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CSE 131 Computer Science 1
Module 1: (basics of Java)
Welcome to the World of Java
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Eclipse
 Integrated
Development Environment (IDE)
» “smart” editing of Java programs – continuous syntax checks
» compiles program files automatically as necessary
» integrated program testing and debugging tools
» has plug-ins for version control system (Subversion)
 Can
be a powerful tool for managing large projects
» but, can also be a bit overwhelming and mysterious at first
» watch the first few Eclipse videos on the web site before lab
 works
the same way in Windows, Mac and Linux
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Your First Program
public class HelloWorld {
public static void main(String[] args) {
System.out.println("Hello, world!");
}
}
» defines a class called HelloWorld (a class defines an object)
» class contains a single method called main
• in Java, the main method is the starting point of a program
• the main method has an array of arguments (not used here)
• Any program has one and only one main() method
» program “prints” a brief message and halts
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Today: understand basic building blocks
of JAVA
 You
will be able to
» Output a slogan on the screen
» Tell if a year is a Leap Year
» Calculate the area of a circle
» Output anything you like
» Do all kinds of calculation
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Basic unit: variable
A
variable in Java is a piece of
information stored in a computer’s
memory: like a jar holding food
 Each variable has a type
» Like different kinds of jars (different
food, size, usage, etc.)
 Main
types:
int a;
double b;
String s;
boolean x;
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Basic Data Types in Java
Type (keyword)
Meaning
Examples
Primitive
Operators
int
Integer
3, 0, -5, 256
+, -, *, /
double
Decimal number
1.5, -3.1415, 0.0
+, -, *, /
boolean
true or false
true, false
&&, ||, !
(return boolean:
<,>,<=,>=,!=,
==)
String
a sequence of
characters
“hello world”
+ (concatenation)
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Basic Definitions
type
Type.
A category of values and a set of operations on such values.
Variable. A name that refers to a value of a declared type.
Assignment statement. Associates a value with a variable.
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The int type
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Integers
This surprises most
students. Don’t feel
bad: experts forget
this too sometimes.
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Key points
 Assignment
statement
Variable = Expression ;
(For example: a = b+ 5/3; )
 Expression’s
type must match the variable’s type (will
come back to this point)
 Sequential
execution
» The order of statements matters
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The double type
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Example: compute the area of a circle
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String: a sequence of characters
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String operation
 Meaning
of characters
depends on context.
 Example: report +, -,
*, / of two integers
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Boolean
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Booleans
data type. Useful to control the logic of
a program.
boolean
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Comparisons
Comparisons.
Take two operands of one type (int
or double) and produce a result of type boolean.
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Leap Year
Q.
Is a given year a leap year?
A. Yes if either (i) divisible by 400 or (ii) divisible
by 4 but not 100.
public class LeapYear {
public static void main(String[] args) {
int year = 2012;
boolean isLeapYear;
// divisible by 4 but not 100
isLeapYear = (year % 4 == 0) && (year % 100 != 0);
// or divisible by 400
isLeapYear = isLeapYear || (year % 400 == 0);
System.out.println(isLeapYear);
}
}
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Data Type Conversion
 Operations
must be applied to same types, with the
following exceptions
 Java automatically converts some types of values to
make them compatible
int i, j; double x, y;
i = 5; j = i/2;
y = 2; x = i/y;
- As long as there is a double in an expression, the type of the
expression is promoted to double
 Combining
strings and numeric values
int i = 5;
String S = “two plus two =” + i;
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Basic Expressions in Java
Expression
Value
Type
3+5
8
int
7/2.0
3.5
double
7/2
3
int
3+5-2/4+2
10
int
3+5-2.0/4+2
9.5
double
(3+5-2)/(4+2)
1
int
“Hello”+”there”
“Hellothere”
String
“I have “+5+5+” toes” “I have 55 toes”
String
true && false
false
boolean
false || true
true
boolean
!true
false
boolean
(true&&false)||(!false)
true
boolean
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Variable types
A
variable can only hold values of the proper type
int x; (default 0)
int x = 1;
…
x = false; (error! Type-checking problem)
 Any
expression of type T can appear in an expression
where a value of type T is required
» E.g. int x = 3+5;
 int
x = 7/2.0; (type error)
 int x= (int) (7/2.0) (cast, x has value 3)
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Method
 What
if we need to find areas of different circles?
» Do we have write the code over and over again?
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Procedural abstraction (method)
 Find
areas of circles
 A method is like a blackbox (input  output)
» Hide details of computation from outside
A
java method
double circleArea(double radius) {
return Math.PI * radius * radius;
}
 Use
it:
double area = circleArea(10.0);
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Method Call and Return
 Java
methods used to decompose program into parts
» allows us to solve a problem once and conveniently re-use the
solution many times
 You
can invoke the method by using it in an expression
» Example: area = circleArea(2.0);
» The number of input parameters should match the method
definition
» The types of input parameters should match the method
definition
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Method for Length of Hypotenuse
 We
know C=√A2+B2
 Forms the basis of the Java method
C
A
B
double hypotenuse(double A, double B) {
return Math.sqrt(A * A + B * B);
}
» Math.sqrt(x) returns the square root of x
• part of a library of mathematical functions, including
sin(x), cos(x), tan(x), log(x), exp(x)
» Signature of a method: double hypotenuse(double, double)
defines method name, return type, types of arguments
» Invocation of a method must match its signature
 Example:
double h = hypotenuse(3, 4);
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