使用可选的空检查 [英] Null check using Optional

问题描述

我想使用Optional Utility在JDK8中执行空检查.这是我正在编写的代码，给我一个错误:

I want to perform the null check in JDK8 using Optional utility. Here is my code I am writing which giving me an error:

java.util.Optional stringToUse = java.util.Optional.of(childPage.getContentResource().getValueMap().get("jcr:description").toString());
stringToUse.ifPresent(description = stringToUse);

此处可以显示"jcr:description".如果存在，我想在description变量中使用该值，如果为null，则只需设置空白String作为描述.也可以在这里使用Lambda表达式吗?谢谢

Here "jcr:description" can be present or not. And if its present I want to use that value in description variable and if null the simply set blank String for description. Also can Lambda expression also can be use here? Thanks

推荐答案

如果get("jcr:description")的结果可以为null，则不应在其上调用toString()，因为没有任何内容，Optional可以如果使用前的操作已经失败并出现NullPointerException.

If the result of get("jcr:description") can be null, you shouldn’t invoke toString() on it, as there is nothing, Optional can do, if the operation before its use already failed with a NullPointerException.

您想要的，可以使用:

Optional<String> stringToUse = Optional.ofNullable(
    childPage.getContentResource().getValueMap().get("jcr:description")
).map(Object::toString);

然后您可以将其用作

if(stringToUse.isPresent())
    description = stringToUse.get();

如果不执行任何操作"是不存在该值的预期操作.或者，您可以为这种情况指定一个后备值:

if "do nothing" is the intended action for the value not being present. Or you can specify a fallback value for that case:

description = stringToUse.orElse("");

然后，总是使用jcr:description的字符串表示形式或空字符串来分配description.

then, description is always assigned, either with the string representation of jcr:description or with an empty string.

如果description不是局部变量，而是字段，则可以使用stringToUse.ifPresent(string -> description = string);.但是，我不建议这样做.

You can use stringToUse.ifPresent(string -> description = string);, if description is not a local variable, but a field. However, I don’t recommend it.

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