如何将 Java 字符串转换为 ASCII 字节数组？

Using the getBytes method, giving it the appropriate Charset (or Charset name).

Example:

String s = "Hello, there.";
byte[] b = s.getBytes(StandardCharsets.US_ASCII);

If more control is required (such as throwing an exception when a character outside the 7 bit US-ASCII is encountered) then CharsetDecoder can be used:

private static byte[] strictStringToBytes(String s, Charset charset) throws CharacterCodingException {
ByteBuffer x  = charset.newEncoder().onMalformedInput(CodingErrorAction.REPORT).encode(CharBuffer.wrap(s));
byte[] b = new byte[x.remaining()];
x.get(b);
return b;
}

Before Java 7 it is possible to use: byte[] b = s.getBytes("US-ASCII");. The enum StandardCharsets, the encoder as well as the specialized getBytes(Charset) methods have been introduced in Java 7.

String s = "ASCII Text";
byte[] bytes = s.getBytes("US-ASCII");

There is only one character wrong in the code you tried:

Charset characterSet = Charset.forName("US-ASCII");
String string = "Wazzup";
byte[] bytes = String.getBytes(characterSet);
^

Notice the upper case "String". This tries to invoke a static method on the string class, which does not exist. Instead you need to invoke the method on your string instance:

byte[] bytes = string.getBytes(characterSet);

Convert string to ascii values.

   String test = "ABCD";


for ( int i = 0; i < test.length(); ++i ) {
char c = test.charAt( i );
int j = (int) c;
System.out.println(j);
}

The problem with other proposed solutions is that they will either drop characters that cannot be directly mapped to ASCII, or replace them with a marker character like ?.

You might desire to have for example accented characters converted to that same character without the accent. There are a couple of tricks to do this (including building a static mapping table yourself or leveraging existing 'normalization' defined for unicode), but those methods are far from complete.

Your best bet is using the junidecode library, which cannot be complete either but incorporates a lot of experience in the most sane way of transliterating Unicode to ASCII.

If you happen to need this in Android and want to make it work with anything older than FroYo, you can also use EncodingUtils.getAsciiBytes():

byte[] bytes = EncodingUtils.getAsciiBytes("ASCII Text");

If you are a guava user there is a handy Charsets class:

String s = "Hello, world!";
byte[] b = s.getBytes(Charsets.US_ASCII);

Apart from not hard-coding arbitrary charset name in your source code it has a much bigger advantage: Charsets.US_ASCII is of Charset type (not String) so you avoid checked UnsupportedEncodingException thrown only from String.getBytes(String), but not from String.getBytes(Charset).

In Java 7 there is equivalent StandardCharsets class.

Try this:

/**
* @(#)demo1.java
*
*
* @author
* @version 1.00 2012/8/30
*/


import java.util.*;


public class demo1
{
Scanner s=new Scanner(System.in);


String str;
int key;


void getdata()
{
System.out.println ("plase enter a string");
str=s.next();
System.out.println ("plase enter a key");
key=s.nextInt();
}


void display()
{
char a;
int j;
for ( int i = 0; i < str.length(); ++i )
{


char c = str.charAt( i );
j = (int) c + key;
a= (char) j;


System.out.print(a);
}


public static void main(String[] args)
{
demo1 obj=new demo1();
obj.getdata();
obj.display();
}
}
}

In my string I have Thai characters (TIS620 encoded) and German umlauts. The answer from agiles put me on the right path. Instead of .getBytes() I use now

  int len = mString.length(); // Length of the string
byte[] dataset = new byte[len];
for (int i = 0; i < len; ++i) {
char c = mString.charAt(i);
dataset[i]= (byte) c;
}

I found the solution. Actually Base64 class is not available in Android. Link is given below for more information.

byte[] byteArray;
byteArray= json.getBytes(StandardCharsets.US_ASCII);
String encoded=Base64.encodeBytes(byteArray);
userLogin(encoded);

Here is the link for Base64 class: http://androidcodemonkey.blogspot.com/2010/03/how-to-base64-encode-decode-android.html

To convert String to ASCII byte array:

String s1 = "Hello World!";
byte[] byteArray = s1.getBytes(StandardCharsets.US_ASCII);
// Now byteArray is [72, 101, 108, 108, 111, 32, 87, 111, 114, 108, 100, 33]

To convert ASCII byte array to String:

String s2 = new String(byteArray, StandardCharsets.US_ASCII));