Newport Robotics Group
Fundamentals of ObjectOriented Robot Programming in
Java
Day 3
10/9/2014
T/Th, 6:30 8:30 PM
Big Picture School
Day 1 Review Exercise
Display
Display
the sum of 4 numbers
the remainder when the sum is
divided by 3
Object-Oriented Programming
(OOP)
Object
Oriented Programming (OOP) is a
methodology that breaks programs into
discrete, reusable units of code called
objects which attempt to model things in
the real world
Class Blueprint that defines properties
and behaviors applying to a set of objects
Object A specific instance of a class
But what does a class do?
A class defines 3 things about a type of
object:
Instance Fields - Information the object
contains (data)
Methods - What the object does
Constructors - How the object is made
What does the code look like?
Code
is a series of keywords and
instructions that tell the computer what to
do
In Java, whitespace does not tell the
computer to stop doing something,
although you need at least one space
between keywords
Generally, we put each statement on its own
line to make it easy to read
semicolon at the end of a statement tells
the compiler where the statement ends;
Primitive Types
boolean
true or false (Is the cow
awake?)
int an integer (How many spots on a
cow?)
double a precise floating-point number
(How many pounds does the cow weigh?)
Others
float less-precise floating-point number
char a text character, such as: a $ 6
A few more we probably dont need to know
Variables
We
use variables to store data in the
computers memory
Before we can use a variable, we must
declare it to the compiler
A declaration consists of a data type and a
variable name (plus a semicolon).
Examples:
boolean awake;
int spots;
camelCase
When
naming things in Java, we have to
keep things as one word
Generally, it is best if variable names are
as descriptive as possible
For variables, leave the 1st letter
lowercase
Capitalize the 1st letter of each word
This system is called camelCase (for the
hump at the beginning of each word)
Example: robotPositionOnField
Default Values
Once
you declare a variable with a
primitive type, it contains its default value
Default
values:
boolean: false
int: 0
double: 0.0
float: 0.0f
(f indicates its a float and not a double)
Assignment
Once we have a variable, we can use it!
Use the equals sign (=) to assign a value
on the right to a variable on the left
int studentCount;
studentCount = 17;
Now studentCount holds the value 17!
Initialization
Declaring
a variable and then setting its value
is tedious. We can do this faster on one line:
int studentCount;
studentCount = 17;
int studentCount = 17;
These two statements are equivalent!
When you declare a variable, you should almost
always initialize it right away.
What if I want to explain my
code?
We can explain what code does or make notes
about it using comments
Code after two slashes (//) will be ignored by the
Java compiler if it is on the same line
Any code between /* and */ will be ignored, even if
it is on multiple lines.
Example:
x = 2; /* This is a multi-line
comment explaining my code. */
y = 3; // Here is a single-line comment.
b = true;
Comparison Operators
Output boolean
Equality: ==
values true or false
Greater than: >
Less than: <
Greater than or equal:
Less than or equal: <=
Not
equal: !=
>=
Boolean Operators
AND
OR
&& ||
true &&
true ==
true
true &&
false
==
false
false
&& true
==
false
true ||
true ==
true
true ||
false
== true
NOT
false
|| true
== true
!true == false
Math Operators
Addition +
Subtraction
Multiplication *
Division /
Remainder of Division %
Increment (add 1) ++
Decrement (subtract 1)
--
Assignment/Math Operators
To
add 6 to a number, we could write:
int x = 5;
x = x + 6;
Or
we could use the += operator:
int x = 5;
x += 6;
adds a number at right to the value of x
and stores the result back in the variable x
A similar thing is true for -=, *=, /=, and
%=
+=
Strings
A group of characters (chars) that form
Use quotation marks to make a String
text
Example:
The quick brown fox
jumps over the lazy dog.
You
can display a String on the screen using
System.out.println(show this);
The main Method
When
you run a program, the first thing that
happens is the main method runs
Code between the curly braces will be
executed
Dont worry about the details of the first
line it will make more sense later (and
Eclipse will automatically generate it for
you)
public static void main(String[] args)
{
// ...Code you put here will run!
Sample Code
This
code will compute the sum of three
integers and print it on the screen.
int a = 5;
int b = 7;
int c = -1;
int sum = a + b + c;
System.out.println(sum);
Where we left off
Your Turn
Write
code that does the following:
Declares two integer variables
Initializes the two variables
Declares a variable to hold an average
Assigns the average of the two integers to the
average variable
Prints the result
A Possible Solution
int a = 1;
int b = 4;
double average = (a + b) / 2.0;
System.out.println(average);
Methods
Organize our codes functions
What is a method?
method is a block of code that performs
some operation
Think of it as a machine that both inputs
and outputs data
A
accessor method provides access to
data
A mutator method changes data
An
Methods
Think
of a method from our earlier
example: We need to find the average of
two integers.
What are the inputs of the method?
What is the output of the method?
What does the method do?
What is the method called?
Method Name
A
method name must describe what the
method does accurately and succinctly
Use camelCase starting with a lowercase
letter
Example:
sumOfThreeInts
Parameters
to a method are called parameters
or arguments
Just as before, we need to tell the
compiler what data types the parameters
are by declaring them
Separate parameters by commas
Inputs
Example:
int a, int b
Return Type
When
a method outputs a value, we say
that it returns that value
We must specify to the compiler what
data type the method outputs
This is called the return type
If
a method has no output, its return type
is called void
Access Modifiers
method should specify an access
modifier to control who can use the
method
The most common access modifiers are
public and private
Public methods can be used by anyone
Private methods can only be used
internally
Example: In an ATM machine, the method
enterPIN should be public (users can
press buttons), but the method
A
Method Signature
method signature is the first line of
any method and tells the compiler what
the method does
The method body, or code that runs when
the method is used, must follow the
contract set up by the method signature
Example: if the signature says that the
method outputs an int, the body cannot
output a boolean.
The
Method Signature
We
can make a method signature by putting
the following parts together:
Access Modifier
Return Type
Method Name
Parameters (in parentheses)
If there are no parameters, use empty parentheses (). Do not put
void.
Example:
public int sumOfThreeInts(int a, int b, int c)
Access
Modifier
Return
Type
Method
Name
Parameter
s
Method Body
The
body of the method is a block of code
between two curly braces
The method body uses the inputs and
generates the output
The return statement indicates the output
public int sumOfThreeInts(int a, int b, int c)
{
Method
Signature
// ...(Method body goes here)
return something; //something is an int
}
Implementation
When
we write the code for a method, we
say that we implement that method
public int sumOfThreeInts(int a, int b, int c)
{
int sum = a + b + c;
Implementation
return sum;
}
Calling Methods
Now
we have created our method, or
machine. However, just because it knows
what to do doesnt mean it gets used
Example: A washing machine may have all the
code it needs to run, but nothing will happen
until the user puts clothes in and turns it on
To
make a method run, we call it.
How to Call a Method
To
call a method, put the method name
and values for any parameters in
parentheses
Example: sumOfThreeInts(5, 2, 1)
In this case, the method will run with a having a
value of 5, b with a value of 2, and c with a value
of 1.
However, we dont do anything with the
value the method returns in this case
Using a Method Call
We
can make use of method calls by
storing their return values in a variable.
Example:
int x = sumOfThreeInts(1, 4, -3);
In this example, after the line of code runs, x
will have a value of 2.
Any Questions?
Before you try it?
Implement Your First Method
Write
the method for the average of two
integers complete with a method
signature and implementation.
Call your method from the main method.
Store the return value from your method
in a variable you declare in main
Print out the return value from your
method at the end of the main method.
Possible Solution
public double average(int a, int b){
double a = a+b;
a = a/2;
return a;
}
public static void main(String[] args){
System.out.println(average(6,9));
//prints out 7.5
}
Lets Turn it up a Notch
Write
a program that takes in 3 doubles
and returns in the difference between the
mean and the sum.
Call your method from main(String[] args)
Possible Solution
public double a(double a, double b, double c){
return (a+b+c)*2.0/3.0;
}
public static void main(String[] args){
System.out.println(a(5.5,7.5,9));
}
Lets turn it up a Notch
(Challenge)
Write
a program that takes in 2 integers
and returns the larger.
No if statements.
Can use Math in javas library
Possible Solution
public static int(int a, int b){
return(Math.abs(a-b)+a+b)/2;
}
Lets Turn it up (Bigger Challenge)
Extend
the problem to a method that
takes in an array of integers and returns
the largest one.
Possible Solution
public static int max(int[] a, int i){
if(i == 1){
return(Math.abs(a[0]-a[1])+a[0]+a[1]);
}
int[] b = new int[2];
b[0] = a[i];
b[1] = max(a,i-1);
return max(b,1);
}
Lets Turn it up some more
(Even Bigger Challenge)
Write
a method that takes in an integer n,
and outputs all the reduced fractions from
0 to 1 in increasing order with
denominator less than or equal to n.
