== vs equals, and traversal

The most exam-famous String trap:

String a = "cat";
String b = new String("cat");
System.out.println(a == b);        // false! == compares references
System.out.println(a.equals(b));   // true  — content comparison

== asks "same object?"; .equals asks "same characters?". Always use .equals for String content. (compareTo returns 0 exactly when equals is true — it also tells you ordering, which ==/equals cannot.)

Traversal patterns — memorize all three, the exam cycles through them:

// 1. by index (when you need the position)
for (int i = 0; i < s.length(); i++) {
    char c = s.charAt(i);
}

// 2. substring walk (when you need neighboring pieces)
for (int i = 0; i + 1 < s.length(); i++) {
    String pair = s.substring(i, i + 2);
}

// 3. first-to-last scan with early exit
int pos = s.indexOf('x');
if (pos != -1) { /* found */ }

Concatenation builds new strings: "ab" + "cd", s + 42, 42 + s all work — numbers convert automatically.