如何在java中根据null检查字符串?

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如何在 java 中根据空值检查字符串?我正在使用

How can I check a string against null in java? I am using

stringname.equalsignorecase(null)

但它不起作用.

推荐答案

string == null 比较对象是否为空.string.equals("foo") 比较该对象内部的值.string == "foo" 并不总是有效，因为您试图查看对象是否相同，而不是它们所代表的值.

string == null compares if the object is null. string.equals("foo") compares the value inside of that object. string == "foo" doesn't always work, because you're trying to see if the objects are the same, not the values they represent.

更长的答案:

Longer answer:

如果您尝试此操作，它将不起作用，正如您所发现的:

If you try this, it won't work, as you've found:

String foo = null; if (foo.equals(null)) { // That fails every time. }

原因是foo为null，所以不知道.equals是什么；那里没有可以调用 .equals 的对象.

The reason is that foo is null, so it doesn't know what .equals is; there's no object there for .equals to be called from.

您可能想要的是:

String foo = null; if (foo == null) { // That will work. }

在处理字符串时保护自己免受空值的典型方法是:

The typical way to guard yourself against a null when dealing with Strings is:

String foo = null; String bar = "Some string"; ... if (foo != null && foo.equals(bar)) { // Do something here. }

这样，如果 foo 为 null，它就不会评估条件的后半部分，一切正常.

That way, if foo was null, it doesn't evaluate the second half of the conditional, and things are all right.

如果您使用的是字符串文字(而不是变量)，最简单的方法是:

The easy way, if you're using a String literal (instead of a variable), is:

String foo = null; ... if ("some String".equals(foo)) { // Do something here. }

如果你想解决这个问题，Apache Commons 有一个类 - StringUtils - 提供空安全的字符串操作.

If you want to work around that, Apache Commons has a class - StringUtils - that provides null-safe String operations.

if (StringUtils.equals(foo, bar)) { // Do something here. }

另一个回应是在开玩笑，说你应该这样做:

Another response was joking, and said you should do this:

boolean isNull = false; try { stringname.equalsIgnoreCase(null); } catch (NullPointerException npe) { isNull = true; }

请不要那样做.你应该只对异常的错误抛出异常；如果你期待一个空值，你应该提前检查它，而不是让它抛出异常.

Please don't do that. You should only throw exceptions for errors that are exceptional; if you're expecting a null, you should check for it ahead of time, and not let it throw the exception.

在我看来，这有两个原因.首先，异常很慢；检查 null 很快，但是当 JVM 抛出异常时，它需要很多时间.其次，如果你只是提前检查空指针，代码会更容易阅读和维护.

In my head, there are two reasons for this. First, exceptions are slow; checking against null is fast, but when the JVM throws an exception, it takes a lot of time. Second, the code is much easier to read and maintain if you just check for the null pointer ahead of time.