Java是否为类的字段分配内存，即使它们尚未初始化？

Question

I'm trying to perform a small memory optimisation for my Java game. It is a bit unclear to me how does Java allocate memory when it comes to fields:

public class Test {
    private HashMap<String, String> info;

    public Test(boolean createInfo) {
        if (createInfo) {
            info = new HashMap<String, String>();
        }
    }

}

As you can observe, the HashMap info is initialised if you pass true to Test 's constructor.

Does new Test(true) take up more memory than new Test(false) ?

Which leads to the more general question:

When you create a field in a class, does Java "reserve" the necessary memory for such field in case it is initialised, or will it do nothing until you actually initialise it?

There is this question: Is memory allocated for unused fields in Java? which seems to be almost exactly what I am looking for, but they seem to be asking what already instantiated fields that are unused, whereas I am asking for uninstantiated fields that may or may not be used.

Answer 1

Does new Test(true) take up more memory than new Test(false) ?

Yes.

When you create a field in a class, does Java "reserve" the necessary memory for such field in case it is initialised, or will it do nothing until you actually initialise it?

There are two things here - the HashMap reference, and the HashMap object. Both of them require memory. When a class instance is created, memory is allocated for all its instance variable. In case of int type field, it will allocate 4 bytes, similarly for a reference, it will allocate some memory for that reference (I don't know exactly how much it is). But surely it will allocate memory for the fields.

Then when you initialize the fields in the constructor, the actual HashMap object is created, which will again take up memory to store the object.

Answer 2

The field itself has space for a reference to an object, and that space is allocated when the containing class ( Test ) is instantiated, even if the value placed in that field is null . Your example code doesn't create an extra object whose reference will go in that field when you pass in false .