ComparableTriangle未实现Comparable方法致Collections.sort报错求助

问题原因 #

你代码的核心问题是Comparable接口的泛型声明与重写的compareTo方法参数不匹配：

• ComparableTriangle类声明实现了Comparable<Triangle>，这要求必须重写int compareTo(Triangle o)方法
• 但你实际编写的是int compareTo(ComparableTriangle param)，这不是对接口方法的重写，只是同名重载方法。JVM在运行时找不到接口要求的实现方法，因此抛出异常。
• 你尝试改成implements Comparable（无泛型）时，需要重写int compareTo(Object o)，但仍使用ComparableTriangle作为参数，依旧未满足接口要求，所以报错持续。

解决方案 #

推荐第一种更简洁的修改方式：

方式一：调整泛型类型为当前类

将类声明中的implements Comparable<Triangle>改为implements Comparable<ComparableTriangle>，让compareTo的参数类型与接口要求完全匹配：

class ComparableTriangle extends Triangle implements Comparable<ComparableTriangle>, Cloneable {
  /** Construct a ComparableTriangle with specified properties */
  public ComparableTriangle(double side1, double side2, double side3) {
    super(side1, side2, side3);
  }
  // Compares the object (t) to the parameter being input (t2)
  @Override public int compareTo(ComparableTriangle param) {
    if (this.getArea() > param.getArea()) { // this > t1
      System.out.println("Triangle 1 is bigger.");
      return 1;
    }
    else if (this.getArea() < param.getArea()) {
      System.out.println("Triangle 2 is bigger.");
      return -1;
    }
    else {
      System.out.println("Both triangles are equal.");
      return 0;
    }
  }
  // Clones the object input. Throws exception if not supported.
  protected Object clone() throws CloneNotSupportedException {
    {  
        System.out.println(this.getArea());
        System.out.println(this.getPerimeter());
        return super.clone();
    }
  }
}

方式二：保持泛型为Triangle，修改compareTo参数类型

如果需要和所有Triangle子类比较，可将compareTo的参数改为Triangle并添加类型检查：

class ComparableTriangle extends Triangle implements Comparable<Triangle>, Cloneable {
  /** Construct a ComparableTriangle with specified properties */
  public ComparableTriangle(double side1, double side2, double side3) {
    super(side1, side2, side3);
  }
  // Compares the object (t) to the parameter being input (t2)
  @Override public int compareTo(Triangle param) {
    if (!(param instanceof ComparableTriangle)) {
        throw new IllegalArgumentException("仅支持与ComparableTriangle实例比较");
    }
    ComparableTriangle other = (ComparableTriangle) param;
    if (this.getArea() > other.getArea()) { // this > t1
      System.out.println("Triangle 1 is bigger.");
      return 1;
    }
    else if (this.getArea() < other.getArea()) {
      System.out.println("Triangle 2 is bigger.");
      return -1;
    }
    else {
      System.out.println("Both triangles are equal.");
      return 0;
    }
  }
  // Clones the object input. Throws exception if not supported.
  protected Object clone() throws CloneNotSupportedException {
    {  
        System.out.println(this.getArea());
        System.out.println(this.getPerimeter());
        return super.clone();
    }
  }
}

补充说明 #

单独创建对象调用compareTo时正常，是因为你调用的是自己重载的compareTo(ComparableTriangle)方法，但Collections.sort内部会调用接口定义的标准compareTo方法，而你未正确实现该方法，因此运行时触发异常。

内容的提问来源于stack exchange，提问作者DrDallas