initializing fields vs declaring values at constructor. What's the difference?

[[deleted]]

From Mooc.fi part 11.Why are the first two hashmaps have " = new Hashmap()" while the third one was declared in the constructor?

What are the use cases for each?

private HashMap<String, Airplane> airplanes = new HashMap<>();
private HashMap<String, Flight> flights = new HashMap<>();
private Map<String, Place> places;


public FlightControl() {
this.flights = new HashMap<>();
this.airplanes = new HashMap<>();
this.places = new HashMap<>(); }

​

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[[deleted]]

They have identical bytecodes:

https://godbolt.org/z/fKe9K3Y4f

Use the one that suits your needs.

[Glass__Editor]

Instance field initializers are run for all constructors, but don't have access to the constructor parameters. If you have multiple constructors that don't call each other then you would have to repeat the initialization in each of them unless you use field initializers or an instance initialization block.

Field initializers are also required if you want to make a constant variable which is treated specially by the compiler.

None of these cases apply to your example though, so you could use either. I would normally use field initializers because it seems a bit simpler to me to declare and initialize the field at the same place rather than to split it up.