条件OR运算符的短路行为（||） [英] short-circuiting behavior of conditional OR operator(||)

问题描述

有条件的算子和&&和||按照
进行短路 http：// docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html ，这意味着有时不需要评估第二个操作数。

Both conditional operators && and || are short-circuited as per
http://docs.oracle.com/javase/tutorial/java/nutsandbolts/op2.html, which means the second operand does not need to be evaluated at times.

有人可以提供一个示例，其中条件OR（||）运算符会被短路吗？

短路行为使用条件AND（&&）运算符非常简单：

Short-circuiting behavior is pretty straightforward with the conditional-AND(&&) operator as in:

if（false&&（1> 0））然后是第二个操作数：（1> 0）不需要进行评估，但似乎无法找到/想到条件OR的示例。

if(false && (1 > 0)) then the second operand:(1 >0) would not need to be evaluated but can't seem to find/think of an example for conditional-OR.

推荐答案

当第一个操作数为真时，或运算符被短路。所以，

The or operator is short-circuited when the first operand is true. So,

String foo = null;
if (true || foo.equals("")) {
    // ...
}

不会抛出 NullPointerException 。

正如@prajeesh正确在评论中指出，在实际代码中使用短路的方法是在处理可能返回null的API时阻止 NullPointerException 。因此，例如，如果有一个 readStringFromConsole 方法返回当前可用的字符串，或者如果用户没有输入任何内容，则返回null，我们可以写

As @prajeesh rightly points out in the comments, on way that short-circuiting is used in real code is to prevent a NullPointerException whenever you are dealing with an API that might return null. So, for instance, if there is a readStringFromConsole method that returns either the currently available string or null if the user doesn't type anything, we could write

String username = readStringFromConsole();
while (username == null || username.length() == 0) {
    // No NullPointerException on the while clause because the length() call
    // will only be made if username is not null

    System.out.println("Please enter a non-blank name");
    username = readStringFromConsole();
}

// Now do something useful with username, which is non-null and of nonzero length

作为旁注，返回用户输入的API应该在用户没有输入任何内容时返回空字符串，并且不应返回null。返回null是一种说没有任何可用的方式，而返回空字符串是一种说用户没有输入任何内容的方式，因此是首选。

As a side note, an API that returns user input should return the empty string whenever the user doesn't type anything, and shouldn't return null. Returning null is a way of saying "there is nothing available," while returning the empty string is a way of saying "the user didn't type anything" and so is preferred.

这篇关于条件OR运算符的短路行为（||）的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！