Question

I have 2 pieces of code from Java:

class Test1 {
  index = 1;
  int index;
}

versus

class Test2 {
  { index = 1; }
  int index; 
}

the first one failed when compiling, I guess it’s because the member “index” is used before it is defined? The second one is error-free. I don’t understand why. Can you guys spare some minutes and help me? Thanks a lot!

Answer 1

The second one compiles, because that { … } inner construction present in the second code is a so-called initializer block. This block is copied into each constructor by Java compiler.

And, as any other class method, it can use a property defined in that class; it shouldn’t matter where exactly this property is defined – above or before this block.

The first one does not compile, because it simply does not fit into the language specification for class bodies and member declarations, which defines basically seven allowed things:

1. InstanceInitializer (the construct used here)
2. StaticInitializer (similar to first, but executed on class loading)
3. ConstructorDeclaration
4. FieldDeclaration
5. MethodDeclaration
6. ClassDeclaration
7. InterfaceDeclaration