Decimal10 to Decimal2 etc...

Decimal.java

import java.util.ArrayList;
import java.util.List;
import java.util.Collections;

class Decimal {

	static int DECIMAL_10 = 20;

	public static void main(String[] args) {
		System.out.println(convertDecimal(DECIMAL_10, 2)); // 10100
		System.out.println(convertDecimal(DECIMAL_10, 8)); // 24
		System.out.println(convertDecimal(DECIMAL_10, 16)); // 14
	}

	private static String convertDecimal(int num_10, int convertDecimalNum) {
		List<Integer> decimal = new ArrayList<>();
		int num = num_10;
		while (num > 0){
			decimal.add(num % convertDecimalNum);
			num = num / convertDecimalNum;
		}
		Collections.reverse(decimal);
		return ArrayToString(decimal);
	}

	private static String ArrayToString(List<Integer> array) {
		StringBuilder buf = new StringBuilder();
		for(int index : array){
			buf.append(String.valueOf(index));
		}
		return buf.toString();
	}
}