Initialize final variable before constructor in Ja

问题:

Is there a solution to use a final variable in a Java constructor? The problem is that if I initialize a final field like:

private final String name = "a name";

then I cannot use it in the constructor. Java first runs the constructor and then the fields. Is there a solution that allows me to access the final field in the constructor?

回答1:

I do not really understand your question. That

public class Test3 {
    private final String test = "test123";

    public Test3() {
        System.out.println("Test = "+test);
    }

    public static void main(String[] args) {
        Test3 t = new Test3();
    }
}

executes as follows:

$ javac Test3.java && java Test3
Test = test123

回答2:

Do the initialization in the constructor, e.g.,

private final String name;
private YourObj() {
    name = "a name";
}

Of course, if you actually know the value at variable declaration time, it makes more sense to make it a constant, e.g.,

private static final String NAME = "a name";

回答3:

We're getting away from the question.

Yes, you can use a private final variable. For example:

public class Account {
    private final String accountNumber;
    private final String routingNumber;

    public Account(String accountNumber, String routingNumber) {
        this.accountNumber = accountNumber;
        this.routingNumber = routingNumber;
    }
}

What this means is that the Account class has a dependency on the two Strings, account and routing numbers. The values of these class attributes MUST be set when the Account class is constructed, and these number cannot be changed without creating a new class.

The 'final' modifier here makes the attributes immutable.

回答4:

Marking it static, will allow you to use it in the constructor, but since you made it final, it can not be changed.

private static final String name = "a_name";

is is possible to use a static init block as well.

private static final String name;

static { name = "a_name"; }

If you are trying to modify the value in the constructor, then you can't assign a default value or you have to make it not final.

private String name = "a_name";
Foo( String name )
{
    this.name = name;
}

or

private final String name;

Foo( String name )
{
    if( s == null )
       this.name = "a_name";
    else
       this.name = name;
}

回答5:

In this case, you can mark the field as 'static' also.

回答6:

Another possiblity is to initialize the field in an instance initializer blocK:

public class Foo {
        final String bar;

        {
                System.out.println("initializing bar");
                bar = "created at " + System.currentTimeMillis();
        }

        public Foo() {
                System.out.println("in constructor. bar=" + bar);

        }

        public static void main(String[] args) {
                new Foo();
        }
}

回答7:

In that case, you might as well make it static, too. And Java convention is to name such constants in ALL_CAPS.

回答8:

private static final String name = getName();

where getName() is a static function that gets you the name.

回答9:

I cannot use it in the constructor, while java first runs the constructor an then the fields...

This is not correct, fields are evaluated first, otherwise you couldn't access any default values of members in your constructors, since they would not be initialized. This does work:

public class A {
    protected int member = 1;
    public A() {
        System.out.println(member);
    }
}

The keyword final merely marks the member constant, it is treated as any other member otherwise.

EDIT: Are you trying to set the value in the constructor? That wouldn't work, since the member is immutable if defined as final.