A typical scenario when creating a new class is to declare and initialize some fields:
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public class Values { private int value1 = 1; private int value2 = 2; } |
I realize that initial values of these fields are correlated and I update the code:
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public class CorrelatedValues { private int value1 = 1; private int value2 = value1 + 1; } |
If I’m not careful, I can even try:
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<br />public class ForwardReferences { private int value1 = value2 + 1; private int value2 = 1; } |
but this won’t compile because it is a forward reference during field initialization:
- The declaration of an instance variable in a class or interface C appears textually after a use of the instance variable;
- The use is a simple name in either an instance variable initializer of C or an instance initializer of C;
- The use is not on the left hand side of an assignment;
- C is the innermost class or interface enclosing the use.
It is easy to understand even without this documentation because it’s commons sense. I need to declare a variable before I can use it. This was clear for me until I saw this code:
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public class DeclarationAndInitializationGoCrazy { private int value = this.value + 1; public static void main(String... args) { System.out.println(new DeclarationAndInitializationGoCrazy().value); } } |
Surprisingly this code compiles and displays:
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1 |
The question is why is this working? I would expect a compile time error. After reading more careful the rules about forward reference during field initialization, the catch is on the second line:
- The use is a simple name in either an instance variable initializer of C or an instance initializer of C;
In this example I’m using this.value and not the simple name value and that’s why it works. What really happens when creating a new instance of DeclarationAndInitializationGoCrazy is:
- Field
valueis set to default value 0 - Field
valueis initialized with 0 + 1 which is 1
I can go really crazy and try something like:
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public class DeclarationAndInitializationReallyGoCrazy { private int value1 = this.value2 + 1; private int value2 = this.value1 + 1; public static void main(String... args) { System.out.println(new DeclarationAndInitializationReallyGoCrazy().value1); System.out.println(new DeclarationAndInitializationReallyGoCrazy().value2); } } |
that compiles and displays:
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1 2 |
It is the same explanation as on previous example:
- Fields
value1andvalue2are set to default value 0 - Field
value1is initialized with 0 + 1 which is 1 - Field
value2is initialized with 1 + 1 which is 2
I don’t see now any practical reason why I would do this in a real project code, at least it was a fun thing to do.
Image credit: cristofer angello caballero thorne, CC0 Public Domain