public static void main(String[] args) {
String s1 = "ab" + "c";
String s2 = "abc";
String s3="abcd";
System.out.println(s1 == s2+null);
System.out.println(s1 == s3.substring(0, 3));
System.out.println(s1.equals(s3.substring(0, 3))+"+"+"null"+"+"+null);
}
}