If statement with return in JML

i need to set up a postcondition which ensures to return null if size_ is 0. Based on

 if(size_ == 0)
  return null;

how can i do that in jml? any ideas? Following doesn't work:

//@ e开发者_开发问答nsures size_ == null ==> \return true;

thanks in advance

Try

//@ ensures size_ == null ==> \result == true;

Example:

//@ ensures size_ == null ==> \result == true;
public boolean sizeUndefined() {
    if (size_ == null)
        return true;

    return size_.length() > 0;
}

You could also simply write it like this:

//@ ensures size_ == null ==> \result;

Here is the documentation for \result:

3.2.14 \result
Within a normal postcondition or a modification target of a non-void method, the special identifier \result is a specification expression whose type is the return type of the method. It denotes the value returned by the method. \result is allowed only within an ensures, also_ensures, modifies, or also_modifies pragma that modifies the declaration of a non-void method.

First of all: what type is size_, Object or primitive(int)?

Secondly, what the return type of the method? Object or primitive(boolean)?

You cannot compare a primitive type to null, or return null where a primitive type is supposed to be returned. If we assume size_ is int and the return is boolean then the the post-condition would be

//@ ensures size_ == 0 ==> \result;