Java StringBuilder codePointBefore()方法及示例

StringBuilder类 的 codePointBefore() 方法以一个索引为参数，返回StringBuilder所包含的String中指定索引之前的字符的 “Unicode编号”。索引指的是char值（Unicode代码单位），索引的值必须在0到length-1之间。如果(index – 1)处的char值在低代用范围内，(index – 2)处的char值不是负值，且在高代用范围内，那么该方法将返回代用对的补充码位值。如果在索引-1处的char值是一个未配对的低代用品或高代用品，则返回代用品的值。

语法

public int codePointBefore(int index)

参数： 该方法接受一个int类型的参数 index ，代表要返回的unicode值后面的字符的索引。
返回值： 该方法返回给定索引前的字符的 “unicode数 “。
异常： 当index为负数或大于等于length()时，该方法抛出 IndexOutOfBoundsException 。下面的程序演示了StringBuilder类的codePointBefore()方法：

例1 :

// Java program to demonstrate// the codePointBefore() Method. class GFG {    public static void main(String[] args)    {        // create a StringBuilder object        // with a String pass as parameter        StringBuilder            str            = new StringBuilder("WelcomeGeeks");         // print string        System.out.println("String is "                           + str.toString());         // get unicode of char at index 1        // using codePointBefore() method        int unicode = str.codePointBefore(2);         // print char and Unicode        System.out.println("Unicode of character"                           + " at position 1 = " + unicode);         // get unicode of char at index 10        // using codePointBefore() method        unicode = str.codePointBefore(11);         // print char and Unicode        System.out.println("Unicode of character"                           + " at position 10 = "                           + unicode);    }}

输出

String is WelcomeGeeksUnicode of character at position 1 = 101Unicode of character at position 10 = 107

例2： 演示IndexOutOfBoundsException

// Java program to demonstrate// exception thrown by codePointBefore() Method. class GFG {    public static void main(String[] args)    {        // create a StringBuilder object        // with a String pass as parameter        StringBuilder            str            = new StringBuilder("WelcomeGeeks");         try {             // get unicode of char at position length + 2            int unicode = str.codePointBefore(                str.length() + 2);        }         catch (Exception e) {            System.out.println("Exception: " + e);        }    }}

输出

Exception: java.lang.StringIndexOutOfBoundsException: String index out of range: 14

**参考资料: ** https://docs.oracle.com/javase/10/docs/api/java/lang/StringBuilder.html#codePointBefore(int)