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Lecture 3Program Elements

Instructors: Fu-Chiung Cheng

( 鄭福炯 )Associate Professor

Computer Science & EngineeringTatung 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

byteshortintlong

Storage

8 bits16 bits32 bits64 bits

Min Value

-128-32,768-2,147,483,648< -9 x 1018

Max Value

12732,7672,147,483,647> 9 x 1018

Note that Built-in types (primitives) are not object handles handles: call by reference primitives: call by value

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Primitive Data Types

• Floating point:

Type

floatdouble

Storage

32 bits64 bits

ApproximateMin Value

-3.4 x 1038

-1.7 x 10308

ApproximateMax Value

3.4 x 1038

1.7 x 10308

S Exponent 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.

• Wrapper classes are useful in situations where you need an object instead of a primitive type• They also contain some useful methods

Primitive Typeint

doublechar

boolean

Wrapper ClassIntegerDouble

CharacterBoolean

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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)

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:

• print and println methods write to standard output

Stream

System.inSystem.outSystem.err

Purpose

reading inputwriting outputwriting errors

Default Device

keyboardmonitormonitor

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Input and Output

• Escape sequences: a special sequence of characters preceded by a backslash (\)

Escape Sequence

\t\n\"\'\\

Meaning

tabnew line

double quotesingle quotebackslash

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Types of Output Stream

ByteArrayOutputStrem

FileOutputStrem

PipeOutputStrem

Block of Memory

File

Pipe(to another thread)

Writes to

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Types of Input Stream

ByteArrayInputStrem

FileInputStrem

PipeInputStrem

Block of Memory

File

Pipe(to another thread)

Reads from

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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} // class Addition2

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Controlling Program Flow

• Essentially same as C/C++• if statement: if (condition)

statement;• Relational operators:

Operator

==!=<<=><=

Meaning

equal tonot equal to

less thanless than or equal to

greater thangreater than or equal to

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Controlling Program Flow

• if-else statementif (condition) statement1;

else statement2;

• while statement: while (condition)

statement;

Note: Avoid infinite loop (logic error)

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Controlling Program Flow

• if-else statementif (condition) statement1;

else statement2;

• while statement: for statemetn: while (condition) for (e1;e2;e3)

statement; statement;

Note: Avoid infinite loop (logic error)

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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."); } // method main} // class Right_Triangle

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Conclusion

完成 Lecture 3 休息十分鐘！

• 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