Head First Java a brain-Friendly Guide Kathy Sierra and Bert Bates

To see if two references are the same (which means they refer to the
same object on the heap) use the == operator
Remember, the == operator cares only about the pattern of bits in the
variable. The rules are the same whether the variable is a reference or
primitive. So the == operator returns true if two reference variables refer to
the same object! In that case, we don’t know what the bit pattern is (because
it’s dependent on the JVM, and hidden from us) but we 
do
know that
whatever it looks like, 
it will be the same for two references to a single
object
.
Foo a = new Foo(); 
Foo b = new Foo(); 
Foo c = a; 
if (a == b) { // false } 
if (a == c) { // true } 
if (b == c) { // false }

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 SHARPEN YOUR PENCIL
What’s legal?
Given the method below, which of the method calls listed on the right
are legal? Put a checkmark next to the ones that are legal. (Some
statements are there to assign values used in the method calls).
int calcArea(int height, int width) { 
return height * width; 
} 
int a = calcArea(7, 12); 
short c = 7; 
calcArea(c,15); 
int d = calcArea(57); 
calcArea(2,3); 
long t = 42; 
int f = calcArea(t,17); 
int g = calcArea(); 
calcArea(); 
byte h = calcArea(4,20); 
int j = calcArea(2,3,5);

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BE the compiler
Each of the Java files on this page represents a complete source file.
Your job is to play compiler and determine whether each of these files
will compile. If they won’t compile, how would you fix them, and if they