public class Main {

    public static void main(String args[]) {

        String ret = "a";

        while (true) {

            ret += ret;

            System.out.println(ret.length());
        }
    }
}

e o erro :

Exception in thread "main" java.lang.OutOfMemoryError
    at java.lang.StringBuilder.ensureCapacityImpl(StringBuilder.java:339)
    at java.lang.StringBuilder.append(StringBuilder.java:205)
    at Main.main(Main.java:9)