如何抓取ClassCastException？ [英] How do I catch ClassCastException?

问题描述

所以，

  try {
 restoredItem =（T）decode.readObject（）; 
} catch（ClassCastException e）{
 //此时不需要崩溃，
 //只是让用户知道错误的文件已被传递。 
} 
  

但是这不会因为异常被捕获。你会建议什么？

解决方案

问题中的代码应该给你一个未经检查的投射警告。收听-Xlint。

所有的编译器知道T是它的边界，它可能没有（除了显式扩展Object和超类型的null类型）。所以有效地运行时的转换是（Object） - 不是非常有用。

你可以做的是传递参数类型的类的一个实例（假设它是't generic）。

  class MyReader< T> {
 private final Class< T> clazz中; 
 MyReader（Class< T> clazz）{
 if（clazz == null）{
 throw new NullPointerException（）; 
} 
 this.clazz = clazz; 
} 
 public T restore（String from）{
 ... 
 try {
 restoredItem = clazz.cast（decoder.readObject（））; 
 ... 
 return restoredItem; 
} catch（ClassCastException exc）{
 ... 
} 
} 
} 
  

或作为通用方法：

  public< T> T恢复（类< T> clazz，String from）{
 ... 
 try {
 restoredItem = clazz.cast（decoder.readObject（））; 
 ... 
  

I'm trying to catch a ClassCastException when deserializing an object from xml.

So,

try {
    restoredItem = (T) decoder.readObject();
} catch (ClassCastException e){
    //don't need to crash at this point,
   //just let the user know that a wrong file has been passed.
}

And yet this won't as the exception doesn't get caught. What would you suggest?

解决方案

The code in the question should give you an unchecked cast warning. Listen to -Xlint.

All the compiler knows about T is its bounds, which it probably doesn't have (other than explicitly extending Object and a super of the null type). So effectively the cast at runtime is (Object) - not very useful.

What you can do is pass in an instance of the Class of the parameterised type (assuming it isn't generic).

class MyReader<T> {
    private final Class<T> clazz;
    MyReader(Class<T> clazz) {
        if (clazz == null) {
            throw new NullPointerException();
        }
        this.clazz = clazz;
    }
    public T restore(String from) {
        ...
        try {
            restoredItem = clazz.cast(decoder.readObject());
            ...
            return restoredItem;
        } catch (ClassCastException exc) {
            ...
        }
    }
}

Or as a generic method:

    public <T> T restore(Class<T> clazz, String from) {
        ...
        try {
            restoredItem = clazz.cast(decoder.readObject());
            ...

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