Q.Predict the output of the following code and explain each result:

String s = "Commerce";  
System.out.println(s.length());  
System.out.println(s.substring(0, 3));  
System.out.println(s.toUpperCase());  
System.out.println(s.indexOf("m"));  

Each line calls a different String method; remember indexes start at 0 and the original string never changes. …

Take each statement in turn, with s = "Commerce". Remember that indexes start at 0: C=0, o=1, m=2, m=3, e=4, r=5, c=6, e=7.

s.length() → 8. length() returns the number of characters. 'Commerce' has 8 letters, so the result is 8.

s.substring(0, 3) → "Com". substring(a, b) returns the characters from index a up to but not including index b. So it takes indexes 0, 1 and 2 — 'C', 'o', 'm' — giving "Com". (Index 3 is not included.)

s.toUpperCase() → "COMMERCE". This returns a new string with every letter in capitals. Note the original s is unchanged, because strings are immutable. …

s.charAt(2) would return the single character 'm' at index 2, a rel …

Expecting substring(0, 3) to include index 3 ('Comm') — the end index is exclusive. Thinking toUpperCase() changes s itself — it returns a new string; the original stays the same. …