private String DateTransfer(String validEndTime){
        try {
            if(validEndTime.length() == 8){
                validEndTime = validEndTime.substring(0, 4) + "." + validEndTime.substring(4, 6) + "." + validEndTime.substring(6, 8);
            }
        } catch (Exception e) {

        }
        return validEndTime;
    }
 

substring：

public String substring(int beginIndex, int endIndex) {
        if (beginIndex < 0) {
            throw new StringIndexOutOfBoundsException(beginIndex);
        }
        if (endIndex > value.length) {
            throw new StringIndexOutOfBoundsException(endIndex);
        }
        int subLen = endIndex - beginIndex;
        if (subLen < 0) {
            throw new StringIndexOutOfBoundsException(subLen);
        }
        return ((beginIndex == 0) && (endIndex == value.length)) ? this
                : new String(value, beginIndex, subLen);
    }
 

substring中，引數beginIndex 和endIndex 

擷取字串比如前四位，要從0開始到4結束

因為subLen是endIndex-beginIndex，所以如果想要擷取前四位 一定是4-0