Lecture 3: Topics

1
Lecture 3 Topics

• If-then-else
• Operator precedence
• While loops
• Static methods
• Recursion

2
Boolean expressions continued

• If I can get an outdoor court tomorrow and its
  not raining or I can get an indoor court, we can
  play tennis
• English can be ambiguous, but in programming, AND
  always has higher precedence than OR

3
If-then-else

• if (i gt j)
• if (a gt b)
• ____
• else // which if does it match?
• ____
• The else clause matches the most recent
  unmatched if in the same block

4
Whats the difference?

• boolean x
• if (x true)
• ______
• if (x)
• ______
• if (x true)
• ______

5
Whats the difference?

• Between x and x
• They are both short for x x1, but they have
  different meanings in y x and y x
• The first means
• y x
• x
• while the second means
• x
• y x
• Potentially confusing, better to use the longer
  versions in my opinion

6
Mixing assignments in one line

• a b c 5
• assignment operators are evaluated right to left,
  but other binary operators with same precedence
  are evaluated left to right, such as a b c
• Very confusing! Better to avoid mixing
  assignment statements in one line in my opinion
• OK to use a5 instead of a a 5, etc, but
  this is not necessary

7
Operator precedence

• postfix , - -
• unary , -, prefix , - - (avoid)
• casting
• !
• ,/,
• binary ,-
• lt, lt, gt, gt
• , !
• (avoid)
• (avoid)
• (avoid)
• , , -, , /, (I avoid except )
• Use (,) when needed AND ALSO FOR CLARITY!

8
While loop demo

• Extracting the letters from a string

9
Static methods

• In older programming languages these were called
  procedures (when they did not return a value)
  and functions (when they returned a value)
• Modifiers static, public, private
• Return value void, or a type
• The formal parameters (or parameters) are the
  ones listed in the method header
• The actual parameters (or arguments) are the ones
  provided when the method is called
• Together, the name of the method and its list of
  parameter types constitute the method signature

10
Why do we need methods?

• To organize programs into manageable chunks of
  code
• Abstraction, encapsulation
• Always think of how to simplify code by using
  methods appropriately

11
Local variables and their scope

• A variable declared inside a method is local to
  the method and cannot be accessed outside it
• A variable declared in a loop or a block
  cannot be accessed outside it
• Example
• for (int i0 ilt10 i)
• System.out.println(i) // error, i is not
  accessible

12
Method overloading

• When two methods have the same name but different
  signatures
• Many examples in the Java libraries

13
The call stack

• Stack of info about the method call history,
  including space for local variables

14
Recursion (chapter 19!)

• When a method calls itself
• classic example factorial
• but factorial can just as well be programmed with
  a loop
• recursion really useful when the method makes two
  recursive calls
• bad example Fibonacci (easy to write recursive
  code but it is very inefficient)