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Java wasn't the first to do that Objective-C (10? years before) had interfaces. Even C++ has that with multiple inheritance - some parents can just be interfac

Java wasn't the first to do that Objective-C (10? years before) had interfaces. Even C++ has that with multiple inheritance - some parents can just be interfaces. As to whether Smalltalk needs interfaces see https://stackoverflow.com/a/7979852/151019 https://stackoverflow.com/a/7979852/151019 and https://www.jot.fm/issues/issue_2002_05/article1/ https://www.jot.fm/issues/issue_2002_05/article1/

lmm 1y ago
Objective-C and Smalltalk were always niche languages, at least by comparison to Java and C#, and I think Smalltalk fans underestimate the value of many things. C++ does not (or at least did not at the time) have a concept of interfaces. There was a pattern in some development communities for defining interfaces by writing classes that followed particular rules, but no first-class support for them in the language.
- kgeist 1y ago
  >no first-class support for them in the language. An interface is just a base class none of whose virtual functions have implementations. C++ has first class support for it. The only thing C++ lacks is the "interface" keyword.
  - gpderetta 1y ago
    The main reason (other than self-documentation) that some other languages separate interfaces form normal classes is that they only support multiple inheritance for interfaces. C++ doesn't have this restriction, so interfaces would add very little.
    - colejohnson66 1y ago
      Because multiple inheritance causes the diamond problem. C# forces you to solve it by declaring one method as the "canonical" and the others as "explicit interface implementations" (only accessible if the variable/receiver is typed as that interface).
      - layer8 1y ago
        The diamond problem strictly speaking only has to be one when the common base class has constructor arguments. While a Java-style interface construct makes it easy to prevent, it also imposes much stronger restrictions than the above. It would have been possible to only impose the above restriction. Yes, there are failure cases with separate compilation, but the way Java dynamically loads classes, that would be similar to how when the JVM loads a class file that is supposed to be an interface, it discovers that instead it has been changed to a class.
- lelanthran 1y ago
  > C++ does not (or at least did not at the time) have a concept of interfaces. There was a pattern in some development communities for defining interfaces by writing classes that followed particular rules, but no first-class support for them in the language. Your distinction between "first class support for interfaces" and "C++ support for interfaces" looks like an artificial one to me. Other than not requiring the keyword "interface", what is it about the C++ way of creating an interface that makes it not "first class support"?
- layer8 1y ago
  Java having an explicit ”interface” construct is one thing I didn’t like about it, because it muddles the notion of a class implicitly having an interface (a notion that clearly exists in C and C++, by way of header files if nothing else) with that construct, while on the other hand there is no a-priori reason to have a distinction between Java’s interfaces and pure abstract classes. Both specify an interface to be implemented. And Java 8+ muddles its concept further by allowing default methods and static members. The important thing is to distinguish between interface and implementation, and that is relevant to any class, whether it implements a separately defined interface or not.
  - pasc1878 1y ago
    Java (following Objective-C) does need a differentiation between interface and pure abstract class - this is because it is single inheritance - a class can have any one class to inherit from but it can have many interfaces.
    - layer8 1y ago
      That doesn’t follow. The restriction could have been defined in terms of allowing at most one parent class to be non-pure-abstract. And there are lesser restrictions hat would have been conceivable as well. Java is single-inheritance only in the sense of the particular interface–class distinction it makes. For example, since Java 8 you can inherit method implementations from multiple interfaces.
- pasc1878 1y ago
  Java's object model is based on Objective-C's so a direct descendant and Objective-C's object model is based on Smalltalk so there is a direct connection there.
igouy 1y ago
Also: 2003 Traits: Composable Units of Behaviour https://www.cs.cmu.edu/~aldrich/courses/819/Scha03aTraits.pdf https://www.cs.cmu.edu/~aldrich/courses/819/Scha03aTraits.pd... " Traits as described in this paper are implemented in Squeak [22], an open-source dialect of Smalltalk-80."

lmm 1y ago

Objective-C and Smalltalk were always niche languages, at least by comparison to Java and C#, and I think Smalltalk fans underestimate the value of many things. C++ does not (or at least did not at the time) have a concept of interfaces. There was a pattern in some development communities for defining interfaces by writing classes that followed particular rules, but no first-class support for them in the language.

kgeist 1y ago
>no first-class support for them in the language. An interface is just a base class none of whose virtual functions have implementations. C++ has first class support for it. The only thing C++ lacks is the "interface" keyword.
- gpderetta 1y ago
  The main reason (other than self-documentation) that some other languages separate interfaces form normal classes is that they only support multiple inheritance for interfaces. C++ doesn't have this restriction, so interfaces would add very little.
  - colejohnson66 1y ago
    Because multiple inheritance causes the diamond problem. C# forces you to solve it by declaring one method as the "canonical" and the others as "explicit interface implementations" (only accessible if the variable/receiver is typed as that interface).
    - layer8 1y ago
      The diamond problem strictly speaking only has to be one when the common base class has constructor arguments. While a Java-style interface construct makes it easy to prevent, it also imposes much stronger restrictions than the above. It would have been possible to only impose the above restriction. Yes, there are failure cases with separate compilation, but the way Java dynamically loads classes, that would be similar to how when the JVM loads a class file that is supposed to be an interface, it discovers that instead it has been changed to a class.
lelanthran 1y ago
> C++ does not (or at least did not at the time) have a concept of interfaces. There was a pattern in some development communities for defining interfaces by writing classes that followed particular rules, but no first-class support for them in the language. Your distinction between "first class support for interfaces" and "C++ support for interfaces" looks like an artificial one to me. Other than not requiring the keyword "interface", what is it about the C++ way of creating an interface that makes it not "first class support"?
layer8 1y ago
Java having an explicit ”interface” construct is one thing I didn’t like about it, because it muddles the notion of a class implicitly having an interface (a notion that clearly exists in C and C++, by way of header files if nothing else) with that construct, while on the other hand there is no a-priori reason to have a distinction between Java’s interfaces and pure abstract classes. Both specify an interface to be implemented. And Java 8+ muddles its concept further by allowing default methods and static members. The important thing is to distinguish between interface and implementation, and that is relevant to any class, whether it implements a separately defined interface or not.
- pasc1878 1y ago
  Java (following Objective-C) does need a differentiation between interface and pure abstract class - this is because it is single inheritance - a class can have any one class to inherit from but it can have many interfaces.
  - layer8 1y ago
    That doesn’t follow. The restriction could have been defined in terms of allowing at most one parent class to be non-pure-abstract. And there are lesser restrictions hat would have been conceivable as well. Java is single-inheritance only in the sense of the particular interface–class distinction it makes. For example, since Java 8 you can inherit method implementations from multiple interfaces.
pasc1878 1y ago
Java's object model is based on Objective-C's so a direct descendant and Objective-C's object model is based on Smalltalk so there is a direct connection there.

igouy 1y ago

Also: 2003 Traits: Composable Units of Behaviour https://www.cs.cmu.edu/~aldrich/courses/819/Scha03aTraits.pdf https://www.cs.cmu.edu/~aldrich/courses/819/Scha03aTraits.pd... " Traits as described in this paper are implemented in Squeak [22], an open-source dialect of Smalltalk-80."