我当前正在执行一项任务，该任务需要将唯一标识符附加到字符串，然后在redshift中使用该名称创建一个表。目前，我正在使用id.hashcode（）生成标识符（如果不存在，我的查询就会创建表）来执行任务。但是我最近得到了1个ID，其生成的哈希码是负值，并且Redshift不允许查询中使用“-”（生成的样本名称：abc-1234）。
如何唯一标识每个传入的ID，如果该ID的表存在，则不应再次创建该表。
答案 0 :(得分：0)
首先，根据定义，任何哈希码都不会比字符串本身唯一，除非您使用更多数据像时间戳一样生成它。
根据要附加到字符串的唯一标识符的类型以及需要的唯一性，您有几种可能。
1ˢᵗ）可以使用时间戳，机器IP地址和随机数，通过类似于以下方法来生成非常接近的通用唯一标识符：
public String createUniqueKey() {
int iRnd;
long lSeed = System.currentTimeMillis();
Random oRnd = new Random(lSeed);
String sHex;
StringBuffer sUUID = new StringBuffer(32);
byte[] localIPAddr = new byte[4];
try {
// 8 characters Code IP address of this machine
localIPAddr = InetAddress.getLocalHost().getAddress();
sUUID.append(byteToStr[((int) localIPAddr[0]) & 255]);
sUUID.append(byteToStr[((int) localIPAddr[1]) & 255]);
sUUID.append(byteToStr[((int) localIPAddr[2]) & 255]);
sUUID.append(byteToStr[((int) localIPAddr[3]) & 255]);
}
catch (UnknownHostException e) {
// Use localhost by default
sUUID.append("7F000000");
}
// Append a seed value based on current system date
sUUID.append(Long.toHexString(lSeed));
// 6 characters - an incremental sequence
sUUID.append(Integer.toHexString(iSequence.incrementAndGet()));
iSequence.compareAndSet(16777000, 1048576);
do {
iRnd = oRnd.nextInt();
if (iRnd>0) iRnd = -iRnd;
sHex = Integer.toHexString(iRnd);
} while (0==iRnd);
// Finally append a random number
sUUID.append(sHex);
return sUUID.substring(0, 32);
} // createUniqueKey()
2ⁿᵈ）：您可能希望创建随时间增加或减少的一系列键，以便对其进行排序：
private static AtomicInteger iSequence = new AtomicInteger(1048576);
/**
* Create a universal unique key which increases over time
* @return String of 32 characters length
*/
public String createTimeDependentKey() {
char pad[];
int padLen;
StringBuilder retval = new StringBuilder(32);
final long seed = System.currentTimeMillis();
Random rnd = new Random(seed);
// 10 characters time dependent part
String timePart = String.valueOf(Long.MAX_VALUE-seed);
padLen = timePart.length()-10;
pad = new char[padLen];
java.util.Arrays.fill(pad, '0');
retval.append(pad).append(timePart);
// 6 characters sequential
retval.append(Integer.toHexString(iSequence.incrementAndGet()));
iSequence.compareAndSet(16777000, 1048576);
// 8 characters IP dependent part
try {
byte[] localIPAddr = InetAddress.getLocalHost().getAddress();
retval.append(byteToStr[((int) localIPAddr[0]) & 255]);
retval.append(byteToStr[((int) localIPAddr[1]) & 255]);
retval.append(byteToStr[((int) localIPAddr[2]) & 255]);
retval.append(byteToStr[((int) localIPAddr[3]) & 255]);
}
catch (UnknownHostException e) {
retval.append("7f000000");
}
// 8 characters random part
String randomPart = String.valueOf(rnd.nextInt(Integer.MAX_VALUE));
padLen = timePart.length()-8;
retval.append(randomPart).append(generateRandomId(padLen, "0123456789abcdef", Character.LOWERCASE_LETTER));
return retval.toString();
}
/**
* Generate a reverse timestamp plus random int identifier of 32 characters length
* The identifier will be composed of Long.MAX_VALUE-Current Time in Milliseconds followed by a random 32 bits integer
* @return Numeric identifier of 32 characters length padded with zeros at the left
*/
public String generateReverseTimestampId() {
String sTs = Str.leftPad(String.valueOf(Long.MAX_VALUE-System.currentTimeMillis()),'0',20);
String sRd = Str.leftPad(String.valueOf(new Random().nextInt(Integer.MAX_VALUE)),'0',10);
return Str.leftPad(sTs+sRd,'0',32);
}
3ʳᵈ）您可能想要创建仅具有给定字符集的伪唯一键：
public String generateRandomId(int iLength, String sCharset, byte byCategory ) throws StringIndexOutOfBoundsException {
if (iLength<=0)
throw new StringIndexOutOfBoundsException("Uid.generateRandomId() identifier length must be greater than zero");
if (iLength>4096)
throw new StringIndexOutOfBoundsException("Uid.generateRandomId() identifier length must be less than or equal to 4096");
if (sCharset!=null) {
if (sCharset.length()==0) throw new StringIndexOutOfBoundsException("Uid.generateRandomId() character set length must be greater than zero");
} else {
sCharset = "abcdefghjkmnpqrstuvwxyz23456789";
}
if (byCategory!=Character.UNASSIGNED && byCategory!=Character.UPPERCASE_LETTER && byCategory!=Character.LOWERCASE_LETTER)
throw new IllegalArgumentException("Uid.generateRandomId() Character category must be one of {UNASSIGNED, UPPERCASE_LETTER, LOWERCASE_LETTER}");
int iCsLen = sCharset.length();
StringBuilder oId = new StringBuilder(iLength);
Random oRnd = new Random();
for (int i=0; i<iLength; i++){
char c = sCharset.charAt(oRnd.nextInt(iCsLen));
if (byCategory==Character.UPPERCASE_LETTER)
c = Character.toUpperCase(c);
else if (byCategory==Character.LOWERCASE_LETTER)
c = Character.toLowerCase(c);
oId.append(c);
}
return oId.toString();
}
您还将需要此数组
private static String[] byteToStr = {
"00","01","02","03","04","05","06","07","08","09","0a","0b","0c","0d","0e","0f",
"10","11","12","13","14","15","16","17","18","19","1a","1b","1c","1d","1e","1f",
"20","21","22","23","24","25","26","27","28","29","2a","2b","2c","2d","2e","2f",
"30","31","32","33","34","35","36","37","38","39","3a","3b","3c","3d","3e","3f",
"40","41","42","43","44","45","46","47","48","49","4a","4b","4c","4d","4e","4f",
"50","51","52","53","54","55","56","57","58","59","5a","5b","5c","5d","5e","5f",
"60","61","62","63","64","65","66","67","68","69","6a","6b","6c","6d","6e","6f",
"70","71","72","73","74","75","76","77","78","79","7a","7b","7c","7d","7e","7f",
"80","81","82","83","84","85","86","87","88","89","8a","8b","8c","8d","8e","8f",
"90","91","92","93","94","95","96","97","98","99","9a","9b","9c","9d","9e","9f",
"a0","a1","a2","a3","a4","a5","a6","a7","a8","a9","aa","ab","ac","ad","ae","af",
"b0","b1","b2","b3","b4","b5","b6","b7","b8","b9","ba","bb","bc","bd","be","bf",
"c0","c1","c2","c3","c4","c5","c6","c7","c8","c9","ca","cb","cc","cd","ce","cf",
"d0","d1","d2","d3","d4","d5","d6","d7","d8","d9","da","db","dc","dd","de","df",
"e0","e1","e2","e3","e4","e5","e6","e7","e8","e9","ea","eb","ec","ed","ee","ef",
"f0","f1","f2","f3","f4","f5","f6","f7","f8","f9","fa","fb","fc","fd","fe","ff" };
可以找到here的完整实现。