Convertir les bases dans le système de numération

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public class ConversionBase { 
 
	public static String convertJava(String number, int from, int to) { 
		return Integer.toString(Integer.parseInt(number, from),to); 
	} 
	 
	public static String convert(String number, int from, int to) { 
		Objects.requireNonNull(number,"Number must be not null"); 
		return convert(convert(number, from),to); 
	} 
 
	public static int convert(String number, int radix) { 
		Objects.requireNonNull(number,"Number must be not null"); 
		if ( radix<Character.MIN_RADIX || radix>Character.MAX_RADIX ) { 
			throw new NumberFormatException("Illegal radix: " + radix); 
		}  
		boolean minus=false; 
		if ( number.trim().isEmpty() ) { 
			throw new NumberFormatException("For input string: \"" + number + "\""); 
		} 
		else if ( number.startsWith("-")) { 
			minus = true; 
			number = number.substring(1); 
		} 
		int result=0; 
		int pow=1; 
		final char[] chars=number.toCharArray(); 
		for(int i=chars.length-1; i>=0; i--) {  
			if (pow<=0) throw new NumberFormatException("For input string: \"" + number + "\""); 
			final int value = valueOf(number, chars[i]); 
			if ( value<radix ) { 
				result+=value*pow; 
			} 
			else { 
				throw new NumberFormatException("For input string: \"" + number + "\""); 
			} 
			pow*=radix; 
		} 
		return minus?-result:result; 
	} 
	 
	private final static int MAX_DIGIT_10=(int)Math.log(Integer.MAX_VALUE)+1; 
	public static String convert(int number, int radix) { 
		if ( radix<Character.MIN_RADIX || radix>Character.MAX_RADIX ) { 
			throw new NumberFormatException("Illegal radix: " + radix); 
		} 
		boolean minus=false; 
		if ( number==0 ) { 
			return "0"; 
		} 
		else if ( number<0 ) { 
			minus=true; 
			number=-number; 
		} 
		final StringBuilder stringBuilder=new StringBuilder(MAX_DIGIT_10+(minus?1:0));  
		while(number>0) { 
			stringBuilder.insert(0, valueOf(number%radix)); 
			number/=radix; 
		}  
		if ( minus ) { 
			stringBuilder.insert(0,'-'); 
		} 
		return stringBuilder.toString(); 
	} 
 
	private static int valueOf(String number, char c) { 
		if ( '0'<=c && c<='9') { 
			return c-'0'; 
		} 
		else { 
			c = Character.toLowerCase(c); 
			if ( c>='a' && c<='z' ) { 
				return 10+c-'a'; 
			} 
			else { 
				throw new NumberFormatException("For input string: \"" + number + "\""); 
			} 
		} 
	} 
	 
	private static char valueOf(int c) { 
		if ( 0<=c && c<=9) { 
			return (char)('0'+c); 
		} 
		else { 
			return (char)('a'+(c-10)); 
		} 
	} 
	 
	public static void main(String[] args) { 
		test("123",10,2); 
		test("123",8,2); 
		test("123",10,10); 
		test("123",4,10);  
		test("1111111111111111111111111111111",2,10); 
		test("FF01",16,2); 
		test("abcd",16,10); 
		test("abcd",20,10); 
		test("-42",10,4); 
		test("-42",10,2); 
	} 
 
	private static void test(String number, int from, int to) { 
		System.out.println("Convert "+number+" from " + from + " to " + to +":"); 
		System.out.println("Java: " + convertJava(number, from, to)); 
		System.out.println("Self: " + convert(number, from, to)); 
	} 
	 
}