Default Methods

17 March 2016 by Lukhnos Liu

We are pleased to announce the support for Java 8 default methods in J2ObjC. The compiler also translates static methods in interfaces now. Together they make Java interfaces more useful, and, when combined with lambdas, enable a wide range of new idioms and programming styles. To translate this new language feature, pass the command line argument -source 8 to j2objc.

The Java Tutorials has a good introduction to default and static methods in interfaces. Another good introduction is Richard Warburton's Java 8 Lambdas. Here we give three short examples to show why this language feature is useful.

Providing New Features that Build Upon Essential Methods

Default methods enable us to keep the requirements of an interface minimal while still providing good default implementations that build upon those essential methods. Imagine you have an interface for objects (for example, appliances) whose date and time can be set:

interface DateTimeSettable {
  void setDate(int year, int month, int day);
  void setTime(int hour, int min);
}

The interface does not have a way to set date and time in one go. An API designer used to have two choices: one is to keep the API minimal and require everyone who needs to set both date and time to make two separate method calls; the other is to add a new setDateTime() method and require every class that implements DateTimeSettable to also implement setDateTime(). The second approach can have rippling effects if your version 1 API only has setDate() and setTime() and gets widely adopted (that is, your interface gets implemented by a lot of classes – a sign of your API's success!) but now your version 2 API starts to require the new setDateTime() implementation (and risks the ire of your existing v1 adopters).

With default methods, we can keep the API minimal while still providing a richer feature set:

interface DateTimeSettable {
  void setDate(int year, int month, int day);
  void setTime(int hour, int min);

  default void setDateTime(int year, int month, int day, int hour, int min) {
    setDate(year, month, day);
    setTime(hour, min);
  }
}

Every class that implements DateTimeSettable now gets a new default method, setDateTime(), for free. An implementing class can also override that method to provide a more specific implementation.

Adding Utility Methods to an Interface

Static methods in an interface serve the same purposes as those in a class. We can use them to provide utility or factory methods. Before this, we had to create another utility class for an interface. Now we can just add them to the same interface. For example, let's design a helper method that applies the same date and time to a group of objects using Joda Time's DateTime:

interface DateTimeSettable {
  static void setDateTime(Collection<DateTimeSettable> objs, DateTime dateTime) {
    for (DateTimeSettable obj : objs) {
      obj.setDateTime(...);
    }
  }

  // ...
}

Pairing Functional Interfaces with Default Methods

Default methods really shine when you pair them with functional interfaces. Imagine you have a Predicate<T> that tells you something about the object of type T:

interface Predicate<T> {
  boolean test(T t);
}

Now we have a Student class:

class Student {
  public int yearGraduated = -1;
  public boolean knowsObjC = false;
}

We can create property filters to help us find graduated students or those who know Objective-C:

Predicate<Student> graduated = (s) -> s.yearGraduated != -1;
Predicate<Student> knowsObjC = (s) -> s.knowsObjC;

Suppose we want to filter students who are still in school. One way to do it is to create a different predicate:

Predicate<Student> notGraduated = (s) -> s.yearGraduated == -1;

Similarly, we can create another predicate for students who have graduated and know Objective-C:

Predicate<Student> objCGraduate = (s) -> s.yearGraduated != -1 && s.knowsObjC;

But there's a better way. We can remove such error-prone repetition by using two new default methods in Predicate<T>:

interface Predicate<T> {
  boolean test(T t);
  default Predicate<T> negate() {
    return (x) -> !test(x);
  }
  default Predicate<T> and(Predicate<T> o) {
    return (x) -> test(x) && o.test(x);
  }
}

New predicates can be easily created this way:

Predicate<Student> notGraduated = graduated.negate();
Predicate<Student> objCGraduate = graduated.and(knowsObjC);

And to create a filter for intern candidates who knows Objective-C:

Predicate<Student> objCIntern = knowsObjC.and(graduated.negate());

Enabling Default Methods in J2ObjC

In previous J2ObjC versions, supplying -source 8 did nothing for default method or static method declarations in an interface. The compiler now handles them when the proper language level is set.

How J2ObjC Translates Default Methods

Objective-C does not have the notion of default methods. Since default methods can only call methods in the declaring interface or in its super-interfaces, we first turn a default method into a C function that takes a self parameter. So if our interface looks like this:

interface P {
  default void f(String x) {
    g(x);
  }

  void g(String y);
}

The translated default method will be a function P_fWithNSString that takes a self as its first argument:

void P_fWithNSString_(id<P> self, NSString *x) {
  [self gWithNSString:x];
}

Then, for every class that implements interface P, we add default method shims that call their actual implementations. So for a class Q:

class Q implements P {
  @Override
  public void g(String y) { ... }
}

The Objective-C method -[Q fWithNSString:] (the default method that Q gets from P) is implemented as:

@implementation Q
// ...

- (void)fWithNSString:(NSString *)arg0 {
  P_fWithNSString_(self, arg0);
}

// ...
@end

Of course, if Q overrides f, the overriding method takes precedence, and no shim will be generated in such case.

Making Use of the Companion Class

When we added reflection support to interfaces, we decided to generate one companion class for each interface. In Objective-C terms, this means for each translated @protocol we generate one @interface under the same name. Objective-C allows a protocol and a class to share the name (NSObject is the best example), and we took advantage of this feature to store the reflection metadata of an interface.

It turns out that companion class is essential in supporting static interface methods as well as default methods in functional interfaces (see the Predicate<T> example above). Internally, static methods in an interface are translated to C functions, like static field getters and setters are. Still, we want to maintain a good level of Objective-C interoperability, and therefore we also generate a class method shim in the interface's companion class. So if an interface P has a static method f:

interface P {
  static void f(String x) { ... }
}

The translated Objective-C declarations will be:

@protocol P < NSObject, JavaObject >
@end

@interface P : NSObject < P >
+ (void)fWithNSString:(NSString *)x;
@end

And the implementation of +[P fWithNSString:] is just a wrapper class method that calls the actual C function that implements the original P.f().

Default Methods in Lambdas

When a lambda is created from a functional interface such as the Predicate<T> example above, the lambda will also get the default methods from that functional interface. Previously, all our lambdas are runtime-created classes that subclass NSObject with runtime-added protocol conformance. We didn't want to inject default methods when we create a class in runtime, as it would be inefficient.

Once again, companion classes come to the rescue. We add default method shims to companion classes, as if they were normal classes that implement an interface with default methods. When a lambda gets created, and if the lambda is from a functional interface, the lambda will be a subclass of that functional interface's companion class, and thus it will have all the shims that in turn call the actual default method implementations.