如何在java中检查字符串中的null？

Question

如何在java中检查字符串中的null？我在用

stringname.equalsignorecase(null)

但它不起作用.

Answer 1

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.

Longer answer:

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

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

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.

What you probably wanted was:

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

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.
}

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

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.
}

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.
}

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

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

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,有两个原因.首先,例外情况很慢; 检查null很快,但是当JVM抛出异常时,需要花费很多时间.其次,如果您只是提前检查空指针,则代码更容易阅读和维护.

Answer 2

s == null

不行吗？

Answer 3

当然有效.你错过了代码的重要部分.你只需要这样做:

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

;)

Answer 4

使用TextUtils方法.

TextUtils.isEmpty(str):如果字符串为null或0-length,则返回true.参数:str要检查的字符串返回:如果str为null或零长度,则返回true

  if(TextUtils.isEmpty(str)) {
        // str is null or lenght is 0
    }

以下是此方法的源代码

 /**
     * Returns true if the string is null or 0-length.
     * @param str the string to be examined
     * @return true if str is null or zero length
     */
    public static boolean isEmpty(CharSequence str) {
        if (str == null || str.length() == 0)
            return true;
        else
            return false;
    }

Answer 5

如果我们看一下equalsIgnoreCase方法的实现,我们会发现这个部分:

if (string == null || count != string.count) {
    return false;
}

因此,false如果参数是,它将始终返回null.这显然是正确的,因为它应该返回的唯一情况true是在a上调用equalsIgnoreCase null String,但是

String nullString = null;
nullString.equalsIgnoreCase(null);

肯定会导致NullPointerException.

因此equals方法不是为了测试对象是否为null而设计的,只是因为你无法调用它们null.