Java程序辅导

17 Feb 2004 1 of 20
Engineering Software 1
Week 3
Data types and control statements
Prepared by
Stavros Dimitriou
Engineering Software 1 - Week 3 2 of 2017 Feb 2004
Engineering Software 1
Contact details
Stavros Dimitriou
dimitrsa@lsbu.ac.uk
Tower Block - Room T820
Tel.: 0207 815 7580
Unit’s information can be found on the local network at eent3
http://eent3.sbu.ac.uk/staff/dimitrsa/
Engineering Software 1 - Week 3 3 of 2017 Feb 2004
Objectives of session
„ In the preceding session, you learned how to create, compile and
run a simple Java program and you were introduced to the structure 
of a program (identifier, variables, methods, symbols, Java 
keywords).
In this session, 
„ (a) you will be introduced to the rest of the data types and operation 
and,
„ (b) you will learn various selection and loop control statements
Engineering Software 1 - Week 3 4 of 2017 Feb 2004
A Java program 
Compute the area of a circle
Algorithm
1. Read in the radius
2. Compute the area using the following formula:
area = radius x radius x π
3. Display the area 
Engineering Software 1 - Week 3 5 of 2017 Feb 2004
Compute the area of a circle
// ComputeArea.java: Compute the area of a circle
public class ComputeArea {
Public static void main(String[] args) {
Double radius;
Double area;
// Assign a radius
Radius = 20;
// Compute area
Area = radius * radius * 3.14159;
// Display results
System.out.println(“The area for the circle of radius” +  
radius + “ is ” + area);
}
}
Engineering Software 1 - Week 3 6 of 2017 Feb 2004
Identifiers
„ Every entity has a name. Java uses special symbols called 
identifiers to name programming entities as variables, constants, 
methods, classes and packages.
Rules for naming identifiers:
„ An identifier is a sequence of characters that consists of letters, 
digits, underscores (_), and dollar signs ($)
„ An identifier must start with a letter, an underscore (_), or a dollar 
signs ($). It cannot start with a digit.
„ An identifier  cannot be a reserved word (See Java keywords)
„ An identifier  cannot be true, false or null.
„ An identifier can be of any length.
Java is case sensitive, ie. A and a are different entities
Engineering Software 1 - Week 3 7 of 2017 Feb 2004
Identifiers and Java Keywords
abstract
class
extends
implements
import
instanceof
interface
native
new
package
private
protected
public
super
this
volatile
break
case
catch
default
do
else
finally
for
if
return
switch
synchronised
throw
throws
try
while
boolean
byte
char
double
false
final
float
int
long
short
static
transient
true
void
ObjectsControlData
Engineering Software 1 - Week 3 8 of 2017 Feb 2004
Variables
„ Variables are used to store data input, data output or intermediate 
data. In our example program, radius and area are variables of 
double-precision, floating-point type.
„ To use a variable, you declare it by telling the compiler the name of 
the variable as well as what type of data it represents (variable 
declaration). 
„ Here are some examples:
int x;                   //declare x to be an integer variable;
double radius;   // declare radius to be a double variable;
char a;               // declare a to be a character variable;
Engineering Software 1 - Week 3 9 of 2017 Feb 2004
Assignments Statements
„ After a variable is declared, you can assign a value to it by using an 
assignment statement and the syntax is as follows:
variable = expression;
„ An expression represents a computation involving values, variables 
and operators that evaluates to a value.
„ Examples:
int x =1;                                // Assign 1 to variable x;
double radius = 1.0;            // Assign 1.0 to variable radius;
a = ‘A’;                                 // Assign ‘A’ to variable a;
x = 5*(3/2)+3*2;                   // Assign the value of the expression to x
Area = radius * radius * 3.14159;       //Compute area
Engineering Software 1 - Week 3 10 of 2017 Feb 2004
Constants
„ The value of a variable may change during the execution of the 
program, but a constant represents permanent data that never 
change. In our ComputeArea example program, π is a constant.
„ Here is the syntax for declaring a constant:
final datatype CONSTANTNAME = VALUE;
„ The word final is a Java keyword which means that the constant 
cannot be changed.
„ Example
final double PI = 3.14159;  //Declare a constant
Engineering Software 1 - Week 3 11 of 2017 Feb 2004
Numeric data Types
„ Every data type has a range of values. The compiler allocates 
memory space to store each variable or constant according to its data 
type.
„ Java has six numeric types: four for integers and two for floating 
points numbers.
64-bit IEEE 754-1.7E308 to 1.7E308double
32-bit IEEE 754-3.4E38 to 3.4E38float
64-bit signed(-263) to (263) long
32-bit signed(-231) to (231) int
16-bit signed(-215) to (215) short
8-bit signed(-27) to (27) byte
Storage SizeRangeName
Engineering Software 1 - Week 3 12 of 2017 Feb 2004
Numeric Operators
„ The arithmetic operators for numeric data types include addition (+), 
subtraction (-), multiplication (*), division (/) and remainder (%).
„ Examples:
int i1= 34 +1;                    // i1 becomes 35
double d1 = 34.0 – 0.1;   // d1 becomes 33.9
long i2 = 300 * 30;           // i2 becomes 9000
double d2 = 1.0 /2.0;       // d2 becomes 0.5
int i3 = 1 / 2;                    // i3 becomes 0
byte i4 = 20 % 3             // i4 becomes 2
„ Shortcut Operators (e.g. i = i + 8; or i += 8;)
Engineering Software 1 - Week 3 13 of 2017 Feb 2004
Further data types and operators
„ Numerical Type Conversions
„ Character data type (char)
„ Casting between char and numeric types
„ Boolean data type (less tan, greater than, equal to…)
„ Convert strings to numbers (ie. Convert a string into an int or double
value)
Engineering Software 1 - Week 3 14 of 2017 Feb 2004
Control Statements
„ Program control specifies the order in which statements are 
executed in a program.
„ Java provides selection statements that let you choose actions with 
two or more atlernatives.
„ In our ComputeArea example program we can use selection 
statements in the following pseudocode:
If the radius is negative
the program displays a message indicating a wrong input;
else
the program computes the area and displays the result;
Engineering Software 1 - Week 3 15 of 2017 Feb 2004
Selection Control Statements
if … else statements
„ A simple if statement executes an action only if the condition is true.
„ If the condition is false, nothing is done
„ Here is the syntax for this type of statement:
if (booleanExpression) {
Statement(s) for the true case;
}
else {
Statements for the false case
}
Engineering Software 1 - Week 3 16 of 2017 Feb 2004
Selection Control Statements
Nested if statements
„ The statement in an if or if … else statement can be any legal Java 
statement, including another if or if … else statement.
Switch statements
„ Java provides a switch statement to handle multiple conditions 
efficiently.
„ You can use switch statement to replace the nested if statement
switch (switch expression) {
case value1: statement(s)1;
break;
...
Engineering Software 1 - Week 3 17 of 2017 Feb 2004
Selection Control Statements
Conditional expressions
„ Example:
if (x > 0)
y = 1; 
else
y = -1;
„ This statement assigns a to y if x is greater than 0 and -1 to y if x is 
less than or equal to 0.
„ Alternatively:
y = (x > 0) ? 1 ; -1;
Expression: booleanExpression ? Expression1 : expression2;
Engineering Software 1 - Week 3 18 of 2017 Feb 2004
Loop statements
„ Loops are structures that control repeated executions of a block of 
statements.
The while loop
while (loop continuation condition) {
//loop body;
}
The do-while loop
„ Is a variation of the while loop; its syntax is given below:
do {
// loop body;
} while (loop continuation condition);
Engineering Software 1 - Week 3 19 of 2017 Feb 2004
Loop statements
The for loop
„ Syntax:
for (initial action; loop continuation condition;
action after each iteration) {
// loop body;
}
„ A for loop generally uses a variable to control how many times the 
loop body is executed and when the loop terminates.
Engineering Software 1 - Week 3 20 of 2017 Feb 2004
Questions ? …Comments
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