Initialize final variable before constructor in Java

Question

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?

I assume that you want to change the value on the constructor right ? — webclimber, Mar 24 '09 at 14:25
I'm voting to close because, from my understanding, it turns out this is not really a problem at all. What the OP is trying to do should work. — Tim Frey, Mar 24 '09 at 15:02
The constructor implicitly executes the field initialisation immediately before calling the super constructor. — Tom Hawtin - tackline, Mar 24 '09 at 15:05
@Tom, actually, all field initialization occurs after the superclass constructor call. Try looking at the bytecode and see. Under the JVM specification, it is permissible to initialize the fields before the ctor call, but Javac never does so. — Antimony, Sep 10 '12 at 21:25
@Antimony Oops, yes. Actually the outer this of an inner class is available during the call to super (`-target 1.4` or later). — Tom Hawtin - tackline, Sep 11 '12 at 00:52

score 17 · Accepted Answer · answered Mar 24 '09 at 14:20

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

Hank Gay · Answer 2 · 2009-03-24T14:26:59.063

16

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";

edited Mar 24 '09 at 14:26

answered Mar 24 '09 at 14:18

Hank Gay

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score 6 · Answer 3 · edited Apr 28 '15 at 08:05

6

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.

edited Apr 28 '15 at 08:05

Eun

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answered Sep 09 '11 at 13:59

Steve Gelman

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Basically, you get one shot at setting the values in the constructor. After the constructor completes, the attributes are constant. – Steve Gelman Sep 09 '11 at 14:02
2

I get an error "cannot assign a value to final variable value". – Martin Konicek Jun 27 '12 at 15:10

sfossen · Answer 4 · 2009-03-24T15:04:36.633

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;
}

score 2 · Answer 5 · answered Mar 24 '09 at 14:18

2

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

answered Mar 24 '09 at 14:18

Tim Frey

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a way without a static field? – Tobias Mar 24 '09 at 14:23
@Tobiask: Why don't you want a static field? – sfossen Mar 24 '09 at 14:25
It's immutable, so you might as well make it static. – Adam Jaskiewicz Mar 24 '09 at 14:25
1

It's immutable. There's no shared mutable state, thus no concurrency issues. You'd be right if it didn't have that final on it. – Adam Jaskiewicz Mar 24 '09 at 18:17

score 2 · Answer 6 · answered Mar 24 '09 at 14:55

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();
        }
}

score 1 · Answer 7 · answered Mar 24 '09 at 14:18

1

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

answered Mar 24 '09 at 14:18

Adam Jaskiewicz

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score 1 · Answer 8 · answered Mar 24 '09 at 14:21

1

private static final String name = getName();

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

answered Mar 24 '09 at 14:21

daanish.rumani

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Beware of this construct if getName() does anything other than return a constant. You can find the logic used by getName() may not have been initialized. – DJClayworth Mar 24 '09 at 15:56

score 0 · Answer 9 · answered Mar 24 '09 at 14:51

0

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.

answered Mar 24 '09 at 14:51

soulmerge

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Technically it is the constructor that is (implicitly) executing the field initialisation logic. – Tom Hawtin - tackline Mar 24 '09 at 15:06
Well, ok. The fields get evaluated first nonetheless. – soulmerge Mar 24 '09 at 15:34

Initialize final variable before constructor in Java

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