public class AsciiUtil {

 public static final char SBC_SPACE = 12288; // 全角空格 12288

 public static final char DBC_SPACE = 32; //半角空格 32

 // ASCII character 33-126 <-> unicode 65281-65374
 public static final char ASCII_START = 33;

 public static final char ASCII_END = 126;

 public static final char UNICODE_START = 65281;

 public static final char UNICODE_END = 65374;

 public static final char DBC_SBC_STEP = 65248; // 全角半角转换间隔

 public static char sbc2dbc(char src){
  if (src == SBC_SPACE) {
   return DBC_SPACE;
  }

  if (src >= UNICODE_START && src <= UNICODE_END) {
   return (char) (src - DBC_SBC_STEP);
  }

  return src;
 }

 /**
  * Convert from SBC case to DBC case
  *
  * @param src
  * @return DBC case
  */
 public static String sbc2dbcCase(String src) {
  if (src == null) {
   return null;
  }
  char[] c = src.toCharArray();
  for (int i = 0; i < c.length; i++) {
   c[i] = sbc2dbc(c[i]);
  }
  return new String(c);
 }

 public static char dbc2sbc(char src){
  if (src == DBC_SPACE) {
   return SBC_SPACE;
  }
  if (src <= ASCII_END) {
   return (char) (src + DBC_SBC_STEP);
  }
  return src;
 }

 /**
  * Convert from DBC case to SBC case.
  *
  * @param src
  * @return SBC case string
  */
 public static String dbc2sbcCase(String src) {
  if (src == null) {
   return null;
  }

  char[] c = src.toCharArray();
  for (int i = 0; i < c.length; i++) {
   c[i] = dbc2sbc(c[i]);
  }

  return new String(c);
 }
}

@Test
 public void testDBC(){
  System.out.println(AsciiUtil.sbc2dbcCase("ｈｅｌｌｏ ｗｏｒｌｄ"));
  System.out.println(AsciiUtil.dbc2sbcCase("hello world"));
 }

hello world
ｈｅｌｌｏ　ｗｏｒｌｄ