我想将代表十六进制值（大写或小写）的char转换为字节，例如

'0'->0, '1' -> 1, 'A' -> 10, 'a' -> 10, 'f' -> 15 etc...

我将非常频繁地调用此方法，因此性能非常重要。是否有比使用预初始化HashMap<Character,Byte>从中获取值更快的方法？

回答

似乎 在使用开关盒和Jon Skeet的直接计算解决方案之间是一个折衷-但是，开关盒解决方案似乎略有下降。
格雷格的数组方法胜出。这是各种方法的200,000,000次运行的性能结果（以毫秒为单位）：

Character.getNumericValue:
8360

Character.digit:
8453

HashMap<Character,Byte>:
15109

Greg's Array Method:
6656

JonSkeet's Direct Method:
7344

Switch:
7281

多谢你们！

基准方法代码

乔恩·斯基特（JonSkeet），您过去的竞争对手，在这里，;-)

public class ScratchPad {

    private static final int NUMBER_OF_RUNS = 200000000;

    static byte res;

    static HashMap<Character, Byte> map = new HashMap<Character, Byte>() {{
        put( Character.valueOf( '0' ), Byte.valueOf( (byte )0 ));
        put( Character.valueOf( '1' ), Byte.valueOf( (byte )1 ));
        put( Character.valueOf( '2' ), Byte.valueOf( (byte )2 ));
        put( Character.valueOf( '3' ), Byte.valueOf( (byte )3 ));
        put( Character.valueOf( '4' ), Byte.valueOf( (byte )4 ));
        put( Character.valueOf( '5' ), Byte.valueOf( (byte )5 ));
        put( Character.valueOf( '6' ), Byte.valueOf( (byte )6 ));
        put( Character.valueOf( '7' ), Byte.valueOf( (byte )7 ));
        put( Character.valueOf( '8' ), Byte.valueOf( (byte )8 ));
        put( Character.valueOf( '9' ), Byte.valueOf( (byte )9 ));
        put( Character.valueOf( 'a' ), Byte.valueOf( (byte )10 ));
        put( Character.valueOf( 'b' ), Byte.valueOf( (byte )11 ));
        put( Character.valueOf( 'c' ), Byte.valueOf( (byte )12 ));
        put( Character.valueOf( 'd' ), Byte.valueOf( (byte )13 ));
        put( Character.valueOf( 'e' ), Byte.valueOf( (byte )14 ));
        put( Character.valueOf( 'f' ), Byte.valueOf( (byte )15 ));
        put( Character.valueOf( 'A' ), Byte.valueOf( (byte )10 ));
        put( Character.valueOf( 'B' ), Byte.valueOf( (byte )11 ));
        put( Character.valueOf( 'C' ), Byte.valueOf( (byte )12 ));
        put( Character.valueOf( 'D' ), Byte.valueOf( (byte )13 ));
        put( Character.valueOf( 'E' ), Byte.valueOf( (byte )14 ));
        put( Character.valueOf( 'F' ), Byte.valueOf( (byte )15 ));
    }};
    static int[] charValues = {0, 1, 2, 3, 4, 5, 6, 7, 8, 9, -1, -1, -1, -1, -1, -1, -1, 10, 11, 12, 13,14,15,-1,-1,-1,-1,-1,-1,-1,-1,-1,
                    -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,10, 11, 12, 13,14,15};
    static char[] cs = new char[]{'0','1','2','3','4','5','6','7','8','9','a','b','c','d','e','f','A','B','C','D','E','F'};

    public static void main(String args[]) throws Exception {
        long time = System.currentTimeMillis();
        for( int i = 0; i < NUMBER_OF_RUNS; i++ ) {
            res = getNumericValue( i );
        }
        System.out.println( "Character.getNumericValue:" );
        System.out.println( System.currentTimeMillis()-time );
        time = System.currentTimeMillis();
        for( int i = 0; i < NUMBER_OF_RUNS; i++ ) {
            res = getDigit( i );
        }
        System.out.println( "Character.digit:" );
        System.out.println( System.currentTimeMillis()-time );
        time = System.currentTimeMillis();
        for( int i = 0; i < NUMBER_OF_RUNS; i++ ) {
            try {
                res = getValueFromArray( i );
            } catch (IllegalArgumentException e) {
            }
        }
        System.out.println( "Array:" );
        System.out.println( System.currentTimeMillis()-time );
        time = System.currentTimeMillis();
        for( int i = 0; i < NUMBER_OF_RUNS; i++ ) {
            res = getValueFromHashMap( i );
        }
        System.out.println( "HashMap<Character,Byte>:" );
        System.out.println( System.currentTimeMillis()-time );
        time = System.currentTimeMillis();
        for( int i = 0; i < NUMBER_OF_RUNS; i++ ) {
            char c = cs[i%cs.length];
            res = getValueFromComputeMethod( c );        
        }
        System.out.println( "JonSkeet's Direct Method:" );
        System.out.println( System.currentTimeMillis()-time );
        time = System.currentTimeMillis();
        for( int i = 0; i < NUMBER_OF_RUNS; i++ ) {
            res = getValueFromSwitch( i );

        }
        System.out.println( "Switch:" );
        System.out.println( System.currentTimeMillis()-time );
    }

    private static byte getValueFromSwitch( int i ) {
        byte res;
        char ch = cs[i%cs.length];
        switch( ch ) {
            case '0':
                res = 0;
                break;
            case '1':
                res = 1;
                break;
            case '2':
                res = 2;
                break;
            case '3':
                res = 3;
                break;
            case '4':
                res = 4;
                break;
            case '5':
                res = 5;
                break;
            case '6':
                res = 6;
                break;
            case '7':
                res = 7;
                break;
            case '8':
                res = 8;
                break;
            case '9':
                res = 9;
                break;
            case 'a':
            case 'A':
                res = 10;
                break;
            case 'b':
            case 'B':    
                res = 11;
                break;
            case 'c':
            case 'C':    
                res = 12;
                break;
            case 'd':
            case 'D':    
                res = 13;
                break;
            case 'e':
            case 'E':    
                res = 14;
                break;
            case 'f':
            case 'F':    
                res = 15;
                break;
            default:
                throw new RuntimeException("unknown hex character: " + ch );
        }
        return res;
    }

    private static byte getValueFromComputeMethod( char c ) {
        byte result = 0;
        if (c >= '0' && c <= '9')
        {
            result =  (byte)(c - '0');
        }
        if (c >= 'a' && c <= 'f')
        {
            result = (byte)(c - 'a' + 10);
        }
        if (c >= 'A' && c <= 'F')
        {
            result =  (byte)(c - 'A' + 10);
        }
        return result;
    }

    private static byte getValueFromHashMap( int i ) {
        return map.get( Character.valueOf( cs[i%cs.length] ) ).byteValue();
    }

    private static byte getValueFromArray( int i ) {
        char c = cs[i%cs.length];
        if (c < '0' || c > 'f') {
            throw new IllegalArgumentException();
        }
        byte result = (byte)charValues[c-'0'];
        if (res < 0) {
            throw new IllegalArgumentException();
        }
        return result;
    }

    private static byte getDigit( int i ) {
        return (byte)Character.digit( cs[i%cs.length], 16 );
    }

    private static byte getNumericValue( int i ) {
        return (byte)Character.getNumericValue( cs[i%cs.length] );
    }

}