Java：填充了接口元素的数据结构 - 如何强制实现接口的类实现compareTo（）？

Question

I got a built in interface called Bicycle (and I am not allowed to add or change anything in it),

and I created a data structure filled with Bicycle elements in it.

I need that the Bicycle elements will be able to be sorted in different ways, the decision how exactly they will be sorted in the data structure has to be upon the class that implements the interface Bicycle. (or any other way that doesn't have to make any change in Bicycle interface)

I read on the internet that I need to write something like this:

Public class BicycleDataStructure <Bicycle extends Comparable<Bicycle>>

and then I can use compareTo() function on Bicycle elements in the data structure. But, When I create a class like:

SportBicycle implements Bicycle

Java doesn't tell me that I need to add implementation of compareTo() function, so how it will know by which parameter to compare it? I think I do something wrong.. Thank you for your help.

Edit: code -

public class BicycleDataStructure<T extends Comparable<Bicycle>> {
protected List<Bicycle> dataStructure;
protected Bicycle max;

public BicycleDataStructure() {
    this.dataStructure= new ArrayList<>();
    this.max = null;

}

public void addBicycle(Bicycle b1) {
    int i;
    for (i = 0; i < dataStructure.size(); i++) {
        Bicycle b = dataStructure.get(i);

        if (b1.compareTo(b) < 0) {
            this.dataStructure.add(i, b1);
            break;
        }

    }

}

and the interface-

public interface Bicycle{

public int getModel();

public String getColor();

public String getBrand();

public int getSize();

}

Answer 1

Currently, Bicycle is being treated as a type parameter, instead do this:

class BicycleDataStructure <T extends Comparable<Bicycle>> {...}

Now T is the type parameter and Bicycle refers to the interface type.

Answer 2

Even if you can't modify the Bicycle interface, you can extend it.

public interface ComparableBicycle extends Bicycle, Comparable<ComparableBicycle> {
}

Your SportBicycle class can implement ComparableBicycle and still be an instance of Bicycle. The compiler will require you to implement compareTo() .

This will result in an error:

public class SportBicycle implements ComparableBicycle {

    // ... Implementations of Bicycle methods ...
}

Until you add compareTo() :

public class SportBicycle implements ComparableBicycle {
    /* (non-Javadoc)
     * @see java.lang.Comparable#compareTo(java.lang.Object)
     */
    @Override
    public int compareTo(ComparableBicycle o) {
        // TODO: Make the comparison
        return 0;
    } 

    // ... Implementations of Bicycle methods ...
}

Answer 3

First, you should change the generic type of your data structure class as follows:

public class BicycleDataStructure<T extends Bicycle & Comparable<T>>

This is called an intersection bound type , and it tells the compiler that the data structure will hold instances of a type that is both a subtype of Bicycle and Comparable<T> . This avoids the need to create a subtype of Bicycle just to express that it's also Comparable of that subtype.

Then, you should make both your dataStructure and max attributes use the generic type T :

protected List<T> dataStructure;
protected T max;

Then, in your add method, the b1 argument and b local variable should also be of type T . This is the code:

public class BicycleDataStructure<T extends Bycycle & Comparable<T>> {

    protected List<T> dataStructure;

    protected T max;

    public BicycleDataStructure() {
        this.dataStructure = new ArrayList<>();
        this.max = null;    
    }

    public void addBicycle(T b1) {
        for (int i = 0; i < dataStructure.size(); i++) {
            T b = dataStructure.get(i);

            if (b1.compareTo(b) < 0) {
                this.dataStructure.add(i, b1);
                break;
            }   
        }    
    }
}