关于List子类调用subList(int from,int end)出现类型转换异常

本文详细阐述了在Java中使用List子类时,调用subList函数可能遇到的ClassCastException问题,解释了原因并提供了修正方法。

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发现List子类在调用subList(int from,int end)函数时出现java.lang.ClassCastException: java.util.SubList cannot be cast to java.util.LinkedList异常,遂将这部分功能提出出来做测试,测试代码如下:
public class TestJavaDemo{
   
    public static void main(String[] args) {
        LinkedList<Integer> list=new LinkedList<Integer>();
        for(int i=0;i<10;i++){
            list.add(i);
        }
        list=(LinkedList<Integer>) list.subList(0, 8);
        for(int temp:list){
            System.out.println(temp+"\t");
        }
    }
}
运行程序,出现以下异常:
java.lang.ClassCastException: java.util.SubList cannot be cast to java.util.LinkedList,本人以为LinkedList即是List子类,则应该可以调用该方法的啊,却出现这样的异常,遂上网寻找答案。原来subList函数返回的是List类型,强制转换的过程中,会随即的转换为其任意的一个子类,若转换成的子类不是LinkedList类型,则会出现上述的类型转换错误。所以在以为的强转过程中,尤须注意。
可以做如下修改:
//list=(LinkedList<Integer>) list.subList(0, 8);
-->修改为:List<Integer> listBuffer=list.subList(0, 8);
或者list=new LinkedList<Integer>(list.subList(0, 8));
package com.example.kucun2.entity.data; import android.annotation.SuppressLint; import android.util.Log; import com.google.gson.Gson; import java.beans.PropertyChangeEvent; import java.beans.PropertyChangeListener; import java.lang.reflect.InvocationTargetException; import java.util.ArrayList; import java.util.Collection; import java.util.Iterator; import java.util.List; import java.util.ListIterator; /** * 支持数据变更自动同步的泛型集合类 * 基于SynchronizableEntity的事件机制实现数据变更自动同步 * * @param <T> 必须是SynchronizableEntity或其子类 */ public class SynchronizedList<T extends SynchronizableEntity> implements List<T> { // 使用ArrayList作为底层存储 private final List<T> list = new ArrayList<>(); public Class<T> getEntityType() { return entityType; } private Class<T> entityType; public SynchronizedList(Class<T> T ) { this.entityType = T; } // 实体变更监听器 private final EntityChangeListener<T> entityChangeListener = new EntityChangeListener<>(); @Override public int size() { return list.size(); } @Override public boolean isEmpty() { return list.isEmpty(); } @Override public boolean contains(Object o) { return list.contains(o); } @Override public Iterator<T> iterator() { return new SyncedIterator<>(list.iterator(), entityChangeListener); } @Override public Object[] toArray() { return list.toArray(); } @Override public <E> E[] toArray(E[] a) { return list.toArray(a); } // 添加元素时注册监听器 @SuppressLint("SuspiciousIndentation") @Override public boolean add(T element) { if (element != null) { element.addPropertyChangeListener(entityChangeListener); } return list.add(element); } // 移除元素时注销监听器 @Override public boolean remove(Object o) { if (o instanceof SynchronizableEntity) { ((SynchronizableEntity) o).removePropertyChangeListener(entityChangeListener); } return list.remove(o); } @Override public boolean containsAll(Collection<?> c) { return list.containsAll(c); } // 批量添加时逐个注册监听器 @Override public boolean addAll(Collection<? extends T> c) { for (T element : c) { if (element != null) { element.addPropertyChangeListener(entityChangeListener); } } return list.addAll(c); } @Override public boolean addAll(int index, Collection<? extends T> c) { for (T element : c) { if (element != null) { element.addPropertyChangeListener(entityChangeListener); } } return list.addAll(index, c); } // 批量移除时逐个注销监听器 @Override public boolean removeAll(Collection<?> c) { for (Object o : c) { if (o instanceof SynchronizableEntity) { ((SynchronizableEntity) o).removePropertyChangeListener(entityChangeListener); } } return list.removeAll(c); } @Override public boolean retainAll(Collection<?> c) { // 先移除未保留元素的监听器 for (T element : list) { if (!c.contains(element)) { element.removePropertyChangeListener(entityChangeListener); } } return list.retainAll(c); } // 清空集合时移除所有监听器 @Override public void clear() { for (T element : list) { element.removePropertyChangeListener(entityChangeListener); } list.clear(); } @Override public T get(int index) { return list.get(index); } // 替换元素时更新监听器 @Override public T set(int index, T element) { T oldElement = list.get(index); if (oldElement != null) { oldElement.removePropertyChangeListener(entityChangeListener); } if (element != null) { element.addPropertyChangeListener(entityChangeListener); } return list.set(index, element); } // 在指定位置添加元素时注册监听器 @Override public void add(int index, T element) { if (element != null) { element.addPropertyChangeListener(entityChangeListener); } list.add(index, element); } // 移除元素时注销监听器 @Override public T remove(int index) { T removed = list.remove(index); if (removed != null) { removed.removePropertyChangeListener(entityChangeListener); } return removed; } @Override public int indexOf(Object o) { return list.indexOf(o); } @Override public int lastIndexOf(Object o) { return list.lastIndexOf(o); } @Override public ListIterator<T> listIterator() { return new SyncedListIterator<>(list.listIterator(), entityChangeListener); } @Override public ListIterator<T> listIterator(int index) { return new SyncedListIterator<>(list.listIterator(index), entityChangeListener); } @Override public List<T> subList(int fromIndex, int toIndex) { return list.subList(fromIndex, toIndex); } /** * 内部实体变更监听器 * 监听属性变化并触发同步操作 */ private static class EntityChangeListener<T extends SynchronizableEntity> implements PropertyChangeListener { @Override public void propertyChange(PropertyChangeEvent evt) { @SuppressWarnings("unchecked") T entity = (T) evt.getSource(); entity.sync(); // 触发同步操作 } } /** * 增强的Iterator实现 * 移除元素时自动注销监听器 */ private static class SyncedIterator<T extends SynchronizableEntity> implements Iterator<T> { private final Iterator<T> iterator; private final EntityChangeListener<T> listener; private T current; public SyncedIterator(Iterator<T> iterator, EntityChangeListener<T> listener) { this.iterator = iterator; this.listener = listener; } @Override public boolean hasNext() { return iterator.hasNext(); } @Override public T next() { current = iterator.next(); return current; } @Override public void remove() { if (current != null) { current.removePropertyChangeListener(listener); } iterator.remove(); } } /** * 增强的ListIterator实现 * 支持添加/设置元素时的监听器管理 */ private static class SyncedListIterator<T extends SynchronizableEntity> implements ListIterator<T> { private final ListIterator<T> iterator; private final EntityChangeListener<T> listener; private T lastReturned; public SyncedListIterator(ListIterator<T> iterator, EntityChangeListener<T> listener) { this.iterator = iterator; this.listener = listener; } @Override public boolean hasNext() { return iterator.hasNext(); } @Override public T next() { lastReturned = iterator.next(); return lastReturned; } @Override public boolean hasPrevious() { return iterator.hasPrevious(); } @Override public T previous() { lastReturned = iterator.previous(); return lastReturned; } @Override public int nextIndex() { return iterator.nextIndex(); } @Override public int previousIndex() { return iterator.previousIndex(); } @Override public void remove() { if (lastReturned != null) { lastReturned.removePropertyChangeListener(listener); } iterator.remove(); } @Override public void set(T e) { if (lastReturned != null) { lastReturned.removePropertyChangeListener(listener); } if (e != null) { e.addPropertyChangeListener(listener); } iterator.set(e); lastReturned = e; } @Override public void add(T e) { if (e != null) { e.addPropertyChangeListener(listener); } iterator.add(e); } } }package com.example.kucun2.entity.data; import android.util.Log; import com.example.kucun2.entity.Information; import com.example.kucun2.function.MyAppFnction; import java.beans.PropertyChangeListener; import java.beans.PropertyChangeSupport; import java.util.HashMap; import java.util.Map; public abstract class SynchronizableEntity implements EntityClassGrassrootsid { public boolean isPreservedObject() { return this.getId() != null && this.getId() == -1; } public enum SyncState { NEW, MODIFIED, DELETED ,PRESERVED} SyncState state = SyncState.NEW; private static boolean syncEnabled = true; public static void setSyncEnabled(boolean enabled) { syncEnabled = enabled; } // 设置状态的方法 public void setState(SyncState state) { this.state = state; } // 使用标准属性变更支持类 private transient PropertyChangeSupport changeSupport = new PropertyChangeSupport(this); // 属性缓存(可选,用于优化) private final Map<String, Object> propertyCache = new HashMap<>(); // 添加属性变更监听器 public void addPropertyChangeListener(PropertyChangeListener listener) { if (changeSupport == null) { changeSupport = new PropertyChangeSupport(this); } changeSupport.addPropertyChangeListener(listener); } // 移除属性变更监听器 public void removePropertyChangeListener(PropertyChangeListener listener) { if (changeSupport != null) { changeSupport.removePropertyChangeListener(listener); } } // 触发属性变更通知(子类在setter中调用) protected void firePropertyChange(String propertyName, Object oldValue, Object newValue) { if (changeSupport != null && changeSupport.hasListeners(propertyName)) { // 更新属性缓存(可选) propertyCache.put(propertyName, newValue); // 触发变更事件 changeSupport.firePropertyChange(propertyName, oldValue, newValue); } } // 获取属性当前值(可选,用于setter中获取旧值) protected <T> T getCachedProperty(String propertyName) { return (T) propertyCache.get(propertyName); } // 初始化属性缓存(可选,在构造函数中调用) protected void initPropertyCache() { // 初始化所有字段到缓存 for (java.lang.reflect.Field field : getClass().getDeclaredFields()) { try { field.setAccessible(true); propertyCache.put(field.getName(), field.get(this)); } catch (Exception e) { // 忽略初始化异常 } } } /** * 添加url * @param type 操作增删改查 * @return 返回相对url */ public String getEndpoint(String type){ //从String.xml获取url Log.d("getEndpoint", "getEndpoint: "+"url_"+type+"_"+this.getClass().getSimpleName().toLowerCase()); return MyAppFnction.getStringResource("string","url_"+type+"_"+this.getClass().getSimpleName().toLowerCase()); } public void sync() { if (!syncEnabled) return; // 全局控制开关 if (this.state == null) return; try { switch (this.state) { case NEW: createToServer(); break; case MODIFIED: updateToServer(); break; case DELETED: deleteFromServer(); break; } } catch (Exception e) { Log.e("SyncError", "Sync failed for " + getClass().getSimpleName(), e); } } /** * 创建实体到后端 */ private void createToServer() { String endpoint = MyAppFnction.getStringResource("string", "url") + getEndpoint("add"); Information<SynchronizableEntity> requestData = new Information<SynchronizableEntity>(); // 使用Volley、Retrofit或OkHttp执行POST请求 // 伪代码示例: ApiClient.post(endpoint, requestData, new ApiClient.ApiCallback<SynchronizableEntity>() { @Override public void onSuccess(Information<SynchronizableEntity> e) { // 解析服务器返回的ID Log.d("onSuccess",e.toJson()); if (e.getData() != null) { setId(e.getData().getId()); // 设置服务器返回的ID } state = SyncState.MODIFIED; state = SyncState.MODIFIED; } }); } /** * 更新实体到后端 */ private void updateToServer() { String endpoint = MyAppFnction.getStringResource("string", "url") + getEndpoint("update") + "/" + getId(); // 获取实体ID Information<SynchronizableEntity> requestData = new Information<SynchronizableEntity>(); ApiClient.post(endpoint, requestData, new ApiClient.ApiCallback<SynchronizableEntity>() { @Override public void onSuccess(Information<SynchronizableEntity> e) { // 解析服务器返回的ID Log.d("onSuccess",e.toJson()); // String serverId = parseServerId(response); // if (serverId != null) { // // 更新本地ID // setId(serverId); // 需要实体实现setId方法 // } // state = SyncState.MODIFIED; } }); } /** * 从后端删除实体 */ private void deleteFromServer() { String endpoint = MyAppFnction.getStringResource("string", "url") + getEndpoint("delete") + "/" + getId(); ApiClient.delete(endpoint, new ApiClient.ApiCallback<SynchronizableEntity>() { @Override public void onSuccess(Information<SynchronizableEntity> e) { Log.d("SyncDelete", "Successfully deleted " + getId()); } }); } /** * 将实体转换为请求映射 */ protected Information<SynchronizableEntity> toRequestMap(){ return Information.newSuccess(this); } } 数据同步
最新发布
06-13
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