Java 优先级队列和类似的接口
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Java priority queues and comparable interface
提问by JohnnyHunter
I've just been learning about priority queues and thought I'd try how it behaves with comparable interface.
我刚刚在学习优先级队列，并认为我会尝试使用可比较的接口来表现它的行为。
Code Snippet:
代码片段：
import java.util.PriorityQueue;
class kinga implements Comparable<Double> {
double time=909.909;
double d;
public kinga(double a) {
this.d=a;
}
public int compareTo(Double d) {
return Double.compare(d, time);
}
public static void main(String arg[]) {
PriorityQueue<kinga> r=new PriorityQueue<kinga>();
r.add( new kinga(4545.45));
r.add( new kinga(45.4));
r.add( new kinga(1235.45));
System.out.println(r.poll()+" "+r.poll()+" "+r.poll());
}
}
It compiles but gives me Exception in thread "main"java.lang.ClassCastException: kinga cannot be cast to java.lang.Double.
它编译但在线程 "main" 中给了我异常java.lang.ClassCastException: kinga cannot be cast to java.lang.Double。
What is wrong here. Can somebody tell me how comparable and priority queues work?
这里有什么问题。有人能告诉我可比队列和优先队列是如何工作的吗？
采纳答案by Katona
kingashould be comparable with kinga, not Double, so:
kinga应该与 相比kinga，而不是Double，所以：
class kinga implements Comparable<kinga>
which means your compareTomethod has to be changed to this:
这意味着您的compareTo方法必须更改为：
public int compareTo(kinga o) {
return Double.compare(o.d, d);
}
回答by sanbhat
PriorityQueue<kinga>will expect Comparable<kinga>in the addmethod. Passing a Comparable<Dobule>instead, is throwing ClassCastException
PriorityQueue<kinga>将期望Comparable<kinga>在 add方法中。传递一个Comparable<Dobule>而不是，正在抛出ClassCastException
回答by JB Nizet
class kinga implements Comparable<Double>
That doesn't make sense. Although your class will compare fine with Double, Double is unaware of that, and won't compare fine with instances of kinga, which will break the Comparable contract. And since a kinga can't compare with another kinga, you can't use a PriorityQueue<kinga>.
那没有意义。尽管您的类会与 Double 进行比较，但 Double 不知道这一点，并且不会与 kinga 的实例进行比较，这会破坏 Comparable 契约。由于一个kinga 无法与另一个kinga 进行比较，因此您不能使用PriorityQueue<kinga>.
It should be
它应该是
class Kinga implements Comparable<Kinga>
(note the upper-case, to respect the Java naming conventions), which means: Kinga instances are comparable together.
（注意大写，以尊重 Java 命名约定），这意味着：Kinga 实例可以一起比较。
The compareTo method should be
compareTo 方法应该是
@Override
public int compareTo(Kinga other) {
return Double.compare(this.d, other.d);
}
which means: I'm bigger than another Kinga if my dis bigger than the other Kinga's d.
这意味着：如果 my 比另一个 Kinga 大，我d就比另一个 Kinga大d。
回答by Aniket Thakur
Can somebody tell me how comparable and priority queues work?
First get the differencebetween Comparable and Comparator interfaces.
先搞清楚Comparable 和 Comparator 接口的区别。
Now for your question you can do something like below
现在对于您的问题，您可以执行以下操作
First create a Comparatorfor Kinga
首先为 Kinga创建一个Comparator
class comparableKinga implements Comparator<kinga> {
@Override
public int compare(kinga o1, kinga o2) {
return Double.compare(o1.getD(),o2.getD());
}
}
Then create your priority queue with this Comparator in the constructor
然后在构造函数中使用此 Comparator创建您的优先级队列
class kinga {
double d;
public kinga(double a) {
this.d = a;
}
public double getD() {
return this.d;
}
@Override
public String toString() {
return "kinga{" +
"d=" + d +
'}';
}
public static void main(String arg[]) {
PriorityQueue<kinga> r = new PriorityQueue<kinga>(11,new comparableKinga());
r.add(new kinga(4545.45));
r.add(new kinga(45.4));
r.add(new kinga(1235.45));
System.out.println(r.poll() + " " + r.poll() + " " + r.poll());
}
}
Outputis as expected
输出符合预期
kinga{d=45.4} kinga{d=1235.45} kinga{d=4545.45}
