7 ms·

Why do we need "? extends" in Java

32 points by rocky_ex 15y ago

democracy 15y ago
There is a good free tutorial on generics: https://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf https://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf. It explains the design of generics, and tells a bit about why Sun engineers came up with this implementation.
H_E_Pennypacker 15y ago
All about Java Generics in one big FAQ
ssdsa 15y ago
Wow, I'm so glad that I don't need to worry about such a mess when using duck-typing in Python.
- SamReidHughes 15y ago
  Yeah, it's soooooo complicated!
- yummyfajitas 15y ago
  I'm glad that when I refactor code in Java, the type checker catches many of my mistakes. Not as many as in Haskell, but still. In Python, the deploy/run unit tests/spot typo cycle greatly slows me down.
  - Mavrik 15y ago
    Yeah this... I've found out that I subconsciously actively avoid refactoring our (old and in parts terrible) codebase because the tools for it are so bad.
gaving 15y ago
Why do we need Java?
- batista 15y ago
  Because it powers tens of thousands of huge systems in use? How about that? Have you tried programming BEFORE Java, when the same kind of systems were done in C++ with CORBA or similar monstrosities?
- deleted 15y ago
  [deleted]
- zxcvb 15y ago
  Many companies depend on Java for their mission critical infrastructure and applications. I used to work at a world leading investment bank who's entire low latency, high scalable trading platforms were built in Java. Just about every large financial firm that I've worked at plus both telco firms and one management consultancy used Java to build large, fast and scalable applications. You may have no use for Java in your small bubble of a world where you most probably work on insignificant, crappy pay and waste of time projects, but people who work on important stuff in the real world do. I'm sure other people can tell you why Java is important in ways that I cannot. I just get annoyed when I see people slag off a piece of technology just because they don't use it or like it.
- deleted 15y ago
  [deleted]
- lani 15y ago
  ava's design allows app development and performance to be decoupled (to a good extent, when compared to C or C++). Combined with the lower learning curve (again, compared to that which ruled before - aka C++), this commoditizes app development and has hence found much adoption in industry, leaving a small team of specialists to work on performance (& the jvm). Adoption leads to funds leads to research and/or improvements until Java expanded to include much of what needed to be done via programming, in other words, you can get almost everything you want done in java. In other words, Java is an industry standard, or if you will, a household name. So why do we need Java ? same reason as we need division of labour.
myspy 15y ago
We had to learn it when doing an oo programming course at university. And I immediately forgot all about it. I hope when writing Java code sometimes at a job, I can refer to this again. This is all done in the name of type safe programming (I don't know the exact english terminology). <? extends B> is called covariance. It helps to access attributes of B and derived classes with get() but prevents to use set(). ArrayList<? extends Number> list; list = new ArrayList<Integer>(); list = new ArrayList<Double>(); list = new ArrayList<Long>(); list.set(index, myInteger); // throws error Compiler has to know the exact type to set the value, but Long, Double and Integer are all from type Number, so you can read it into a Number variable. Is that correct?
- guard-of-terra 15y ago
  You describe how it works, but not what it means in simple terms. List<Shape> is not a "list of Shapes", it's a "list parametrized with Shape" List<? extends Shape> is a "list parametrized with something that extends Shape" e.g. you can assign it with List<Circle> (you can't assign it to List<Shape> since Shape != Circle) and then you can do operations on it that would not break List<Circle> (can't put Square in it)
  - myspy 15y ago
    Great, thank you. It's interesting to learn, when you don't know how to describe something in simple terms. I blame my lacking college professor and me, not taking enough time to learn it properly.
ronreiter 15y ago
I thought the whole point about Java is that it's supposed to be extra verbose (which is also the reason it sucks a bit). wtf is this shit
- guard-of-terra 15y ago
  Let me tell you why everyone is downvoting you: The "? extends" is not an "extra verbosity". It's actually a sound CS concept, one that you need to make compiler understand how some generic code turns out to be correct while working on partially specified types. IIRC C# didn't have that "extra verbose" thing in its early versions and the developers weren't very happy because you do need it. Not being able to separate "extra verbosity" from "solid CS concepts" makes you lousy at reasoning about programming languages. Might as well stop bothering.
guard-of-terra 15y ago
When we hire developers with proficiency in Java that's one of our common interview questions. We also seek ability to figure out what "? super" means.
- ootachi 15y ago
  I have a feeling you're going to get a bunch of type theorists instead of programmers who get things done if you ask about Java's syntax for use-site contravariance.
  - reginaldo 15y ago
    Not true. You'll get, for instance, people who read Effective Java [1], by Joshua Bloch (who is, IMHO, THE man when it comes to java), and remembered the PECS mnemonics. PECS stands for producer-extends, consumer-super. So, to cite the textbook example, say you are implementing a Stack<E>. It will probably have the methods: public void push(E element); public E pop(); and, for convenience: public void pushAll(Iterable<? extends E> elements); public void popAll(Collection<? super E> destination); The pushAll has "? extends" because the elements Iterable will "produce" elements for the stack. The popAll has super because the destination Collection will "consume" elements from the stack. It is not that hard, is it? Let's note that guard-of-terra is talking about proficiency, not mere familiarity. I believe reading "Effective Java" is a nice way to get closer to the proficient level. Let it be noted that this whole mess exists because generics in Java were implemented with type erasure so their introduction wouldn't break legacy code. I personally think this was a bad idea, but it does show that when a language is evolving, there are a bunch of constraints the designers must be aware of. [1] http://www.amazon.com/Effective-Java-Edition-Joshua-Bloch/dp/0321356683/ref=sr_1_1?ie=UTF8&qid=1332688982&sr=8-1 http://www.amazon.com/Effective-Java-Edition-Joshua-Bloch/dp...
    - ootachi 15y ago
      This doesn't seem to have anything to do with type erasure, does it? It's needed for type-safe generics. Even without type erasure, you still need type safety. If your generics are always covariant (like Dart's are), you break type safety. I think the mistake the Java designers made here was going with use-site variance instead of definition-site variance.

32 points by rocky_ex 15y ago

There is a good free tutorial on generics: https://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf https://java.sun.com/j2se/1.5/pdf/generics-tutorial.pdf. It explains the design of generics, and tells a bit about why Sun engineers came up with this implementation.

H_E_Pennypacker 15y ago

All about Java Generics in one big FAQ

ssdsa 15y ago

Wow, I'm so glad that I don't need to worry about such a mess when using duck-typing in Python.

SamReidHughes 15y ago
Yeah, it's soooooo complicated!
yummyfajitas 15y ago
I'm glad that when I refactor code in Java, the type checker catches many of my mistakes. Not as many as in Haskell, but still. In Python, the deploy/run unit tests/spot typo cycle greatly slows me down.
- Mavrik 15y ago
  Yeah this... I've found out that I subconsciously actively avoid refactoring our (old and in parts terrible) codebase because the tools for it are so bad.

gaving 15y ago

Why do we need Java?

batista 15y ago
Because it powers tens of thousands of huge systems in use? How about that? Have you tried programming BEFORE Java, when the same kind of systems were done in C++ with CORBA or similar monstrosities?
deleted 15y ago
[deleted]
zxcvb 15y ago
Many companies depend on Java for their mission critical infrastructure and applications. I used to work at a world leading investment bank who's entire low latency, high scalable trading platforms were built in Java. Just about every large financial firm that I've worked at plus both telco firms and one management consultancy used Java to build large, fast and scalable applications. You may have no use for Java in your small bubble of a world where you most probably work on insignificant, crappy pay and waste of time projects, but people who work on important stuff in the real world do. I'm sure other people can tell you why Java is important in ways that I cannot. I just get annoyed when I see people slag off a piece of technology just because they don't use it or like it.
deleted 15y ago
[deleted]
lani 15y ago
ava's design allows app development and performance to be decoupled (to a good extent, when compared to C or C++). Combined with the lower learning curve (again, compared to that which ruled before - aka C++), this commoditizes app development and has hence found much adoption in industry, leaving a small team of specialists to work on performance (& the jvm). Adoption leads to funds leads to research and/or improvements until Java expanded to include much of what needed to be done via programming, in other words, you can get almost everything you want done in java. In other words, Java is an industry standard, or if you will, a household name. So why do we need Java ? same reason as we need division of labour.

myspy 15y ago

We had to learn it when doing an oo programming course at university. And I immediately forgot all about it. I hope when writing Java code sometimes at a job, I can refer to this again. This is all done in the name of type safe programming (I don't know the exact english terminology). <? extends B> is called covariance. It helps to access attributes of B and derived classes with get() but prevents to use set(). ArrayList<? extends Number> list; list = new ArrayList<Integer>(); list = new ArrayList<Double>(); list = new ArrayList<Long>(); list.set(index, myInteger); // throws error Compiler has to know the exact type to set the value, but Long, Double and Integer are all from type Number, so you can read it into a Number variable. Is that correct?

guard-of-terra 15y ago
You describe how it works, but not what it means in simple terms. List<Shape> is not a "list of Shapes", it's a "list parametrized with Shape" List<? extends Shape> is a "list parametrized with something that extends Shape" e.g. you can assign it with List<Circle> (you can't assign it to List<Shape> since Shape != Circle) and then you can do operations on it that would not break List<Circle> (can't put Square in it)
- myspy 15y ago
  Great, thank you. It's interesting to learn, when you don't know how to describe something in simple terms. I blame my lacking college professor and me, not taking enough time to learn it properly.

ronreiter 15y ago

I thought the whole point about Java is that it's supposed to be extra verbose (which is also the reason it sucks a bit). wtf is this shit

guard-of-terra 15y ago
Let me tell you why everyone is downvoting you: The "? extends" is not an "extra verbosity". It's actually a sound CS concept, one that you need to make compiler understand how some generic code turns out to be correct while working on partially specified types. IIRC C# didn't have that "extra verbose" thing in its early versions and the developers weren't very happy because you do need it. Not being able to separate "extra verbosity" from "solid CS concepts" makes you lousy at reasoning about programming languages. Might as well stop bothering.

guard-of-terra 15y ago

When we hire developers with proficiency in Java that's one of our common interview questions. We also seek ability to figure out what "? super" means.

ootachi 15y ago
I have a feeling you're going to get a bunch of type theorists instead of programmers who get things done if you ask about Java's syntax for use-site contravariance.
- reginaldo 15y ago
  Not true. You'll get, for instance, people who read Effective Java [1], by Joshua Bloch (who is, IMHO, THE man when it comes to java), and remembered the PECS mnemonics. PECS stands for producer-extends, consumer-super. So, to cite the textbook example, say you are implementing a Stack<E>. It will probably have the methods: public void push(E element); public E pop(); and, for convenience: public void pushAll(Iterable<? extends E> elements); public void popAll(Collection<? super E> destination); The pushAll has "? extends" because the elements Iterable will "produce" elements for the stack. The popAll has super because the destination Collection will "consume" elements from the stack. It is not that hard, is it? Let's note that guard-of-terra is talking about proficiency, not mere familiarity. I believe reading "Effective Java" is a nice way to get closer to the proficient level. Let it be noted that this whole mess exists because generics in Java were implemented with type erasure so their introduction wouldn't break legacy code. I personally think this was a bad idea, but it does show that when a language is evolving, there are a bunch of constraints the designers must be aware of. [1] http://www.amazon.com/Effective-Java-Edition-Joshua-Bloch/dp/0321356683/ref=sr_1_1?ie=UTF8&qid=1332688982&sr=8-1 http://www.amazon.com/Effective-Java-Edition-Joshua-Bloch/dp...
  - ootachi 15y ago
    This doesn't seem to have anything to do with type erasure, does it? It's needed for type-safe generics. Even without type erasure, you still need type safety. If your generics are always covariant (like Dart's are), you break type safety. I think the mistake the Java designers made here was going with use-site variance instead of definition-site variance.