The String API you may use

A String is an object — the first one you use. You never create it with new in exam code; a literal is fine:

String name = "Minh";

The exam's official method set (the Java Quick Reference list):

name.length()          // 4
name.substring(1)      // "inh"  — from index 1 to the end
name.substring(1, 3)   // "in"   — index 1,2 (end index EXCLUDED)
name.charAt(0)         // 'M'   — returns a char, not a String
name.indexOf("i")      // 1     — first position, -1 when absent
name.equals("Minh")    // true
name.compareTo("Nam")  // <0 — "Minh" sorts before "Nam"

Index rules: valid positions are 0 through length() - 1. Out of range → StringIndexOutOfBoundsException at runtime. For substring(lo, hi): lo may equal length() (empty result), but hi may not exceed it.

Immutability: String methods never change the string — they return new ones. name.toUpperCase() alone does nothing; you must store it: name = name.toUpperCase();