Java Interface - constants and static class in normal interface

Question

Consider following interface.

public interface ThirdPartyApiHandler {

    public OperationResult doOperation(OperationInput input);

    public static class OperationResult {
         //members of OpeationResult. metrics after file processing
         private int successfulRecords;
         private int failedRecords;
    }  

    public static class OperationInput {
         //implementations call third party API to process this file.
         private String inputBatchFile; 
    }  

    //Constant which would be same across all implementations.
    public static final int GLOBAL_CONSTANT = 1;
}

Is above interface a bad design?

1. OperationResult and OperationInput are defined as static class. They would be only used by implementations and not anywhere else. Advantage that I see here is - I don't have to create separate files for these two classes. Also they get namespace of parent class.

2. I have read about constant interface. But in this case, I am defining constant in normal interface which are bound to be same across all implementations and would be used in those implementations.

I am using this pattern for first time so wanted to get suggestions.

Answer 1

OperationResult and OperationInput are defined as static inner class. They won't be used anywhere else.

That's OK since they will not be used anywhere else. If they're long than I would prefer to have them in separate classes.

I have read about constant interface. But in this case, I am defining constant in normal interface which are bound to be same across all implementations and would be used in those implementations.

That's a good place to declare such a field.

Answer 2

Having nested classes in interfaces is only matter of additional namespace. This approach help to organize the code when small interfaces are created to support simple data structure.

I recommend you this lecture: Java Tip 75: Use nested classes for better organization.

Note that public and static are redundant in this case so you do not need them. What you need to remember is that having such classes do not limit other developers to use them in other parts of code.

From my point of view, this is a good design but, i would extend and replace the class with interfaces.

public interface ThirdPartyApiHandler {

    OperationResult doOperation(OperationInput input);

    interface OperationResult {
         int getSuccessfulRecords();
         int getFailedRecords();
    }  

    interface OperationInput {
         String getInputBatchFile(); 
    }  

    final int GLOBAL_CONSTANT = 1; //This could be replaced by enum but no need 
}