小数を含む進数変換 https://paiza.io/projects/GETkSMiMN4VjQmgcIxvwxA

Main.java

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.math.BigDecimal;
import java.math.RoundingMode;
import java.util.regex.Pattern;

public class Main {
    
    private static final String CHARS = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ";
    
    public static void main(String[] args) throws Exception {
        String data = "12.345";
        int base = 10;
        
        if(base > 62){
            System.err.println("62進数までです");
            System.exit(1);
        }
        
        int integer = Integer.parseInt(data.split(Pattern.quote("."))[0]);
        BigDecimal few = new BigDecimal("0." + data.split(Pattern.quote("."))[1]);
        
        String newBase = "";
        for(int i = 0;i < 1200;i++){
            few = few.multiply(new BigDecimal(base));
            newBase += changeBase(few.intValue());
            
            few = few.subtract(new BigDecimal(few.intValue()));
        }
        
        // startPos 開始位置
        // length 桁数
        // checkPos 何回目
        
        boolean flag = true;
        String num = "";
        tag: for(int startPos = 0;startPos < 100;startPos++){
            for(int length = 1;length <= 100;length++){
                flag = true;
                for(int checkPos = 1;checkPos <= 10;checkPos++){
                    if( !newBase.substring( startPos, startPos + length ).equals( newBase.substring( startPos + length * checkPos,startPos + length * checkPos + length ) )){
                        flag = false;
                        break;
                    }
                    num = newBase.substring( startPos, startPos + length );
                }
                if(flag){
                    break tag;
                }
            }
        }
        
        System.out.println("整数部: " + data.split(Pattern.quote("."))[0]);
        System.out.println("少数部: " + "." + data.split(Pattern.quote("."))[1]);
        System.out.println("基　数: " + base);
        System.out.println("==============================================================================================================================");
        System.out.println("計算結果: " + Integer.toString(integer,base) + "." + newBase.substring(0,100));
        System.out.println("循環節: " + num);
        
    }

    public static char changeBase(int num){
        return CHARS.charAt(num);
    }
    
}