[英]What's wrong with my wildcard implementation?
我有3个类(PriorityQueue,Assignment,AssignmentLog)和1个接口(IPriorityQueue)。
public interface IPriorityQueue<T extends Comparable<? super T>> {
void add(T newEntry);
T remove();
T peek();
boolean isEmpty();
int getSize();
void clear(); }
PriorityQueue类:
public class PriorityQueue<T extends Comparable<? super T>> implements IPriorityQueue<T> {
private T[] priorityQueue;
private int frontIndex = 0;
private int backIndex = 0;
private static final int DEFAULT_SIZE= 50;...
}
Assignment.java:
public class Assignment implements Comparable<Assignment> {
private String course;
private String task;
private Date dueDate;
@Override
public int compareTo(Assignment other) {
return -dueDate.compareTo(other.dueDate);
}}
AssignmentLog.java:
public class AssignmentLog {
private IPriorityQueue<Assignment> log;
public AssignmentLog(){
log = new PriorityQueue<>();
}
public void addProject(Assignment newAssignment){
log.add(newAssignment);
}
public Assignment getProject(){
return log.peek();
}
我的问题是即使IDE无法识别任何错误,当我运行程序时它会显示异常:
Exception in thread "main" java.lang.ClassCastException: class [Ljava.lang.Object; cannot be cast to class [Ljava.lang.Comparable; ([Ljava.lang.Object; and [Ljava.lang.Comparable; are in module java.base of loader 'bootstrap')
at PriorityQueue.<init>(PriorityQueue.java:13)
at PriorityQueue.<init>(PriorityQueue.java:9)
at AssignmentLog.<init>(AssignmentLog.java:5)
at Main.main(Main.java:13)
第13行: public PriorityQueue(int size){ priorityQueue = (T[])new Object[size]; }
public PriorityQueue(int size){ priorityQueue = (T[])new Object[size]; }
第10行: public PriorityQueue(){ this(DEFAULT_SIZE); }
public PriorityQueue(){ this(DEFAULT_SIZE); }
第5行: public AssignmentLog(){ log = new PriorityQueue<>(); }
public AssignmentLog(){ log = new PriorityQueue<>(); }
第13行: AssignmentLog myHomework = new AssignmentLog();
请解释一下我的任务有什么问题。 我仍然是外卡的新手,所以请给我一些建议
谢谢
您正在将Object[]
向下转换为T[]
。 在Java中向下转换为不是子类的类会抛出ClassCastException
的实例。
从Javadoc for ClassCastException
:
抛出以指示代码已尝试将对象强制转换为不是实例的子类。 例如,以下代码生成ClassCastException:
Object x = new Integer(0); System.out.println((String)x);
在行priorityQueue = (T[])new Object[size];
,当您尝试将Object[]
为T[]
,Java会进行类型检查。 由于Object
不扩展T
,因此转换失败。 但是,您必须记住,除Object
之外的每个类都隐式扩展Object
。
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