Question

1. Assign value to field at the same time with field declaration
2. Assign value to field in the constructor?

Answer 1

What’s the difference between those two ways to initialize fields in Java?

Not much! The main difference is that if you assign it at the point of declaration, it will apply to all constructors. That is, there’s no way you can forget to add the initialization to any future constructors.

Have a look at the official Initializing Fields trail.

May be worth noting that if you initialize the fields at the declaration, it happens before the constructor starts executing.

As a rule of thumb, I usually initialize the “dead simple” fields at the declaration (when there is no doubt what the initial value should be). This “cleans up” For instance

class SomeClass {

    List<Integer> currentNumbers = new ArrayList<Integer>();
    int counter = 0;

    // ...
}

while as if there is the slightest chance that I may want different behaviors in different constructors, I leave it to the constructor.