JavaSound API Sequencer接口

Question

How can the method getSequencer in JavaSound API return an instance of the interface Sequencer?

Sequencer sequencer=MidiSystem.getSequencer();

I have read that we cant create an instance of an interface.

Answer 1

static is a very troublesome concept at best, its greatest akin for trouble at explaining is "volatile" keyword declaration.

You would have less trouble with "synchronized" keyword on a code block than the previous two for explaining their usage parameters and concept!

"static" is not constructed as "new" because it is not a "separate instance" it is already available when compiled in as a static object. All interfaces in Java are abstract but have "static" fields(variables) only , ONLY ONE of those loaded static class instruction version of AKA(A Kind of Alias) "instance" of a class(or alternately interface) will be present at that class hierarchy level on the process in the JVM runtime in that particular "user classes" hierarchy structure of call for any number of classes created that commit call of a static object or a static method (static code DOES NOT MAKE A NEW SEPARATE SET OF INSTRUCTION IF CALLED CONCURRENTLY FROM VARIOUS CLASS COPIES).

With anything "static" there is only one copy in use for all of the program at that calling class on the PID process level in the JVM during runtime.

You cannot instantiate MidiSystem because all its methods are "static" So to use ANY static class to call one of its static methods from it (or the same on an interface) you only use its class name followed by the dot operator on its method you wish to call. Exactly as you have it in the code and syntax you posted. (NOTE "Sequencer" is actually static )

But if you need your variable non static to remove static you cast it if the class type to cast to is non static, only if the class you are casting is not itself an actual "static" compiled class !

eg DriverManager.getDriver() for JDBC database running more than one connection concurrently cannot use static driver copies or there would only be one copy available in use of during runtime with the instructions template (the class byte code for the static class) ! To remove "static" from an object, the object must be cast to non static into a variable of same object type that is not of static notation(declared).

// the getDriver() method is static inside class DriverManager , 
// Driver is an Interface not a class
Driver driver = (Driver) DriverManager.getDriver( configuration.jdbcUrl() );
// After casting, there is now a separate non static reference of Driver 
// interface , so **note that neither MidiSystem or DriverManager class are** 
// actually declared static and both have no constructor and not declared 
// abstract but contain only static methods !

an "INSTANCE" is something you construct , so another one is a new instance ! Interfaces are not constructed, they operate much more alike "abstract" and "static" declarations. Using the class name only is the syntax of calling EITHER abstract or static classes to obtain their methods.

A final point , to refer to an interface as a "data type" is to make a reference variable to represent it because an interface IS a data type (known as an object) the same as a class or abstract class.

So your above code has "Sequencer" interface as a data type. When a class "implements" an interface the class itself can be cast to that interface because it is ALSO that object type. eg

    public class Example implements Extra{.....}
    Extra example = (Extra)new Example();
// next below shorthand implicit cast is compiler dependent
    public class Example implements Extra{.....}
    Extra example = new Example();

If you do not implement an interface in a class the interface can be called into the code with assignment of a reference variable by using a class that has a method that obtains that interface data type. There is a huge relationship between abstract classes and interfaces but they are not the same.

Abstract classes do not have global variables.

Interfaces do have global variables but all of them must be static and final. Abstract classes cannot have any global variables or it would be an " instance of a class" and would then require to be constructed as "new".

Abstract classes have less strict rules on method declaration than interfaces.

Abstract classes can have most class modifiers interfaces are all public

Interfaces have "default" modifier for methods that contain an implementation body of code or must be static method.

In short the variable for Sequencer interface is not an instance variable, it is a reference and (clause for static) you are referring to something defined as "static" so IT MUST be there when the class that calls it starts !