no mapping of the desired type exists for the given key.  
Parameters

     key    a String
Returns
     a long value

So it never returns null, but 0L eventually. However, getLongArray returns null sometimes, so it might be a leftover from using getLongArray?

Answer 3

In Java Long is an object and like any object it can be null. long is a primitive type and cannot be null.

From Java 1.5 on wards Java supports auto-boxing and unboxing meaninng that it's convert between the primitives and objects automatically.

If a method returns long then it's probably safe not checking for null even if you cast it to a Long however if a method returns Long object its a good idea to check for nulls.

Answer 4

Try creating new instance. bundle.getLong(someId) returns the primitive long type not Long . Long l = new Long(bundle.getLong(someId));

And then check if(l != null)

Make sure you have extracted the right long from the Bundle . Try Log ging this Long object see if it is all right.

Log.i("MY LONG: ", String.valueOf(l));