为什么不能在Java 8 lambda表达式中抛出异常？

问题描述

我升级到Java 8，并试图用新的lamdba表达式通过Map替换简单的迭代。循环搜索空值，如果找到空值则抛出异常。旧的Java 7代码如下所示：

I upgraded to Java 8 and tried to replace a simple iteration through a Map with a new lamdba expression. The loop searches for null values and throws an exception if one is found. The old Java 7 code looks like this:

for (Map.Entry<String, String> entry : myMap.entrySet()) {
    if(entry.getValue() == null) {
        throw new MyException("Key '" + entry.getKey() + "' not found!");
    }
}

我尝试将其转换为Java 8看起来像这样：

And my attempt to convert this to Java 8 looks like this:

myMap.forEach((k,v) -> {
    if(v == null) {
        // OK
        System.out.println("Key '" + k+ "' not found!");

        // NOK! Unhandled exception type!
        throw new MyException("Key '" + k + "' not found!");
    }
});

任何人都可以解释为什么 throw 允许这里，这可以如何纠正？

Can anyone explain why the throw statement not allowed here and how this could be corrected?

Eclipse的快速修复建议对我来说不正确，它只是围绕 throw 语句与 try-catch 块：

Eclipse's quick-fix suggestion does not look right to me... it simply surrounds the throw statement with a try-catch block:

myMap.forEach((k,v) -> {
    if(v == null) {
        try {
            throw new MyException("Key '" + k + "' not found!");
        }
        catch (Exception e) {
            e.printStackTrace();
        }
    }
});

推荐答案

您不能抛出检出的异常，因为 void accept（T t，U u）方法在 BiConsumer< T，U> 接口中不会抛出任何异常。而且，如你所知， forEach 需要这样的类型。

You are not allowed to throw checked exceptions because the void accept(T t, U u) method in the BiConsumer<T, U> interface doesn't throw any exceptions. And, as you know, forEach takes such type.

public void forEach(BiConsumer<? super K, ? super V> action) { ... }
                         |
                         V
@FunctionalInterface
public interface BiConsumer<T, U> {
    void accept(T t, U u); // <-- does throw nothing
}

当我们说话时关于检查的例外。但是您可以抛出一个未经检查的异常，例如，一个 IllegalArgumentException ：

That's true when we're talking about checked exceptions. But you are allowed to throw an unchecked exception, for instance, an IllegalArgumentException:

new HashMap<String, String>()
    .forEach((a, b) -> { throw new IllegalArgumentException(); });

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