Re: Statements before super()

Tom Anderson <>
Wed, 10 Feb 2010 21:57:15 +0000
On Wed, 10 Feb 2010, Mike Schilling wrote:

Tom Anderson wrote:

new SeparatedString("foo=bar", '=')

is equivalent to:

new StringPair("foo", "bar)
class SeparatedString {
 public SeparatedString(String str, char separator) {
 // now what?

   this(split(str, separator));
private String[] split(String str, char separator)

private SeparatedString(String[] strings)
   super(strings[0], strings[1]);

I refuse to be concerned about the cost of the allocation of the String

Okay. So now:

/** A string pair combining a search term, and the string resulting from
using the term as the parameter to the supplied query.
class TermAndResult extends StringPair {
  public KeyAndValueString(String term, PreparedStatement query) {
  // and no, it's not Derby or H2

Believe me, i can keep contriving examples all week!

What would, I think, solve this kind of problem neatly would be to let a
constructor delegate object construction to a static factory method, hiding
the use of the factory from the caller.

What would solve the problem is allowing statements before the super call
that don't involve 'this'. This is simple, backwards compatible, and would
be *perfectly safe*!

On compatibility - it's obviously (i think) true that any code valid now
would be valid when the rules were relaxed. Could you change the language
rules without changing the VM spec? The only bit i could find in the VM
spec which bears on what a constructor can do is in 4.8.2 Structural

  * When any instance method is invoked or when any instance variable is
  accessed, the class instance that contains the instance method or
  instance variable must already be initialized.

  * There must never be an uninitialized class instance on the operand
  stack or in a local variable when any backwards branch is taken. There
  must never be an uninitialized class instance in a local variable in code
  protected by an exception handler. However, an uninitialized class
  instance may be on the operand stack in code protected by an exception
  handler. When an exception is thrown, the contents of the operand stack
  are discarded.

  * Each instance initialization method (3.9), except for the instance
  initialization method derived from the constructor of class Object, must
  call either another instance initialization method of this or an instance
  initialization method of its direct superclass super before its instance
  members are accessed. However, instance fields of this that are declared
  in the current class may be assigned before calling any instance
  initialization method.

The most painful thing there is the ban on backward branches; that rules
out loops before the super invocation. The ban on exception handlers could
also be awkward. I don't understand the rationale behind those particular
rules, so i can't really suggest how they could be relaxed.


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evenly distributed. he was talking specifically about finsbury park. --

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