How to determine whether a variable is empty in Java

Preface

Java's StringUtil.isEmpty(str) and "".equals(str) are both methods used to determine whether a string is empty, but they have some different usage scenarios.

StringUtil.isEmpty()

StringUtil.isEmpty(str) is usually used to determine whether the string is null or has a length of 0. If the string is null or the length is 0, it returns true , otherwise return false. Using this approach, you can avoid null pointer exceptions caused by multiple null strings.

"".equals()

"".equals(str) is used to determine whether the string is an empty string (that is, the length is 0). If the string is an empty string, It returns true, otherwise it returns false. This method is suitable for situations where you only need to determine whether a string is an empty string.

It is recommended to use StringUtil.isEmpty(str) to judge the null value of multiple strings. It is recommended to use str.isEmpty() to determine whether a string is an empty string. In this way, you can choose a more appropriate method based on the actual situation, thereby improving the readability and efficiency of the code.

Determine whether multiple strings are empty at the same time

public static boolean isAllEmpty(String... strs) {
    for (String str : strs) {
        if (StringUtil.isNotEmpty(str)) {
            return false;
        }
    }
    return true;
}

In this method, we use variable parameters to receive multiple strings, and then traverse these strings through a loop, As long as one of the strings is found to be not empty, it returns false, indicating that not all of them are empty. Otherwise, true is returned after the loop ends, indicating that all strings are empty.

It should be noted that when judging whether it is empty, the StringUtil.isNotEmpty(str) method is used instead of the StringUtil.isEmpty(str) method. This is because when judging whether multiple strings are all empty, if you use the StringUtil.isEmpty(str) method, as long as one string is null or an empty string, true will be returned and other strings will not be judged. Whether it is empty. Therefore, it is more accurate to use the StringUtil.isNotEmpty(str) method. It only returns true when the string is not null and the length is not 0, which can avoid misjudgment.

Using this method can easily determine whether multiple strings are empty, avoid duplication of code, and improve the readability of the code.

When we need to determine whether multiple strings are all empty, we can use variable parameters to receive these strings. For example:

String str1 = "";
String str2 = null;
String str3 = "hello";
boolean result = isAllEmpty(str1, str2, str3);
System.out.println(result); // 输出false

In this example, we define three strings str1, str2, str3, where str1 is the empty string, str2 is null, and str3 is not empty. Next, we use the isAllEmpty method to check if all three strings are empty. If str3 is not empty, false will be returned when looping to str3, indicating that not all is empty.

If we set str3 to null or an empty string, then the loop will not find any string that is not empty until the end, and finally return true, indicating that all strings are empty. For example:

String str1 = "";
String str2 = null;
String str3 = "";
boolean result = isAllEmpty(str1, str2, str3);
System.out.println(result); // 输出true

In this example, we set both str1 and str2 to null or empty string, and only str3 is also empty, so the isAllEmpty method will return true after the loop ends, indicating that all strings are Is empty.

When we need to determine whether a string is null or has a length of 0, we can use the StringUtil.isEmpty(str) method. For example:

String str = "";
if (StringUtil.isEmpty(str)) {
    System.out.println("字符串为空");
} else {
    System.out.println("字符串不为空");
}

In this example, we define an empty string str, and then use the StringUtil.isEmpty(str) method to determine whether the string is empty. Since the length of the string is 0, the StringUtil.isEmpty(str) method will return true, indicating that the string is empty. Depending on the return value, we can perform different logic.

Another example:

String str = null;
if (StringUtil.isEmpty(str)) {
    System.out.println("字符串为空");
} else {
    System.out.println("字符串不为空");
}

In this example, we set the string str to null. At this time, the StringUtil.isEmpty(str) method will also return true, indicating that the string is empty. . This approach can avoid null pointer exceptions, because when the StringUtil.isEmpty(str) method is executed, even if the string is null, a null pointer exception will not be thrown.

It should be noted that this method is only suitable for situations where you need to determine whether a single string is empty. If you need to determine whether multiple strings are empty, you need to use variable parameters to determine. Please refer to the above example for details.

Even if the string is null, a null pointer exception will not be thrown when the StringUtil.isEmpty(str) method is called. When does a null pointer exception occur?

Null pointer exception usually occurs when a variable or object with a null value is used in a program. For example:

String str = null;
int length = str.length(); // 抛出空指针异常

In this example, we create a string named str and assign it null. Then we use the str.length() method to get the length of the string. At this time, a null pointer exception will be thrown, because when we use the method of the str object, we are actually using the method of an empty object.

Another example:

String[] strs = {"hello", "world", null};
for (String str : strs) {
    int length = str.length(); // 抛出空指针异常
}

In this example, we create an array strs holding strings where the value of the third element is null. Next, we will use a for loop to iterate through the array, and then for each string, call the str.length() method to get its length. Since the third element is null, a null pointer exception is thrown.

It should be noted that although the StringUtil.isEmpty(str) method can avoid null pointer exceptions, in some cases, we may need to determine whether the string is null. At this time, you can use str == null to make a judgment. For example:

String str = null;
if (str == null) {
    System.out.println("字符串为null");
} else if (StringUtil.isEmpty(str)) {
    System.out.println("字符串为空");
} else {
    System.out.println("字符串不为空");
}

在这个例子中，我们先使用str == null来判断该字符串是否为null，如果为null则输出"字符串为null"；否则使用StringUtil.isEmpty(str)方法来判断该字符串是否为空。通过这种方法，即可以预防出现空指针异常，又可以独立处理字符串为空和字符串为null的情况。

注意"“.equals(str)不要写成str.equals(”")，否则当str为null时，就会报错空指针异常。

两种判断，哪种性能更好？

就性能而言，使用StringUtil.isEmpty()相对更高效些。这是因为在判断一个字符串是否为空时，StringUtil.isEmpty()方法只需要进行一次判断，而使用"".equals()方法则需要进行两次判断，一次是判断字符串是否为null，另一次是判断字符串的长度是否为0。

虽然这个差距不大，但是在进行大量字符串判断的场景下，使用StringUtil.isEmpty()可以稍微提升一些性能。在实际开发中，更应该关注代码的可读性和易于维护性。虽然有一些性能差异，但它们微乎其微。

两种判断，哪种可读性和易于维护性都好

从可读性和易于维护性的角度来看，使用StringUtil.isEmpty()的方式更好一些，这是因为这个方法的意义更加清晰明了。使用"".equals()方法，虽然也可以达到同样的效果，但是需要写更多的代码来进行判断，不如直接调用一个专门的工具类方法更加简洁明了。

此外，如果后续需要修改判断条件，只需要在工具类方法中修改即可，而使用"".equals()方法则需要在多处修改，这样就增加了代码的维护难度。因此，在可读性和易于维护性方面，使用StringUtil.isEmpty()的方式更为优秀。

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