In what order are initializer block and variable definitions and etc. executed? (in java)

问题

I have problem understanding the order in which initialization happens. this is the order I assumed:

*Once per 
    1. Static variable declaration
    2. Static block
*Once per object
    3. variable declaration
    4. initialization block
    5. constructor

but according to this code I am obviously wrong:

    class SomethingWrongWithMe
    {
        {
            b=0;         //no. no error here.
            int a = b;   //Error: Cannot reference a field before it is defined.
        }
        int b = 0;
    }

And the error would disappear if I do this:

    class SomethingWrongWithMe
    {
        int b = 0;
        {
            b=0;
            int a = b;   //The error is gone.
        }
    }

I can't figure out why isn't there an error on

    b=0;

回答1:

The Java Language Specification (section 8.3.2.3) says you can use a variable on the left hand side of an expression, i.e. assign to it, before it is declared, but you cannot use it on the right hand side.

All variables are initialized to their default values, then explicit initializers and anonymous blocks are run in the order they are found in the source file. Finally the constructor is called.

Statics are only run once on the first use of a class.

The compile error appears to be a rule of Java rather than something that necessarily makes sense in every case.

回答2:

Variable definitions are not done "before" blocks. They are both done at the same time, in the order that they are defined

class SomethingWrongWithMe {
    {
        b = debug("block 1");
    }
    int b = debug("define");
    {
        b = debug("block 2");
    }
    private int debug(String str) {
        System.out.println(str);
        return 0;
    }
}

Output

block 1
define
block 2

回答3:

First of all, your assumptions are more or less correct, except for the fact that declarations (with initialization, such as int b = 0) and instance initializer blocks are executed in the order they are written.

int b = 0;   // executed first

{
    b = 1;   // executed second
}

int a = b;   // executed third

Also note that the declaration i.e. int b is not executed. The declaration just declares the existence of the variable.

As for the error you got (or, rather the error you didn't get) I agree that it looks strange. I assume that the compiler deals with referencing a variable in an expression and assigning a value to it in different ways. When writing to a variable in an instance initializer, it just checks that the variable is there, while when reading from it, it requires it to be declared above the instance initializer block. I'll see if I can find a reference for that in the JLS.

来源：https://stackoverflow.com/questions/9665297/in-what-order-are-initializer-block-and-variable-definitions-and-etc-executed

标签

java

block

initializer