Public, Static and Arrays

CS61BL Summer 2009

Access Levels (HFJ pg 667)• public – any code anywhere can access the public

thing• protected – same as default + subclasses outside the

package can inherit the protected thing.• default – only code within the same package as the

class with the default thing can access the default thing• private – only code within the same class can access the

private thing.

• By “thing” we mean class, variable, method, constructor

Public vs. Private Variables

• public variables are accessible outside of the class

• private variables are only accessible inside of the class

• When we get to inheritance - if you have a sub-class, it can’t access the private variables

Public vs. Private Methods

• Private methods are not accessible by other classes

• Helper methods should be private

static fields• Global state shared by all instances of the class• Looks like an instance variable declaration + static• Access them as ClassName.fieldName

static methods

static methods

• static methods do not operate on an object

• Has no implicit parameter this• Can NOT access instance variables• Can access static fields• Call the method as ClassName.methodName(…)

static version

Non static version

Announcements

• Join the google group (link from the webpage)• Don’t worry about your quiz scores!!!• Do the reading• PRACTICE, PRACTICE, PRACTICE!

Arrays

Debugging an Array

After the first line:

Debugging an Array

After the

second line:

Debugging an Array

PersonmyAge myNamemyParent

115

Gertrude Baines

mainpeople

Arrays

64 62 60 62 64 64 64 62 62 62

mainnotes

64 62 60 62 64 64 64 62 62 62

mainnotes

68

For Loops

for (initialize; test; increment)

{// loop-body

}

For Loops

for (int i = 0; i<10; i++)

{System.out.println(i + “ “);

}

Awesome For Loops (FOR EACH)

int [] notes = {60, 61, 62};

for (int oneNote : notes)

{System.out.println(oneNote + “ “);

}

64 62 60 62 64 64 64 62 62 62

mainnotesnotes2

0 0 0 0 0 0 0 0 0 0 064 62 60 62 64 64 64 62 62 62 70

Arrays

• Set to the default value for primitive types • Set to null for objectsType [] arrayName = new Type[size];

Type [] arrayName = {value1, value2};

• Arrays don’t change size• Arrays hold one type of thing• Access items in the array using the [] notation

arrayName[pos] = newValue;

Flow of control

• break – breaks from the inner most loop enclosing the break and continues executing at the code after the loop.

• return – causes the method to end immediately and return to the calling method

• continue – transfers control to the header of the inner most enclosing loop.

Code after

Code after Loop

continue

Test Driven Developmentstatic void beAGoodProgrammer()

{while (true) // for each method

{

writeTest();

writeCode();

writeAFewMoreTests();

writeCode();

}

}

Why do Test Driven Development?

• It is a way to brainstorm what your method is going to do.

• It is hard to write test cases• Write your test cases before you’re biased by

how your code is written

How to do Pair Programming?• Share one computer with your partner. • The “Driver” types and uses the mouse.• The “Navigator” watches, discusses and

focuses on the big picture. • Take turns (about every 30 minutes) to switch

roles.

Why to do Pair Programming?• Writing code is easy compared to making code work!

– The bugs are the part that takes a long time

• Clearer programs, better designs, fewer bugs.• Great to talk about at an interview!• “Overcoming difficult problems”• Research Study – Error free code went from 70% to

85% with pairs. So 30% to 15% or a 50% reduction in bugs.

Source: Wikipedia: Pair Programming

Estimating Time• Most people grossly underestimate how long it

takes to program (not thinking about time to debug and test)

• You appear incompetent at programming (not true) if you can’t estimate how long something will take.

• This is one of the hardest things to learn!!! Start to practice now! For each task estimate how long it will take and keep track of how wrong you were

Working with Partners

• Use pair programming!• Talk about other commitments

– Travel plans– Other classes– Jobs

• Assume that if you haven’t seen their code – it doesn’t exist!

• Talk to your partner the moment you get stuck