Java - Static and Final variables

Question

I'm aware this won't work but it's just an example so I can understand why it won't work.

public class MyClass {

    final static JButton b;

 public void myMethod() {
   b = new JButton("Hello!");
 }
 }

1) Why is there an error for the final static JButton b; line? And why does this error disappear when I remove the static keyword?

2) With or without the static keyword, there is an error for the line b = new JButton("Hello!"); I'm wondering why this is? I thought it would be okay to have a final variable instantiated within a method.

Ted Hopp · Accepted Answer · 2012-05-08 21:14:42Z

7

When you declare a static field to be final, it must be initialized statically, either in a static initializer block or with an initializer expression. You cannot initialize it in a constructor.

When you declare a non-static field to be final, it must be initialized in every constructor, or with an initializer expression, or in an instance initializer block.

answered May 8, 2012 at 21:14

Ted Hopp

235k48 gold badges412 silver badges533 bronze badges

Sign up to request clarification or add additional context in comments.

2 Comments

ametren Over a year ago

Just want to clarify for asker, the reason you can't initialize a static final in the constructor is because accessing static properties doesn't require construction of the object.

yshavit Over a year ago

@ametren Not quite -- because if it's not final, you can set it in a constructor (even though it still doesn't require construction of the object). The reason you can't set it in a constructor is that a static final can only be assigned once, whereas a constructor can be invoked many times.

Edwin Dalorzo · Accepted Answer · 2012-05-08 21:19:56Z

1

Since the field is static final it needs to be initialized with the class itself, the compiler is requesting that you provide a value other than the default one. You can either provide a default initializer or provide a value in a initialization block:

Either

public class MyClass {
  final static JButton b = new JButton("Hello!");
}

or

public class MyClass {
  final static JButton b;

  static{
      b = new JButton("Hello!");
  }
}

are valid.

By the time yourMethod is invoked it would be already too late, because it is expected that at that point your class is already initialized, and you would expect the field in question to have been initialized as well.

answered May 8, 2012 at 21:19

Edwin Dalorzo

78.9k25 gold badges151 silver badges211 bronze badges

Comments

creativeby · Accepted Answer · 2012-05-08 21:25:56Z

Some people should reread what static and final truely is..

A static variable is global for every instance of its class. If you have a class Person, with an "int static age = 0" in it and you say age++; in your constructor, age will have in every instance the same value! Means 3x new Person() expects age to be 3, in EVERY Person instance! michael.age == 3 and susan.age == 3 and tom.age == 3

Therefor you need to reserve some space for that variable and have to initialize AND declare it! Because this means "you have ONE variable for all of your instances of a class"...

next, a final variable has to be filled with data on its declaration, because you MAY NOT! edit that variable later! Like a const (constant)

so there is last static final. As you might have learned, this variable is for every instance of a class, initialized on its declaration and is is not changeable overall! Which explains the class comprehensive "const" (constant) variable for java!

There you go!

Joop Eggen · Accepted Answer · 2012-05-08 22:39:10Z

1

With final assignment is only allowed immediately or in the constructor. So only the following is allowed.

class A {
    static final B b = new B();
    static final C c;
    static { // Static initializer block.
         c = new C();
    }
    final B b2 = new B();
    final C c2;
    { // Initializer block.
         c2 = new C();
    }
    final D d2;
    A() {
        d2 = new D();
    }
}

edited May 8, 2012 at 22:39

answered May 8, 2012 at 21:18

Joop Eggen

110k8 gold badges89 silver badges142 bronze badges

4 Comments

jbindel Over a year ago

You could also initialize a non-static final field in an instance initializer block.

Joop Eggen Over a year ago

@Fly didn't I do that for c2? Maybe my "anonymous constructor" misled you.

jbindel Over a year ago

Doh, I didn't see that somehow. It could be the use of the word "constructor" led me astray.

Ted Hopp Over a year ago

What you labeled an "anonymous constructor" is actually an instance initializer block. An anonymous constructor is something completely different. (See Java Language Specification, §15.9.5.1.) In particular, you can have both a constructor and an instance initializer block. As described in JLS §12.5, the instance initializer block executes shortly after the constructor starts executing.

Mik378 · Accepted Answer · 2012-05-08 21:15:48Z

0

A static final variable must be initialized directly (near to the variable) or by using a static block.

A final variable (non-static) must be declared either directly (near to the variable) or by using an initialization block, or in the object constructor.

answered May 8, 2012 at 21:15

Mik378

22.2k15 gold badges94 silver badges191 bronze badges

Collectives™ on Stack Overflow

Java - Static and Final variables

5 Answers 5

2 Comments

Comments

Comments

4 Comments

Comments

Your Answer

Hot Network Questions

Collectives™ on Stack Overflow

5 Answers 5

2 Comments

Comments

Comments

4 Comments

Comments

Your Answer

Sign up or log in

Post as a guest

Related