CODEWITHSUNDEEP
javainterfacesemantics
jameshfisher
jameshfisher

Reputation: 36403

In what context are statements in Java interface definitions executed?

I'm looking at the definition of org.apache.maven.plugin.Mojo:

public interface Mojo
{
    String ROLE = Mojo.class.getName();
    [...]
}

I'm lost. To my knowledge, Java interfaces are a set of method signatures. So what's this line which looks like a statement doing here? What are the semantics? For example:

Upvotes: 2

Views: 71

Answers (4)

ifloop
ifloop

Reputation: 8386

  • When does that line get "executed"?

Any member of an Interface is static and final by default, so String ROLE (which can effectivly be seen as 

public static final String ROLE

will be initialized, as every static member, when the ClassLoader loads the class (by the first time, the class is referenced).

JLS - Loading of Classes and Interfaces

  • In the context in which that line runs, what does Mojo refer to? What is its type?
  • In the context in which that line runs, what does Mojo.class refer to? I assume its type is java.lang.Class?

Mojo is of the type java.lang.Class<Mojo>

  • In what context can I read that ROLE variable? What is the syntax for doing so? What will the variable contain?

As it is implicit public static final you can access it from everywhere by using Mojo.ROLE or through the implementing classes by ClassName.ROLE. As Class#getName() says in the java doc, it will contain the full qualified name of the object: 

org.apache.maven.plugin.Mojo
  • Can I write to that ROLE variable?

No, you cannot, because it is implicit final

JLS - Interfaces

Upvotes: 0

JB Nizet
JB Nizet

Reputation: 691735

All the fields of an interface are implicitely public, static and final. So this is the same as writing

public static final String ROLE = Mojo.class.getName();

It defines a constant, that all the users of the interface can use, as any other constant: Mojo.ROLE. This line is executed when the Mojo interface is initialized by the ClassLoader. Mojo.class is the Mojo class, indeed of type java.lang.Class<Mojo>. Since the package of the class is org.apache.maven.plugin, the value of the constant will be "org.apache.maven.plugin.Mojo".

Look here for the relevant section of the Java language specification.

Upvotes: 3

Ghostkeeper
Ghostkeeper

Reputation: 3050

All fields in an interface are implicitly static and final. That answers some of your questions:

  • The line gets executed during the static execution. That is possibly during compilation, possibly at the launching of the program and possibly when the class loader loads that class. It's a complicated business, but it's the same as all other static fields.
  • Mojo refers to the interface itself, statically. It is a type itself, so it has no type.
  • Mojo.class refers to the java.lang.Class of Mojo.
  • You can read the ROLE variable in any class that implements Mojo. You can only read it statically, like Mojo.ROLE.
  • No, the field is final, so you can't write to it. It is intended for constants only.

Upvotes: 0

Dima
Dima

Reputation: 8652

the variable get defined when class loader loads the class. interface variables are static final and can be accessed statically by any class that implements the interface. 

public class MyClass implements Mojo{
   .
   .
   .
}

access:

public void someMethod(){
  System.out.println(MyClass.ROLE)
}

Upvotes: 0

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