CODEWITHSUNDEEP
javagenericsinterface
pseudoramble
pseudoramble

Reputation: 2561

Writing a method to accept interface type instead of class type

I am trying to create some classes that implement a particular interface (in this case, XYPlottable) and a method that can handle any class implementing that interface.

So far I have the following (that works):

public interface XYPlottable {
    public Number getXCoordinate();
    public Number getYCoordinate();
    public Number getCoordinate(String fieldName);
}

public class A implements XYPlottable {
//Implements the above interface properly
...
}

public class B implements XYPlottable {
//Implements the above interface properly
...
}

This works fine. I also have a method to try and plot anything that's XYPlottable:

public static Frame createPlot(String title, String xAxisLabel, String yAxisLabel,
                               List<XYPlottable> points, boolean fitLine) {

So I attempt to go use it with one of the above concrete classes and it complains about having incompatible types:

List<A> values = _controller.getValues(tripName);
XYPlotter.createPlot("Plot A", "B", "C", values, false);

Here's the exact error:

incompatible types
  required: java.util.List<XYPlottable>
  found:    java.util.List<A>

I hoping I'm just having a moment and missing something really obvious, but maybe I'm having a misunderstanding of how I should be using interfaces.

Upvotes: 11

Views: 5210

Answers (3)

Martin
Martin

Reputation: 7139

You are using the concrete type A in your list of values. This should be a list of XYPlottables e.g

List<XYPlottable> value = _controller.getValues(tripName)

Upvotes: 0

duffymo
duffymo

Reputation: 308753

Try this: 

List<? extends XYPlottable>

in the method declaration.

Generics in Java can be confusing.

http://download.oracle.com/javase/tutorial/java/generics/index.html

Upvotes: 8

Premraj
Premraj

Reputation: 7902

Method declaration like following should work - 

public static Frame createPlot(String title, String xAxisLabel, String yAxisLabel,
                               List<? extends XYPlottable> points, boolean fitLine) {

Note the change in the parameter List<XYPlottable> to List<? extends XYPlottable> - This is called as wildcards.
Read more about generic wildcards here

Upvotes: 22

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