Converting UTF-8 to ISO-8859-1 in Java - how to keep it as single byte

I am trying to convert a string encoded in java in UTF-8 to ISO-8859-1. Say for example, in the string 'âabcd' 'â' is represented in ISO-8859-1 as E2. In UTF-8 it is represented as two bytes. C3 A2 I believe. When I do a getbytes(encoding) and then create a new string with the bytes in ISO-8859-1 encoding, I get a two different chars. â. Is there any other way to do this so as to keep the character the same i.e. âabcd?

If you're dealing with character encodings other than UTF-16, you shouldn't be using java.lang.String or the char primitive -- you should only be using byte[] arrays or ByteBuffer objects. Then, you can use java.nio.charset.Charset to convert between encodings:

Charset utf8charset = Charset.forName("UTF-8");
Charset iso88591charset = Charset.forName("ISO-8859-1");

ByteBuffer inputBuffer = ByteBuffer.wrap(new byte[]{(byte)0xC3, (byte)0xA2});

// decode UTF-8
CharBuffer data = utf8charset.decode(inputBuffer);

// encode ISO-8559-1
ByteBuffer outputBuffer = iso88591charset.encode(data);
byte[] outputData = outputBuffer.array();

byte[] iso88591Data = theString.getBytes("ISO-8859-1");

Will do the trick. From your description it seems as if you're trying to "store an ISO-8859-1 String". String objects in Java are always implicitly encoded in UTF-16. There's no way to change that encoding.

What you can do, 'though is to get the bytes that constitute some other encoding of it (using the .getBytes() method as shown above).

Starting with a set of bytes which encode a string using UTF-8, creates a string from that data, then get some bytes encoding the string in a different encoding:

    byte[] utf8bytes = { (byte)0xc3, (byte)0xa2, 0x61, 0x62, 0x63, 0x64 };
    Charset utf8charset = Charset.forName("UTF-8");
    Charset iso88591charset = Charset.forName("ISO-8859-1");

    String string = new String ( utf8bytes, utf8charset );
    
    System.out.println(string);
    
    // "When I do a getbytes(encoding) and "
    byte[] iso88591bytes = string.getBytes(iso88591charset);
    
    for ( byte b : iso88591bytes )
        System.out.printf("%02x ", b);
    
    System.out.println();
    
    // "then create a new string with the bytes in ISO-8859-1 encoding"
    String string2 = new String ( iso88591bytes, iso88591charset );
    
    // "I get a two different chars"
    System.out.println(string2);

this outputs strings and the iso88591 bytes correctly:

âabcd 
e2 61 62 63 64 
âabcd

So your byte array wasn't paired with the correct encoding:

    String failString = new String ( utf8bytes, iso88591charset );
    
    System.out.println(failString);

Outputs

âabcd

(either that, or you just wrote the utf8 bytes to a file and read them elsewhere as iso88591)

This is what I needed:

public static byte[] encode(byte[] arr, String fromCharsetName) {
	return encode(arr, Charset.forName(fromCharsetName), Charset.forName("UTF-8"));
}

public static byte[] encode(byte[] arr, String fromCharsetName, String targetCharsetName) {
	return encode(arr, Charset.forName(fromCharsetName), Charset.forName(targetCharsetName));
}

public static byte[] encode(byte[] arr, Charset sourceCharset, Charset targetCharset) {

    ByteBuffer inputBuffer = ByteBuffer.wrap( arr );

    CharBuffer data = sourceCharset.decode(inputBuffer);

    ByteBuffer outputBuffer = targetCharset.encode(data);
    byte[] outputData = outputBuffer.array();

    return outputData;
}

If you have the correct encoding in the string, you need not do more to get the bytes for another encoding.

public static void main(String[] args) throws Exception {
	printBytes("â");
	System.out.println(
			new String(new byte[] { (byte) 0xE2 }, "ISO-8859-1"));
	System.out.println(
			new String(new byte[] { (byte) 0xC3, (byte) 0xA2 }, "UTF-8"));
}

private static void printBytes(String str) {
	System.out.println("Bytes in " + str + " with ISO-8859-1");
	for (byte b : str.getBytes(StandardCharsets.ISO_8859_1)) {
		System.out.printf("%3X", b);
	}
	System.out.println();
	System.out.println("Bytes in " + str + " with UTF-8");
	for (byte b : str.getBytes(StandardCharsets.UTF_8)) {
		System.out.printf("%3X", b);
	}
	System.out.println();
}

Output:

Bytes in â with ISO-8859-1
 E2
Bytes in â with UTF-8
 C3 A2
â
â

For files encoding...

public class FRomUtf8ToIso {
    	static File input = new File("C:/Users/admin/Desktop/pippo.txt");
    	static File output = new File("C:/Users/admin/Desktop/ciccio.txt");
    	
    
	public static void main(String[] args) throws IOException {
 
		BufferedReader br = null;
 
		FileWriter fileWriter = new FileWriter(output);
		try {
 
			String sCurrentLine;
 
			br = new BufferedReader(new FileReader( input ));
 
			int i= 0;
			while ((sCurrentLine = br.readLine()) != null) {
				byte[] isoB =  encode( sCurrentLine.getBytes() );
				fileWriter.write(new String(isoB, Charset.forName("ISO-8859-15") ) );
				fileWriter.write("\n");
				System.out.println( i++ );
			}
 
		} catch (IOException e) {
			e.printStackTrace();
		} finally {
			try {
				fileWriter.flush();
				fileWriter.close();
				if (br != null)br.close();
			} catch (IOException ex) {
				ex.printStackTrace();
			}
		}
 
	}
	
	
	static byte[] encode(byte[] arr){
		Charset utf8charset = Charset.forName("UTF-8");
		Charset iso88591charset = Charset.forName("ISO-8859-15");

		ByteBuffer inputBuffer = ByteBuffer.wrap( arr );

		// decode UTF-8
		CharBuffer data = utf8charset.decode(inputBuffer);

		// encode ISO-8559-1
		ByteBuffer outputBuffer = iso88591charset.encode(data);
		byte[] outputData = outputBuffer.array();
		
		return outputData;
	}
	
}

In addition to Adam Rosenfield's answer, I would like to add that ByteBuffer.array() returns the buffer's underlying byte array, which is not necessarily "trimmed" up to the last character. Extra manipulation will be needed, such as the ones mentioned in this answer; in particular:

byte[] b = new byte[bb.remaining()]
bb.get(b);

evict non ISO-8859-1 characters, will be replace by '?' (before send to a ISO-8859-1 DB by example):

utf8String = new String ( utf8String.getBytes(), "ISO-8859-1" );