Why is it illegal to assign a class variable that was just declared?

Question

class A {
    public int someVar;
    someVar = 123;      /* cannot find symbol */
}

Why cant the language see the variable I just declared? And is this unique to Java or this is true in all classful OOP languages?

@SotiriosDelimanolis no, false; you could write `public Foo foo = new Foo()`, and the constructor contains arbitrary code. — djechlin, Sep 03 '13 at 20:08
@djechlin That's an instance initialization statement. Have a look [here](http://stackoverflow.com/questions/1994218/instance-variable-initialization-in-java). It's not the same as executable code. — Sotirios Delimanolis, Sep 03 '13 at 20:10
so is this unique to Java or this applies to all OOP languages? — 1der, Sep 03 '13 at 20:20
@1der It's likely on a per-language basis, but note that python doesn't allow you to put code outside the methods at all. You must define them all with self.variable. They may also be defined outside of the constructor for the first time for a class. Every language does OO differently — Cruncher, Sep 03 '13 at 20:31

score 9 · Answer 1 · answered Sep 03 '13 at 20:06

9

In Java, you can declare instance variables inside a class but outside a method, but statements such as someVar = 123; must be within a method (or a constructor, a static initializer, or an instance initializer).

answered Sep 03 '13 at 20:06

rgettman

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+1, but will add that they may also be within an initialization block – Kon Sep 03 '13 at 20:07
1

@Kon Yes, I've added to my answer to make it more complete. – rgettman Sep 03 '13 at 20:09

score 6 · Answer 2 · edited May 23 '17 at 10:31

You can't have arbitrary statements in a class definition. You can either immediately assign the variable in the declaration:

public int someVar = 123;

Or, assign it in a constructor or other instance scope:

public class A {
  public int someVar;

  public A() {
    someVar = 123;
  }
}
//Or...
public class B {
  public int someVar;
  { someVar = 123; }
}

Note that the second technique uses an instance initializer, which is not always the most immediately clear piece of code.

score 2 · Answer 3 · answered Sep 03 '13 at 20:08

You can not declare someVar = 123; statement directly in class.
It should be instance block or in method or in constructor

 class A {
    public int someVar = 123;
}

or

class A {

    public int someVar ;
    {
          somevar = 123;
    }
}

or

class A {

        public int someVar ;
        A(){
              somevar = 123;
        }
    }

or

class A {

        public int someVar ;
        public void method(){
              somevar = 123;
        }
    }

or `public int somevar = 123;` – djechlin Sep 03 '13 at 20:09 — djechlin, Sep 03 '13 at 20:09

Hot Licks · Answer 4 · 2013-09-03T22:02:11.720

1

But, oddly, you can code

public class AFunnyClass {
    public int someVar;
    {
        someVar = 123;
    }
    public static int anotherVar;
    static {
        anotherVar = 456;
    }
    public int yetAnotherVar = 789;
    public static int adNauseum = 101112;
}

edited Sep 03 '13 at 22:02

answered Sep 03 '13 at 20:12

Hot Licks

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Yes, using an instance initializer and a static initializer. – rgettman Sep 03 '13 at 20:13

score 0 · Answer 5 · answered Sep 03 '13 at 20:13

Maybe you want to use someVar as a static variable. Then you should declare it also static. Using it like:

class A {
    static public int someVar = 123; 
}

That means someVar isn't just a member of an instance of this Class, it becomes a member of the class it self. That also means someVar will be instantiated just once and can be used by all instances of this Class

Why is it illegal to assign a class variable that was just declared?

5 Answers5