Java实现自定义Comparable接口报错：incompatible types不兼容类型

我在Java中使用自定义接口时遇到了类型转换报错，具体细节如下：

自定义Comparable接口 #

public interface Comparable<ContentType> {
    public boolean isGreater ( ContentType pContent );
    public boolean isEqual ( ContentType pContent );
    public boolean isLess ( ContentType pContent );
}

实现接口的Point类 #

public class Point implements Comparable<Point>{
    private double x;
    private double y;

    public Point ( double pX, double pY ) {
        x = pX;
        y = pY;
    }

    public boolean isLess ( Point pContent ) {
        return this.distance() < pContent.distance();
    }

    public boolean isEqual ( Point pContent ) {
        return this.distance() == pContent.distance();
    }

    public boolean isGreater ( Point pContent ) {
        return this.distance() > pContent.distance();
    }

    public double distance() {
        return Math.sqrt( x*x + y*y );
    }
}

排序方法与测试代码 #

public class Algorithms {
    public static List<Comparable> insortComparable( Queue<Comparable> pQueue ) {
        List<Comparable> l = new List<Comparable>();
        while ( !pQueue.isEmpty() ) {
            if ( l.isEmpty() ) {
                l.append(pQueue.front());
                pQueue.dequeue();
            } else {
                l.toFirst();
                while (l.hasAccess() && pQueue.front().isGreater(l.getContent())) {
                    l.next();
                }
                if ( l.hasAccess() ) {
                    l.insert(pQueue.front());
                } else {
                    l.append(pQueue.front());
                }
                pQueue.dequeue();
            }
        }
        return l;
    }

    public static Queue<Point> randomQueuePoint() {
        Queue<Point> q = new Queue<Point>();    
        for ( int i = 0; i < 10; i++ ) {
            q.enqueue( new Point( Math.random()*100, Math.random()) );
        }
        return q;
    }

    public void test() {
        Queue<Point> q = randomQueuePoint();
        List<Point> l = insortComparable(q);
    }
}

错误信息 #

调用insortComparable(q)时编译器抛出如下错误：

incompatible types: Queue cannot be converted to Queue

问题原因 #

Java泛型具备不变性，即使Point实现了Comparable<Point>接口，Queue<Point>和Queue<Comparable>也不存在继承关系，不能直接互相转换，因此传入Queue<Point>类型的参数到要求Queue<Comparable>的方法会触发类型不匹配报错。

解决方案 #

修改insortComparable方法为有界泛型方法，定义泛型参数<T extends Comparable>，适配所有实现了Comparable接口的类，同时保证输入输出类型一致：

public static <T extends Comparable> List<T> insortComparable( Queue<T> pQueue ) {
    List<T> l = new List<T>();
    while ( !pQueue.isEmpty() ) {
        if ( l.isEmpty() ) {
            l.append(pQueue.front());
            pQueue.dequeue();
        } else {
            l.toFirst();
            while (l.hasAccess() && pQueue.front().isGreater(l.getContent())) {
                l.next();
            }
            if ( l.hasAccess() ) {
                l.insert(pQueue.front());
            } else {
                l.append(pQueue.front());
            }
            pQueue.dequeue();
        }
    }
    return l;
}

内容的提问来源于stack exchange，提问作者codefinn