Using the getBytes method, giving it the appropriate Charset (or Charset name).
Example:
String s = "Hello, there.";
byte[] b = s.getBytes(StandardCharsets.US_ASCII);
(Before Java 7: byte[] b = s.getBytes("US-ASCII");)
与恶龙缠斗过久,自身亦成为恶龙；凝视深渊过久,深渊将回以凝视…
