java 如何比较if-else语句中的字符串？
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How to compare Strings in if-else statements?
提问by Gary Deuces Rozanski
I wonder if anyone could give me a pointer with some pre-population in JSP:
我想知道是否有人可以给我一个在 JSP 中预先填充的指针：
So I am attempting to pre-populate a checkbox depending on the value in the customers database record. The checkbox would be checked if there is a 1 in the field, the other option would be a 0 - where the checkbox should be empty.
所以我试图根据客户数据库记录中的值预先填充一个复选框。如果字段中有 1，则复选框将被选中，另一个选项将是 0 - 复选框应为空。
I am currently using this code within the input:
我目前在输入中使用此代码：
<%if (UPDATEDPREFERENCES != null)
{
if (ISNEWSLETTER != "0") {
out.println("checked");
}
}
%>
so it would be:
所以它会是：
<input type='checkbox' class="newsletter" name='temp_ISNEWSLETTER_FIELD' id="temp_ISNEWSLETTER_FIELD" <%if (UPDATEDPREFERENCES != null)
{
if (ISNEWSLETTER != "0") {
out.println("checked");
}
}
%>/>
As it stands, it pre-populates fine. However, it also populates if the field is 0. If I change it to read:
就目前而言，它预填充得很好。但是，如果该字段为 0，它也会填充。如果我将其更改为：
if (ISNEWSLETTER == "1")
then the checkbox is empty whether it's a 1 or 0.
然后复选框是空的，无论是 1 还是 0。
Am I missing something obvious? I am not a developer, I am in NYC and my support is in Belgium - they've all left for the day.
我错过了一些明显的东西吗？我不是开发人员，我在纽约，我的支持在比利时 - 他们都离开了。
回答by BalusC
Stringsare objects, not primitives. Use equals(), not ==/!=. The ==/!=won't check if they contain the same value, but if they point to the same object reference. The equals()will check if they contain the same value.
字符串是对象，而不是原语。使用equals()，而不是==/ !=。该==/!=不会检查它们是否包含相同的值，但如果它们指向同一个对象的引用。该equals()会检查它们是否包含相同的值。
So, instead of if (ISNEWSLETTER != "0")you should use
所以，而不是if (ISNEWSLETTER != "0")你应该使用
if (!ISNEWSLETTER.equals("0"))
and instead of if (ISNEWSLETTER == "1")you should use
而不是if (ISNEWSLETTER == "1")你应该使用
if (ISNEWSLETTER.equals("1"))
This problem is not related to JSP. You would have exactly the same problem when doing so in a normal Java class (where this kind of code actuallybelongs).
此问题与 JSP 无关。在普通 Java 类（这种代码实际上属于其中）中执行此操作时，您会遇到完全相同的问题。
As an alternative, since they represent numbers, you could also just convert ISNEWSLETTERto an int. It's a primitive, so you should be able to use ==/!=the usual way. You can if necessary convert a Stringto an intusing Integer#parseInt(). Or, even more, if it actually represents a boolean state (it can only be trueor false), then you should rather use booleaninstead. You can just use it straight in the ifcondition without any need for comparisons.
作为替代方案，由于它们代表数字，您也可以将其转换ISNEWSLETTER为int. 这是一个原语，所以你应该能够使用==/!=通常的方式。如有必要，您可以将 a 转换String为intusing Integer#parseInt()。或者，甚至更多，如果它实际上表示一个布尔状态（它只能是true或false），那么您应该boolean改为使用。您可以直接在if条件下使用它，而无需进行任何比较。
