Using generic object methods in generic class

Question

I am trying to create a generic class in which the generic type itself can have methods.

This is what I did in Java:

public class DepthFirstPaths <GraphType> implements PathInterface
{
    private final int start;        

    public DepthFirstPaths (GraphType G, int s)
    {
        int vCount = G.numVertex(); //This line is giving errors
        start = s;

        DFS(G, s);
    }

    //Other methods
    ....
}

GraphType can refer to a directed graph type or undirected graph type and this is because DFS does the same thing for both types of graphs given a source vertex. But the above java code gives errors like:

Description: The method numVertex() is undefined for the type GraphType
Resource: DepthFirstPaths.java
Path: /Paths/src/GraphAlgorithms
Location: line 17
Type: Java Problem

Plus other errors relating to using GraphType object methods in the code

What can I do to fix this? What is the best way to do this?

I guess you are porting some C++ code to java...

ZhongYu
– ZhongYu

2013-11-15 22:09:44 +00:00
Commented Nov 15, 2013 at 22:09 — ZhongYu
– ZhongYu, Commented Nov 15, 2013 at 22:09

Luiggi Mendoza · Accepted Answer · 2013-11-15 21:56:42Z

4

This definition

public class DepthFirstPaths <GraphType>

Means your generic type name is GraphType.

To make your class always work with a GraphType, you should mark your generic extends from GraphType:

public class DepthFirstPaths <T extends GraphType>

Then just use T in your code to refer to the generic:

public class DepthFirstPaths <T extends GraphType> implements PathInterface {
    private final int start;        

    public DepthFirstPaths (T G, int s) {
        //This line won't give you errors anymore
        //except if GraphType doesn't have a numVertex method
        int vCount = G.numVertex();
        start = s;

        DFS(G, s);
    }

    //Other methods
    ....
}

answered Nov 15, 2013 at 21:56

Luiggi Mendoza

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4 Comments

smac89 Over a year ago

So say I have the 2 graph classes: undirGraph and dirGraph, do I have to write: public class DepthFirstPaths <T extends undirGraph, dirGraph>?

Luiggi Mendoza Over a year ago

If UndirGraph and DirGraph classes extend from GraphType, you won't need to do that.

smac89 Over a year ago

Aha, I see. So I have to somehow make all Graph objects extend GraphType then I can use your above syntax? So what should GraphType be? interface? Abstract class?

Luiggi Mendoza Over a year ago

That will depend on your design but IMO it should be an interface. You can define such design based on the List interface and ArrayList class implementation.

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