如何检查我的字符串是否等于空? [英] How to check if my string is equal to null?

问题描述

仅当我的字符串具有有意义的值时，我才想执行某些操作.所以，我试过了.

I want to perform some action ONLY IF my string has a meaningful value. So, I tried this.

if (!myString.equals("")) {
doSomething
}

还有这个

if (!myString.equals(null)) {
doSomething
}

还有这个

if ( (!myString.equals("")) && (!myString.equals(null))) {
doSomething
}

还有这个

if ( (!myString.equals("")) && (myString!=null)) {
doSomething
}

还有这个

if ( myString.length()>0) {
doSomething
}

在所有情况下，我的程序 doSomething 尽管我的字符串是空的.它等于 null.那么，这有什么问题?

And in all cases my program doSomething in spite on the fact that my string IS EMPTY. It equals to null. So, what is wrong with that?

添加:

我找到了问题的原因.该变量被声明为字符串，因此，分配给该变量的 null 被转换为 "null"！所以，if (!myString.equals("null")) 有效.

I found the reason of the problem. The variable was declared as a string and, as a consequence, null assigned to this variable was transformed to "null"! So, if (!myString.equals("null")) works.

推荐答案

if (myString != null && !myString.isEmpty()) {
  // doSomething
}

<小时>

作为进一步的评论，您应该了解 equals 合同中的这个术语:

来自 Object.equals(Object):

对于任何非空引用值x，x.equals(null) 应该return false.

For any non-null reference value x, x.equals(null) should return false.

与null比较的方法是使用x == null和x != null.

The way to compare with null is to use x == null and x != null.

此外，如果x == null，x.field 和x.method() 会抛出NullPointerException.

Moreover, x.field and x.method() throws NullPointerException if x == null.

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