Initialization in definition vs. initialization in

问题:

This question already has an answer here:

• What are the best practices for determining the tasks of Constructor, Initialization and Reset methods 5 answers

In Java, but in other OO languages as well, is there a difference between initializing an attribute in its definition, as in

class Example {
    public Sample sample_attribute = new Sample();
}

and using a constructor to initialize it?

class Example {
    public Sample sample_attribute;

    public Example() {
        sample_attribute = new Sample();
    }
}

I could not think of any practical difference, is there one? Otherwise, are there cases in which one method is better than the other, even if they have the same result?

回答1:

The initialization order is matter here.

1. Set fields to default initial values (0, false, null)
2. Call the constructor for the object (but don't execute the body of the constructor yet)
3. Invoke the constructor of the superclass
4. Initialize fields using initializers and initialization blocks
5. Execute the body of the constructor

So, first case will be initialize the variable sample_attribute in 4th step, second will initialize the variable sample_attribute in 5th step. It's all depends on your requirement.

If you want to access any of the variables from Constructor, you need to use 1st case.

回答2:

When you initialize your fields with information which gets passed into the constructor, you have no other choice but to initialize in the constructor. Otherwise, I prefer initialization on the spot as it saves me lines of code I have to read later.

回答3:

These two versions are equivalent. But if new Sample() threw a checked exception you wouldn't be able to initialize it at field declaration