Is there a difference between defining class attributes and initializing them? Are there cases where you want to do one over the other?
Example:
The following code snippets should point out the difference that I mean. I'm using a primitive and an object there:
import Java.util.Random;
public class Something extends Activity {
int integer;
Random random = null;
Something(){
integer = 0;
random = new Random();
....
vs.
import Java.util.Random;
public class Something extends Activity {
int integer = null;
Random random;
Something(){
integer = 0;
random = new Random();
....
Firstly you cannot set a primitive to be null as a primitive is just data where
null
is an object reference. If you tried to compileint i = null
you would get a incompatible types error.Secondly initializing the variables to
null
or0
when declaring them in the class is redundant as in Java, primitives default to0
(orfalse
) and object references default tonull
. This is not the case for local variables however, if you tried the below you would get an initialization error at compile timeExplicitly initializing them to a default value of
0
orfalse
ornull
is pointless but you might want to set them to another default value then you can create a constructor that has the default values for exampleIn Java, initializing is defined explicitly in the language specification. For fields and array components, when items are created, they are automatically set to the following default values by the system:
numbers: 0 or 0.0
booleans: false
object references: null
This means that explicitly setting fields to 0, false, or null (as the case may be) is unnecessary and redundant.
Shanku and Morpheus answered the question correctly. First, you will get a compile error setting your primitive int variable "integer" to null; you can only do that for Objects. Second, Shanku is right that Java assigns default values to instance variables, which are "integer" and "random" in your example code; instance variables are viewable within the class or beyond depending on the scope (public, private, protected, package).
However, default values are not assigned for local variables. For instance, if you assigned a variable in your constructor like "int height;" then it will not be initialized to zero.
I would recommend reading the Java variable documentation, which describe the variables very well and furthermore you could also look over the Java tutorials, which again are great reading material.