Generating 8-character only UUIDs

You can try RandomStringUtils class from apache.commons:

import org.apache.commons.lang3.RandomStringUtils;

final int SHORT_ID_LENGTH = 8;

// all possible unicode characters
String shortId = RandomStringUtils.random(SHORT_ID_LENGTH);

Please keep in mind, that it will contain all possible characters which is neither URL nor human friendly.

So check out other methods too:

// HEX: 0-9, a-f. For example: 6587fddb, c0f182c1
shortId = RandomStringUtils.random(8, "0123456789abcdef"); 

// a-z, A-Z. For example: eRkgbzeF, MFcWSksx
shortId = RandomStringUtils.randomAlphabetic(8); 

// 0-9. For example: 76091014, 03771122
shortId = RandomStringUtils.randomNumeric(8); 

// a-z, A-Z, 0-9. For example: WRMcpIk7, s57JwCVA
shortId = RandomStringUtils.randomAlphanumeric(8); 

As others said probability of id collision with smaller id can be significant. Check out how birthday problem applies to your case. You can find nice explanation how to calculate approximation in this answer.

It is not possible since a UUID is a 16-byte number per definition. But of course, you can generate 8-character long unique strings (see the other answers).

Also be careful with generating longer UUIDs and substring-ing them, since some parts of the ID may contain fixed bytes (e.g. this is the case with MAC, DCE and MD5 UUIDs).