Randomly generated 5-bit case letters or digital __java base

randomly generated 5-bit uppercase or lowercase letters or numbers

Method One: Generate a duplicate

public static void Main (string[] args) {
		Random rand = new Random ();
		char[] letters=new char[]{' A ', ' B ', ' C ', ' D ', ' E ', ' F ', ' G ', ' H ', ' I ', ' J ', ' K ', ' L ', ' M ', ' N ', ' O ', ' P ', ' Q ', ' R ', ' S ', ' T ', '
				U ', ' V ', ' W ', ' X ', ' Y ', ' Z ', ' A ', ' B ', ' C ', ' d ', ' e ', ' f ', ' g ', ' h ', ' I ', '
				j ', ' K ', ' l ', ' m ', ' n ', ' o ', ' P ', ' Q ', ' R ', ' s ', ' t ', ' U ', ' V ', ' w ', ' x ', ' y ', ' z ', ' r ', '
				0 ', ' 1 ', ' 2 ', ' 3 ', ' 4 ', ' 5 ', ' 6 ', ' 7 ', ' 8 ', ' 9 '};
		String str = "";
		int index;
		Boolean[] flags = new boolean[letters.length];//defaults to False for
		(int i=0;i<5;i++) {
			do{
				index = Rand.nextint (letters.length); 
			} while (flags[index]==true);
			char C = letters[index];
			STR + C;
			flags[index]=true;
		}
		System.out.println (str);
	

Method Two: Generate duplicates, similar to method one

public static void Main (string[] args) {
		Random rand = new Random ();
		char[] letters=new char[]{' A ', ' B ', ' C ', ' D ', ' E ', ' F ', ' G ', ' H ', ' I ', ' J ', ' K ', ' L ', ' M ', ' N ', ' O ', ' P ', ' Q ', ' R ', ' S ', ' T ', '
				U ', ' V ', ' W ', ' X ', ' Y ', ' Z ', ' A ', ' B ', ' C ', ' d ', ' e ', ' f ', ' g ', ' h ', ' I ', '
				j ', ' K ', ' l ', ' m ', ' n ', ' o ', ' P ', ' Q ', ' R ', ' s ', ' t ', ' U ', ' V ', ' w ', ' x ', ' y ', ' z ', ' r ', '
				0 ', ' 1 ', ' 2 ', ' 3 ', ' 4 ', ' 5 ', ' 6 ', ' 7 ', ' 8 ', ' 9 '};
		String str = "";
		int index;
		Boolean[] flags = new boolean[letters.length];//defaults to False for
		(int i=0;i<5;i++) {
			do{
				index = Rand.nextint (letters.length); 
			} while (flags[index]==true);
			char C = letters[index];
			STR + C;
			flags[index]=true;
		}
		System.out.println (str);
	

Method Three: Generate Duplicates (recommended option)

public static void Main (string[] args) {
		String str = "";
		Random rand = new Random ();
		for (int i=0;i<5;i++) {
			int num = rand.nextint (3);
			Switch (num) {case
				0:
					char c1 = (char) (Rand.nextint + ' a ');//Generate random lowercase letter 
					str + c1;
					break;
				Case 1:
					char c2 = (char) (rand.nextint () + ' A ');//Generate Random uppercase letters 
					STR + c2;
					break;
				Case 2:
					str = rand.nextint (10);//Generate Random
			number
		}
		System.out.println ("generated 5 random Captcha is:" +str);
	}