将HashMap值添加到TreeSet时出错

问题描述

Here is some sample code...I always seem to get a ClassCastException...anybody point out what I am doing wrong?

c $ c> package com.query;

import java.util.ArrayList;
import java.util.Collection;
import java.util.HashMap;
import java.util.Map;
import java.util.Set;
import java.util.TreeSet;

import org.junit.Test;

导入com.google.common.collect.Sets;


public class ClassCastExceptionTest {

@Test
public void test（）{

A< SomeType> a1 = new A< SomeType>（1）;
A< SomeType> a2 = new A< SomeType>（2）;
A< SomeType> a3 = new A< SomeType>（3）;

Map< String，A< SomeType>> map = new HashMap< String，A< SomeType>>（）;
map.put（A1，a1）;
map.put（A2，a2）;
map.put（A3，a3）;

收集< A< SomeType>> coll = map.values（）;

Set< A< SomeType>> set = Sets.newTreeSet（coll）;

System.out.println（完成。）;

// EXCEPTION ...
//com.query.ClassCastExceptionTest$A不能转换为
//com.query.ClassCastExceptionTest$BaseType
}

私有类A< T扩展了BaseType>延伸Base< T> {

public A（int i）{
super（i）;
}
}

私有类Base< T扩展了BaseType>实现可比< T> {

私人整数ID;

public Base（int id）{
this.id = id;
}

/ **
* @return the id
* /
public Integer getId（）{
return id;

$ b $ **
* @param id设置
* /
的标识b @SuppressWarnings（未使用）
public void setId（int id）{
this.id = id;
}

@Override
public int compareTo（T o）{
return getId（）。compareTo（o.getId（））;


$ b private class SomeType extends BaseType {
$ b $ @Override
public Integer getId（）{
return 0 ;
}

@Override
public int compareTo（BaseType o）{
return this.getId（）。compareTo（o.getId（））;
}
}

私有抽象类BaseType实现了Comparable< BaseType> {

public abstract Integer getId（）;
}

}

package com.query; import java.util.ArrayList; import java.util.Collection; import java.util.HashMap; import java.util.Map; import java.util.Set; import java.util.TreeSet; import org.junit.Test; import com.google.common.collect.Sets; public class ClassCastExceptionTest { @Test public void test() { A<SomeType> a1 = new A<SomeType>(1); A<SomeType> a2 = new A<SomeType>(2); A<SomeType> a3 = new A<SomeType>(3); Map<String, A<SomeType>> map = new HashMap<String, A<SomeType>>(); map.put("A1", a1); map.put("A2", a2); map.put("A3", a3); Collection<A<SomeType>> coll = map.values(); Set<A<SomeType>> set = Sets.newTreeSet(coll); System.out.println("Done."); //EXCEPTION... //com.query.ClassCastExceptionTest$A cannot be cast to //com.query.ClassCastExceptionTest$BaseType } private class A<T extends BaseType> extends Base<T> { public A(int i) { super(i); } } private class Base<T extends BaseType> implements Comparable<T> { private Integer id; public Base(int id) { this.id = id; } /** * @return the id */ public Integer getId() { return id; } /** * @param id the id to set */ @SuppressWarnings("unused") public void setId(int id) { this.id = id; } @Override public int compareTo(T o) { return getId().compareTo(o.getId()); } } private class SomeType extends BaseType { @Override public Integer getId() { return 0; } @Override public int compareTo(BaseType o) { return this.getId().compareTo(o.getId()); } } private abstract class BaseType implements Comparable<BaseType> { public abstract Integer getId(); } }

推荐答案

private class Base<T extends BaseType> implements Comparable<Base> { ... }

而在 Base 与 T ：

private class Base<T extends BaseType> implements Comparable<T> { ... }

这篇关于将HashMap值添加到TreeSet时出错的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持！