Tricks and Traps (GOTCHAS)

The code shown would (suprisingly) print "Boo!". 🐞

See next.

Rectangle rec1, rec2;
rec1 = new Rectangle(10, 5);
rec2 = new Rectangle(10, 5);
if (rec1 == rec2)
	prn("Yay!");
else
	prn("Boo!");

Use 'equals' for Objects

For objects we want to use "equals" as shown.

The code shown would print "Yay!" (this assumes that 'equals' is properly implemented on Rectangle).

Rectangle rec1, rec2;
rec1 = new Rectangle(10, 5);
rec2 = new Rectangle(10, 5);
if (rec1.equals(rec2))
	prn("Yay!");
else
	prn("Boo!");

What is '==' For?

This will print "Yay!" (try it).

Only primitive (tiny) values use '==' for equality. Types including "int", "boolean" and "double".

Note that String is a full-fledged object type (i.e. use "equals" not "==").

Confusing eh? See next.

int a, b;
a = 10;
b = 10;
if (a == b)
	prn("Yay!");
else
	prn("Boo!");

Trick -- Think 'equals' 💡

If not sure which to use, we might first try "equals".

The compiler should give us a clear message if we are dealing with a primitive (that does not support "equals"). Then we could flip to "==".

int a, b;
if (z.equals(y))
	prn("Yay!");
else
	prn("Boo!");