Preface
Recently, when I was developing, I needed to calculate the length of a String array after splicing. I originally planned to write a simple tool class myself to calculate the length. After a colleague reminded me, I found that there is such a useful tool class
.
method one
public static int calculateStrJoinLengthOfListMethod1(List<String> strList){
if(CollectionUtils.isEmpty(strList)){
return 0;
}
// 数组的join方法 “[Hello,world,abc]”
String tmpStr1 = StringUtils.join(strList);
// 注意此处我们不希望使用默认的分隔符 "," "HelloWorldAbc"
String tmpStr2 = StringUtils.join(strList, "");
return tmpStr2.length();
}
Method Two
public static int calculateStrJoinLengthOfListMethod2(List<String> strList){
if(CollectionUtils.isEmpty(strList)){
return 0;
}
StringBuilder builder = new StringBuilder();
strList.forEach(str -> builder.append(str));
return builder.toString().length();
}
Test Methods
package com.yanxml.util.string;
import org.apache.commons.collections.CollectionUtils;
import org.apache.commons.lang3.StringUtils;
import java.util.ArrayList;
import java.util.List;
public class StringArrayUtils {
public static int calculateStrJoinLengthOfListMethod1(List<String> strList){
if(CollectionUtils.isEmpty(strList)){
return 0;
}
// 数组的join方法 “[Hello,world,abc]”
String tmpStr1 = StringUtils.join(strList);
// 注意此处我们不希望使用默认的分隔符 "," "HelloWorldAbc"
String tmpStr2 = StringUtils.join(strList, "");
return tmpStr2.length();
}
public static int calculateStrJoinLengthOfListMethod2(List<String> strList){
if(CollectionUtils.isEmpty(strList)){
return 0;
}
StringBuilder builder = new StringBuilder();
strList.forEach(str -> builder.append(str));
return builder.toString().length();
}
public static void main(String[] args) {
List<String> strList = new ArrayList<>();
strList.add("Hello");
strList.add("World");
strList.add("abc");
// 测试方法1
int lengthByTest1 = calculateStrJoinLengthOfListMethod1(strList);
System.out.println("Test By Method1 - StringUtils.join, length " + lengthByTest1);
// 测试方法2
int lengthByTest2 = calculateStrJoinLengthOfListMethod2(strList);
System.out.println("Test By Method2 - StringBuilder.append, length "+ lengthByTest2);
}
}
# 测试结果
Test By Method1 - StringUtils.join, length 13
Test By Method2 - StringBuilder.append, length 13
Source code analysis
From a user's perspective, it should normally end here. However, from actual development, we have all learned that when we encounter problems, if we go deeper, we will have better understanding and rewards
.
Let’s take a closer look at the method first StringUtils.join.
Join method of array
String tmpStr1 = StringUtils.join(strList);
# org.apache.commons.lang3.StringUtils 类
# 单参数重载方法 Array
public static <T> String join(T... elements) {
return join((Object[])elements, (String)null);
}
# 2个参数重载方法 Array & 间隔符
public static String join(Object[] array, String separator) {
return array == null ? null : join(array, separator, 0, array.length);
}
# 4个参数重载方法 Array & 间隔符 & 开始下标 & 结束下标
public static String join(Object[] array, String separator, int startIndex, int endIndex) {
if (array == null) {
return null;
} else {
if (separator == null) {
separator = "";
}
int noOfItems = endIndex - startIndex;
if (noOfItems <= 0) {
return "";
} else {
StringBuilder buf = new StringBuilder(noOfItems * 16);
for(int i = startIndex; i < endIndex; ++i) {
if (i > startIndex) {
buf.append(separator);
}
if (array[i] != null) {
buf.append(array[i]);
}
}
return buf.toString();
}
}
}
- As you can see, this join method is the same as the two methods. The method used
StringBuilder.appendis just a layer of packaging and has no other use. And both of them are thread-unsafe.
Join method of string
String tmpStr2 = StringUtils.join(strList, "");
# org.apache.commons.lang3.StringUtils 类
public static String join(Iterable<?> iterable, String separator) {
return iterable == null ? null : join(iterable.iterator(), separator);
}
public static String join(Iterator<?> iterator, String separator) {
if (iterator == null) {
return null;
} else if (!iterator.hasNext()) {
return "";
} else {
Object first = iterator.next();
if (!iterator.hasNext()) {
String result = ObjectUtils.toString(first);
return result;
} else {
StringBuilder buf = new StringBuilder(256);
if (first != null) {
buf.append(first);
}
while(iterator.hasNext()) {
if (separator != null) {
buf.append(separator);
}
Object obj = iterator.next();
if (obj != null) {
buf.append(obj);
}
}
return buf.toString();
}
}
}
- What's more interesting is that the results obtained by these two methods are completely different. The main problem may be here, the iteration of the array is through a forced conversion, and the string is processed
return join((Object[])elements, (String)null);usingIterator.
a strange phenomenon
The result is "[Hello, World, Abc]"
// 数组的join方法 “[Hello,world,abc]”
String tmpStr1 = StringUtils.join(strList);
// 注意此处我们不希望使用默认的分隔符 "," "HelloWorldAbc"
String tmpStr2 = StringUtils.join(strList, "");
- It may be that the first calling method triggered the array method, which caused this display
toStringto appear here .[XX,XX,XX]
Reference
[1]. [Method and use of StringUtils.join() method] ( https://www.cnblogs.com/fenghh/p/12175368.html )
[2]. [Which package CollectionUtils belongs to] https://blog .csdn.net/weixin_42114097/article/details/90579980