package .lang3;
中maven的依赖包
<dependency>
<groupId></groupId>
<artifactId>commons-lang3</artifactId>
<version>3.4</version>
</dependency>
1. substringAfter()源码解读
/**
* <p>Gets the substring after the first occurrence of a separator.
* The separator is not returned.</p>
*
* <p>A {@code null} string input will return {@code null}.
* An empty ("") string input will return the empty string.
* A {@code null} separator will return the empty string if the
* input string is not {@code null}.</p>
*
* <p>If nothing is found, the empty string is returned.</p>
*
* <pre>
* (null, *) = null
* ("", *) = ""
* (*, null) = ""
* ("abc", "a") = "bc"
* ("abcba", "b") = "cba"
* ("abc", "c") = ""
* ("abc", "d") = ""
* ("abc", "") = "abc"
* </pre>
*
* @param str the String to get a substring from, may be null
* @param separator the String to search for, may be null
* @return the substring after the first occurrence of the separator,
* {@code null} if null String input
* @since 2.0
*/
public static String substringAfter(final String str, final String separator) {
if (isEmpty(str)) {
return str;
}
if (separator == null) {
return EMPTY;
}
final int pos = str.indexOf(separator);//indexOf() 方法可返回某个指定的字符串值在字符串中首次出现的位置。
if (pos == INDEX_NOT_FOUND) {
return EMPTY;
}
return (pos + ());
}
()源码解读
/**
* <p>Gets the substring after the last occurrence of a separator.
* The separator is not returned.</p>
*
* <p>A {@code null} string input will return {@code null}.
* An empty ("") string input will return the empty string.
* An empty or {@code null} separator will return the empty string if
* the input string is not {@code null}.</p>
*
* <p>If nothing is found, the empty string is returned.</p>
*
* <pre>
* (null, *) = null
* ("", *) = ""
* (*, "") = ""
* (*, null) = ""
* ("abc", "a") = "bc"
* ("abcba", "b") = "a"
* ("abc", "c") = ""
* ("a", "a") = ""
* ("a", "z") = ""
* </pre>
*
* @param str the String to get a substring from, may be null
* @param separator the String to search for, may be null
* @return the substring after the last occurrence of the separator,
* {@code null} if null String input
* @since 2.0
*/
public static String substringAfterLast(final String str, final String separator) {
if (isEmpty(str)) {
return str;
}
if (isEmpty(separator)) {
return EMPTY;
}
final int pos = str.lastIndexOf(separator);//返回指定字符在此字符串中最后一次出现处的索引，如果此字符串中没有这样的字符，则返回 -1。
if (pos == INDEX_NOT_FOUND || pos == () - ()) {
return EMPTY;
}
return (pos + ());
}
总结： 阅读源码进行对比，源码中有注解的部分给与case学习。源码没有你想象中的难读懂。
substringAfter()方法取str内，首次出现分隔符separator后的字符串。
substringAfterLast()方法取str内，最后一个分隔符separator后的字符串。