COMP 14: Primitive Data and Objects
May 24, 2000
Nick Vallidis
Announcements
• Could you get Visual J++? Book?
• Is everyone signed up for an ISIS account?
• Office Hours?
• Lab Assistants’ hours are up
• Too fast?
Review
• What are the rules for a Java identifier?
• What symbol indicates the end of a statement in Java?
• What symbols start and end a section in a Java program?
• What are the two ways of indicating comments?
Outline for today
• Data in Java
• Data Types
• Variables
• Methods
• Strings
• Class libraries: Using Objects
Need 2 things for a program
• Just like baking a cake. You need:– ingredients– a recipe
• in programming, we need:– data– algorithm
• Today we’ll talk about our “ingredients”
How does Java handle data?
• Primitive data– data fundamental to a computer– numbers, characters (because people use them)
• Objects– represent more complex concepts by combining
primitive data– e.g., a car or a hotel
Data Types
• Both primitive data and objects can be described by a data type
• Data types have two parts:– a set of possible values– a set of operations
Data Type example
• Integers
• Set of values:– {…, -2, -1, 0, 1, 2, …}
• Set of operations:– addition, subtraction, multiplication, division
Another example
• Let’s take the car object example
• Values:– more complex: we’ll limit it to speed, amount
of gas in tank, and what gear it’s in
• Operations:– speed up, slow down, change gear, etc.
Objects and Data Types
• You tell the computer the data type for an object by writing a class
• A class contains:– variables to keep track of the value– methods to perform the operations
Variables
• A location in memory associated with an identifier and a specific data type
• Declaration:– tell the compiler to make space for a specific
kind of data and assign it a name
– you can also specify a value for the variable
int sum;
int sum = 0;
Primitive Data Types
• We’ll only use int, double, char, boolean:– int -- 32 bits -- integer from -2 billion to 2
billion (approx.)– double -- 64 bits -- floating point value from
-1.7x10308 to 1.7x10308 (approx)– char -- 16 bits -- one character such as ‘a’,
‘G’, ‘!’, ‘4’, etc. (also things like space, enter)– boolean -- holds true or false
Assignment
• Changes the value held by a variable
• Example:
• Destroys the old value in the variable
sum = 10;
int numBooks = 5;
numBooks = 25;
Constants
• What do we do if the variable’s value never changes?
• We can tell the compiler this:
• final means “I won’t ever change the value of this variable” (thus, it’s value is constant)
• use all capital letters for identifier
final int MAX_BRAINS = 1;
Expressions
• You can combine operators and operands to form expressions (usually as part of an assignment statement)
• for int some operations are: +, -, *, /, %
answer = 2 + 2;
answer = 17 / 3 + 6;
answer = answer * 2;
Remember this?
• You tell the computer the data type for an object by writing a class
• A class contains:– variables to keep track of the value– methods to perform the operations
Methods
• Methods are collections of programming statements that are given a name
• Usually they perform one of the operations of the data type
• Called procedures and functions when not in an object-oriented language
We’ve already seen methods
public class Simple
{
public static void main(String[] args)
{
System.out.println(“Hello!”);
}
} Here we are using a method (making a “call” to that method)
Let’s break it apart
System.out.println (“Hello!”);
objectobject methodmethodInformation provided to the methodInformation provided to the method
(parameters)(parameters)
System.out.println(“Hello!”);
• println is the name of a method that belongs to the System.out object
• It tells the System.out object that we’d like to print Hello! on the screen
• System.out.print is the same, but it doesn’t go to the beginning of the next line
So what is “Hello!”?
• We’re sending this data to the println method, but what is its data type?
• It’s a special case in Java called a string literal
• It’s data type is actually the String class
Objects work like primitive data
• You declare an object variable just like primitive data, but with a class data type:
• So we could do our simple program in a slightly different way if we wanted...
String greeting;
String greeting = “Hello!”;
Simple Program modified
public class Simple
{
public static void main(String[] args)
{
String greeting = “Hello!”;
System.out.println(greeting);
}
}
Things work a little differently...
• When you declare a variable with a class data type, it really just creates a reference
• You have to create an object of that data type (class) -- this is called instantiation
String greeting;
greeting = new String(“Hello!”);
<reference> = new <class>(<parameters>);
You saw a shortcut for strings...
• Before, I did it this way though:
• That’s because strings are so common, that Java lets you use string literals as a shortcut:
String greeting = “Hello!”;
greeting = “Hello!”;
is the same as
gretting = new String(“Hello!”);
Concatenating Strings
• What if we want to print out something long that doesn’t fit on one line?
• Java doesn’t like this:
• string literals must be on one line
System.out.println(“Hello, how
are you this fine day?”);
The solution
• The + operator can also be used to glue two strings together
• This works with primitive data too!
System.out.println(“Hello, how” +
“ are you this fine day?”);
int result = 5;
System.out.println(“Result: “ + result);
Other string “problems”
• What if you want a <return> in the string?
• You can’t do this:
• You could do:
System.out.println(“Hello.
Nice to meet you.”);
System.out.println(“Hello.”);
System.out.println(“Nice to meet you.”);
Escape Sequences
• There’s another way!
• If you put ‘\n’ in the string literal, it becomes a <return> in the output
System.out.println(“Hello.\nNice to meet you.”);
works the same as
System.out.println(“Hello.”);
System.out.println(“Nice to meet you.”);
Escape Sequences
• Called this because the ‘\’ is an escape character, which means we want to do something special
• Two others you should know:– \” -- puts a “ in the string (why do we need it?)– \\ -- puts a \ in the string (why do we need it?)
Class Libraries
• Java comes with a bunch of classes that you can use in your programs (e.g., String)
• a class library is just a name for a set of such classes
• Java puts a bunch of classes together in a package
Packages
• When you want to use a class in a package, you have to let the compiler know
• Even though you don’t have to type it, Java behaves like all programs have this import:
import <class name>
import java.lang.*;
Examples
Homework
• Reading: 2.1-2.7
• P1 (Program1) goes out today and is due Friday. There is no “programming” involved in this assignment. The purpose is to get you used to Visual J++. Additionally, you see how you will hand in your programs.
