This is exactly addressed by the JLS in 15.12.3. “Compile-Time Step 3: Is the Chosen Method Appropriate?”. If the form is TypeName . super . [TypeArguments] Identifier, then: […] If TypeName denotes an interface, let T be the type declaration immediately enclosing the method invocation. A compile-time error occurs if there exists a method, distinct … Read more
I’ve been troubled by similar issues as well when using MethodHandle.Lookup in JDK 8 – 10, which behave differently. I’ve blogged about the correct solution here in detail. This approach works in Java 8 In Java 8, the ideal approach uses a hack that accesses a package-private constructor from Lookup: import java.lang.invoke.MethodHandles.Lookup; import java.lang.reflect.Constructor; import … Read more
As we saw in What is the reason why “synchronized” is not allowed in Java 8 interface methods? and Why is “final” not allowed in Java 8 interface methods?, extending interfaces to define behavior is more subtle than it might first appear. It turns out that each of the possible modifiers has their own story; … Read more
Java doesn’t have extension methods. Default methods are not extension methods. Let’s look at each feature. Default methods Both Java and C# support this feature Problems solved: Many objects may implement the same interface and all of them may use the same implementation for a method. A base class could solve this issue but only … Read more
Update: Seems like it’s really a bug. A class or super-class method declaration always takes priority over a default method! default hello(…) method from the Myinterface allows you to write without errors: ClassB b = new ClassB(); b.hello(); Until runtime, because at runtime hello(…) method from the ClassA takes the highest priority (but the method … Read more
Besides having the possibility of adding methods to the interface in future versions, there is the important point of allowing an interface to stay a functional interface even if it has more than one method. A functional interface has only one non-default abstract method which can be implemented via a lambda expression. One example is … Read more
It’s more or less a question of scope. From the JLS Unlike code appearing in anonymous class declarations, the meaning of names and the this and super keywords appearing in a lambda body, along with the accessibility of referenced declarations, are the same as in the surrounding context (except that lambda parameters introduce new names). … Read more
Doesn’t JDK 1.8 introduce a forward incompatibility for Java source code due to default methods? Any new method in a superclass or interface can break compatibility. Default methods make it less likely that a change in an interface will break compatibility. In the sense that default methods open the door to adding methods to interfaces, … Read more
This is a very interesting issue! It seems like JLS section 12.4.1 ought to cover this definitively. However, the behavior of Oracle JDK and OpenJDK (javac and HotSpot) differs from what’s specified here. In particular, the Example 12.4.1-3 from this section covers interface initialization. The example as follows: interface I { int i = 1, … Read more
The short answer is: it’s safe if you use them safely 🙂 The snarky answer: tell me what you mean by traits, and maybe I’ll give you a better answer 🙂 In all seriousness, the term “trait” is not well-defined. Many Java developers are most familiar with traits as they are expressed in Scala, but … Read more