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 WelcomeGeeks
Unicode of character at position 1 = 101
Unicode 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)