format-str-in-date.java

public class ExFormatationDate {

	public static void main(String[] args) {
		String dt1 = "20200124";
		StringBuilder sb = new StringBuilder(dt1)
				.insert(4,"-")
				.insert(7,"-");
		String output = sb.toString();
		System.out.println(output); 

		
		String dt2  = "20200124";
		String dataf = dt2.replaceAll("(\\d{4})(\\d{2})(\\d{2})", "$1-$2-$3");
		System.out.println(dataf);
	}
}