模板对象的Java 8功能构造函数 [英] Java 8 functional constructor from templated object

问题描述

我正在使用Eclipse Luna服务版本2（4.4.2），Java 8 u51。

我试图创建一个方法来创建一个基于另一个方法参数传递对象。原型被简化为

  public< T> T test（Object param，T instance）{
构造函数<？>构造函数= instance.getClass（）。getConstructors（）[0]; //我实际上选择了一个合适的构造函数
 
 // eclipse reports未处理的异常类型InvocationTargetException
函数< Object，Object> createFun = constructor :: newInstance; 
 
 T result =（T）createFun.apply（param）; 
返回结果; 
 
  

符合函数声明eclipse报告未处理的异常类型InvocationTargetException 编译器错误。我需要 Function 以稍后在流中使用。

我尝试添加各种try / catch块，抛出声明，但没有修复此编译器错误。如何使这段代码工作？

解决方案

你不能从一个函数目标类型，因为它的 apply 方法不会引发异常。所以你需要把它变成一个未经检查的异常，例如通过包装它：

 函数< Object，Object> createFun = o  - > {
尝试{
 return constructor.newInstance（o）; 
 catch（InstantiationException | InvocationTargetException | IllegalAccessException e）{
抛出new RuntimeException（e）; 
} 
}; 
  

另一种方法是引导编译器认为它是一个未经检查的异常，它产生一个更干净的堆栈跟踪vs 。上面的选项：

 函数< Object，Object> createFun = o  - > {
尝试{
 return constructor.newInstance（o）; 
} catch（InstantiationException | InvocationTargetException | IllegalAccessException e）{
 return uncheck（e）; 
} 
}; 
  

使用以下实用程序方法：

  @SuppressWarnings（unchecked）
 public static< E extends Throwable，T> T uncheck（Throwable t）抛出E {
 throw（（E）t）; 
} 
  

I am using Eclipse Luna Service Release 2 (4.4.2), Java 8 u51.

I'm trying to create a method which will create instances of a passed object based on another method parameter. The prototype is simplified to

public <T> T test(Object param, T instance) {
    Constructor<?> constructor = instance.getClass().getConstructors()[0]; // I actually choose a proper constructor

    // eclipse reports "Unhandled exception type InvocationTargetException"
    Function<Object, Object> createFun = constructor::newInstance;

    T result = (T) createFun.apply(param);
    return result;
}

On line with Function declaration eclipse reports Unhandled exception type InvocationTargetException compiler error. I need the Function to later use in a stream.

I tried to add various try/catch blocks, throws declarations, but nothing fixes this compiler error.

How to make this code work?

解决方案

You can't throw a checked exception from a lambda with a Function target type because its apply method does not throw an exception. So you need to make it into an unchecked exception, for example by wrapping it:

Function<Object, Object> createFun = o -> {
  try {
    return constructor.newInstance(o);
  } catch (InstantiationException | InvocationTargetException | IllegalAccessException e) {
    throw new RuntimeException(e);
  }
};

An alternative is to lead the compiler to think it's an unchecked exception, which produces a cleaner stack trace vs. the option above:

Function<Object, Object> createFun = o -> {
  try {
    return constructor.newInstance(o);
  } catch (InstantiationException | InvocationTargetException | IllegalAccessException e) {
    return uncheck(e);
  }
};

With the following utility method:

@SuppressWarnings("unchecked")
public static <E extends Throwable, T> T uncheck(Throwable t) throws E {
  throw ((E) t);
}

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