Transcript Lecture 3 Program Elements

Lecture 3
Program Elements
Instructors:
Fu-Chiung Cheng
(鄭福炯)
Associate Professor
Computer Science & Engineering
Tatung Institute of Technology
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Outline
• Data types
• Variable declaration and use
• Decisions and loops
• Input and output
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Primitive Data Types
• A data type: define values and the operators
• Each value stored in memory is associated with a
particular data type
• primitive data types: predefined data types in Java
Ex: A: integers: byte, short, int, long.
B: float, double.
C: boolean (true or false) (on or off)
D: char (unicode 16 bits for international languages)
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Storage in Programming
Languages
• Registers
• Stack (handles & primitives in Java)
• Heap (all class objects in Java)
• Static storage
• Constant storage
• Non-RAM storage (persistent)
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Primitive Data Types
• Integers:
Type
Storage
Min Value
Max Value
byte
short
int
long
8 bits
16 bits
32 bits
64 bits
-128
-32,768
-2,147,483,648
< -9 x 1018
127
32,767
2,147,483,647
> 9 x 1018
Note that Built-in types (primitives) are not object handles
handles: call by reference
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primitives: call by value
Primitive Data Types
• Floating point:
Type
Storage
Approximate
Min Value
float
double
32 bits
64 bits
-3.4 x 1038
-1.7 x 10308
S
Exponent
Approximate
Max Value
3.4 x 1038
1.7 x 10308
Mantisa
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Primitive Data Types
• char: Unicode character set
• A character set is an ordered list of characters
• The Unicode character set uses sixteen bits per character,
allowing for 65,536 unique characters
• International character set.
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Primitive Data Types
•boolean: a true or false condition (two states: on or off)
• The reserved words true and false are the
only valid values for a boolean type
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Primitive Data Types
•Same operations as C/C++
• Size of each data type is machine independent
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Wrappers for primitive Data
Types
• For each primitive data type there is a corresponding
wrapper class.
Primitive Type
int
double
char
boolean
Wrapper Class
Integer
Double
Character
Boolean
• Wrapper classes are useful in situations where you need
an object instead of a primitive type
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• They also contain some useful methods
Variables
• A variable is an identifier that represents a location in
memory that holds a particular type of data
• Variables must be declared before they can be used
• Syntax of a variable declaration:
data-type variable-name;
For example:
int total; // 4-byte int in stack
int total, count, sum;
int total = 0, count = 20;
float unitPrice = 57.25;
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Scope of Variables
• Block statements:
group of statements delimited by braces
{ // begin of scope 1
int x=1;
System.out.println("x="+x);
{ // begin of scope 2
// int x=3; // can not redine x
int y=2;
System.out.println("x="+x);
System.out.println(“y=”+y);
}
}
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Assignment Statements
• An assignment statement takes the following form:
variable-name = expression;
• The expression is evaluated and the result is stored in
the variable, overwriting the value currently stored
in the variable
• The expression can be a single value or a more
complicated calculation
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Constants
• A constant is similar to a variable except that they keep
the same value throughout their existence
• They are specified using the reserved word final
in the declaration. For example:
final double PI = 3.14159;
final int STUDENTS_COUNT = 25;
• All final are static.
final static double PI = 3.14159;
Better than literal values because:
A. make code more readable by giving meaning to a value
B. use less memory, easy to modification (one place)14
Input and Output
• Java I/O is complicated.
1. Different kind of IO:
Files, console, block of memory, network
2. Different kinds of operations:
Sequential, random-access, binary,
character, integer, by lines, be words, ...
• Java provides a lot of classes to support IO.
Input and Output
• Java I/O is based on input streams and output streams
1. All classes inherit from IS have read methods.
2. All classes inherit from OS have write methods.
• There are three predefined standard streams:
Stream
Purpose
Default Device
System.in
System.out
System.err
reading input
writing output
writing errors
keyboard
monitor
monitor
• print and println methods write to
standard output
Input and Output
• Escape sequences: a special sequence of characters
preceded by a backslash (\)
Escape Sequence
Meaning
\t
\n
\"
\'
\\
tab
new line
double quote
single quote
backslash
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Types of Output Stream
Writes to
ByteArrayOutputStrem
Block of Memory
FileOutputStrem
File
PipeOutputStrem
Pipe(to another thread)
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Types of Input Stream
Reads from
ByteArrayInputStrem
Block of Memory
FileInputStrem
File
PipeInputStrem
Pipe(to another thread)
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Input From Keyboard
• The Java API allows you to create many kinds of streams
to perform various kinds of I/O
• To read character strings, we will convert the
System.in stream to another kind of stream using:
BufferedReader stdin = new BufferedReader
(new InputStreamReader (System.in));
• This declaration creates a new stream called stdin
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Echo.java
import java.io.*;
class Echo {
public static void main (String[] args) throws IOException {
BufferedReader stdin = new BufferedReader
(new InputStreamReader(System.in));
String message;
System.out.println ("Enter a line of text:");
message = stdin.readLine();
System.out.println ("You entered: \"" + message + "\"");
} // method main
} // class Echo
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Open File
public class Scanner {
…
protected static DataInputStream inputStream;
Scanner(String fileName) throws IOException {
inputStream = new DataInputStream
(new FileInputStream (fileName));
}
…
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}
Buffers
• As you type, the characters are stored in an input buffer
• When you press enter, the program begins processing
the data
• Output information is temporarily stored in an
output buffer
• The output buffer can be explicitly flushed
(sent to the screen) using the flush method
• See Python.java
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Numeric Input
• Converting a string into the integer value:
value = Integer.parseInt(my_string);
• A value can be read and converted in one line:
num = Integer.parseInt(stdin.readLine());
C: scanf(“%d\n”, num);
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Addition2.java
import java.io.*;
class Addition2 {
public static void main (String[] args) throws IOException {
BufferedReader stdin = new BufferedReader
(new InputStreamReader(System.in));
int num1, num2;
System.out.println ("Enter a number: ");
num1 = Integer.parseInt (stdin.readLine());
System.out.println ("Enter another number:");
num2 = Integer.parseInt (stdin.readLine());
System.out.println ("The sum is " + (num1+num2));
} // method main
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} // class Addition2
Controlling Program Flow
• Essentially same as C/C++
• if statement:
if (condition)
Operator
statement;
• Relational operators:
==
!=
<
<=
>
<=
Meaning
equal to
not equal to
less than
less than or equal to
greater than
greater than or equal to
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Controlling Program Flow
• if-else statement
if (condition)
statement1;
else
statement2;
• while statement:
while (condition)
statement;
Note: Avoid infinite loop (logic error)
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Controlling Program Flow
• if-else statement
if (condition)
statement1;
else
statement2;
• while statement:
while (condition)
for statemetn:
for (e1;e2;e3)
statement;
Note: Avoid infinite loop (logic error)
statement;
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import java.io.*;
class Right_Triangle { // bad bad bad RightTriangle
public static void main (String[] args) throws IOException {
BufferedReader stdin = new BufferedReader
(new InputStreamReader(System.in));
int hypotenuse_sq; // bad hypotenuseSquare
System.out.println ("Enter side 1:");
int side1 = Integer.parseInt (stdin.readLine());
System.out.println ("Enter side 2:");
int side2 = Integer.parseInt (stdin.readLine());
System.out.println ("Enter the hypotenuse:");
int side3 = Integer.parseInt (stdin.readLine());
hypotenuse_sq = (side1 * side1) + (side2 * side2);
if ((side3*side3) == hypotenuse_sq)
System.out.println ("It is a right triangle.");
else
System.out.println ("It is not a right triangle.");
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} // method main
Conclusion
• primitive data types: predefined data types in Java
• Size of each primitive data type is machine independent
• Beware of scope(variables can not be redefined in blocks)
• Java I/O is complicated.
• IO is based input streams and output streams
完成 Lecture 3 休息十分鐘！
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