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Qiniu Cloud Storage SDK for Java
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package com.qiniu.util;

import com.qiniu.common.Constants;

import java.nio.charset.Charset;
import java.security.MessageDigest;
import java.security.NoSuchAlgorithmException;
import java.util.Collection;

/**
 * 字符串连接工具类
 */
public final class StringUtils {

    private StringUtils() {
    }

    /**
     * @see #join(Object[] array, String sep, String prefix)
     */
    public static String join(Object[] array, String sep) {
        return join(array, sep, null);
    }

    /**
     * @see #join(Object[] array, String sep, String prefix)
     */
    public static String join(Collection list, String sep) {
        return join(list, sep, null);
    }

    /**
     * @see #join(Object[] array, String sep, String prefix)
     */
    public static String join(Collection list, String sep, String prefix) {
        Object[] array = list == null ? null : list.toArray();
        return join(array, sep, prefix);
    }

    /**
     * 以指定的分隔符来进行字符串元素连接
     * 
     * 例如有字符串数组array和连接符为逗号(,)
     * 
     * String[] array = new String[] { "hello", "world", "qiniu", "cloud","storage" };
     * 
     * 那么得到的结果是:
     * 
     * hello,world,qiniu,cloud,storage
     * 
     * 
     *
     * @param array  需要连接的对象数组
     * @param sep    元素连接之间的分隔符
     * @param prefix 前缀字符串
     * @return 连接好的新字符串
     */
    public static String join(Object[] array, String sep, String prefix) {
        if (array == null) {
            return "";
        }

        int arraySize = array.length;

        if (arraySize == 0) {
            return "";
        }

        if (sep == null) {
            sep = "";
        }

        if (prefix == null) {
            prefix = "";
        }

        StringBuilder buf = new StringBuilder(prefix);
        for (int i = 0; i < arraySize; i++) {
            if (i > 0) {
                buf.append(sep);
            }
            buf.append(array[i] == null ? "" : array[i]);
        }
        return buf.toString();
    }

    /**
     * 以json元素的方式连接字符串中元素
     * 
     * 例如有字符串数组array
     * 
     * String[] array = new String[] { "hello", "world", "qiniu", "cloud","storage" };
     * 
     * 那么得到的结果是:
     * 
     * "hello","world","qiniu","cloud","storage"
     * 
     * 
     *
     * @param array 需要连接的字符串数组
     * @return 以json元素方式连接好的新字符串
     */
    public static String jsonJoin(String[] array) {
        int arraySize = array.length;
        int bufSize = arraySize * (array[0].length() + 3);
        StringBuilder buf = new StringBuilder(bufSize);
        for (int i = 0; i < arraySize; i++) {
            if (i > 0) {
                buf.append(',');
            }

            buf.append('"');
            buf.append(array[i]);
            buf.append('"');
        }
        return buf.toString();
    }

    public static boolean isNullOrEmpty(String s) {
        return s == null || "".equals(s);
    }

    public static boolean inStringArray(String s, String[] array) {
        for (String x : array) {
            if (x.equals(s)) {
                return true;
            }
        }
        return false;
    }

    public static byte[] utf8Bytes(String data) {
        return data.getBytes(Constants.UTF_8);
    }

    public static String utf8String(byte[] data) {
        return new String(data, Constants.UTF_8);
    }

    public static String md5Lower(String src) throws NoSuchAlgorithmException {
        MessageDigest digest = MessageDigest.getInstance("MD5");
        digest.update(src.getBytes(Charset.forName("UTF-8")));
        byte[] md5Bytes = digest.digest();
        return Hex.encodeHexString(md5Bytes);
    }
}