Java basics 1 variable (lập TRÌNH NÂNG CAO SLIDE)

ADVANCED PROGRAMMING

JAVA BASICS


A Simple Java Program
public class FirstSample{
public static void main(String[] args)
{
System.out.println("Hello, World!");
}
}

 Java is case sensitive
 The keyword public is called an access modifier
 The keyword class is there to remind you that everything in a Java program must
be inside a class

 The main method in the source file is necessary in order to execute the program
 The System.out.println(..) is used to invoke method println of an object
System.out

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Comments
System.out.println("We will not use 'Hello world!'");
// is this too cute?

/*
This is the first sample program
Copyright (C) by Cay Horstmann and Gary Cornell
*/
public class FirstSample {
public static void main(String[] args) {
System.out.println("We will not use 'Hello, World!'");
}
}
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Compiling and executing the program

The java compiler creates a file called 'First.class' that contains the byte codes

To actually run the program, a java interpreter called java is required to execute the code.

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Passing Command Line Arguments

class CommLineArg {

public static void main (String [] pargs) {
System.out.println("These are the arguments passed to the main method.");
System.out.println(pargs [0]);
System.out.println(pargs [1]);
System.out.println(pargs [2]);
}
}

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Passing Command Line Arguments

Output

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Identifiers
 Identifiers are:
 Text strings that represent variables, methods, classes or labels
 Case-sensitive

 Characters can be digit, letter, '$' or '_'
 Identifiers cannot:
 Begin with a digit
 Be the same as a reserved word.

An_Identifier
a_2nd_Identifier
Go2
$10





An-Identifier
2nd_Identifier
goto
10$

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Legal Identifiers
 Identifiers must start with a letter, a currency character ($), or a connecting
character such as the underscore ( _ ). Identifiers cannot start with a
number!



After the first character, identifiers can contain any combination of letters,
currency characters, connecting characters, or numbers.

 In practice, there is no limit to the number of characters an identifier can contain.
 You can't use a Java keyword as an identifier
 Identifiers in Java are case-sensitive; foo and FOO are two different identifiers.

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Complete List of Java Keywords

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Java Code Conventions
 Classes: the names should typically be nouns
 Dog
 Account
 PrintWriter

 Interfaces: the names should be adjectives
 Runnable

 Serializable

 Methods: the names should be verb-noun pairs.
 getBalance
 doCalculation
 setCustomerName

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Java Code Conventions
 Variables: Like methods, starting with a lowercase letter. Sun recommends short,
meaningful names, which sounds good to us. Some examples:
 buttonWidth
 accountBalance
 myString

 Constants: Java constants are created by marking variables static and final.
They should be named using uppercase letters with underscore characters as
separators:
 MIN_HEIGHT

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The Elements of a class

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Data Types

 Java is a strongly typed language. This means that every variable
must have a declared type. There are eight primitive types in Java

 Four of them are integer types; two are floating-point number types; one
is the character type char, used for characters in the Unicode encoding,
and one is a boolean type for truth values.

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Variables and Data Types (cont.)
 Data types
 Primitive types
 Reference types

 The primitive types are boolean, byte, char, short, int, long, float and double
 All non-primitive types are reference types, so classes, which specify the types of
objects, are reference types

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Primitives: Integers
 Signed whole numbers
 Initialized to zero

Categories:
Categories:
Size:
Size:

a.
a. integer
integer
b.
b. floating
floating

1.
1. byte
byte

2.
2. short
short

Range:
Range:

Size:
Size:
Range:
Range:

11 byte
byte
77
77
-2
-2  22 -- 11

22 bytes
bytes
15
15
15
15
-2
-2  22 -- 11

c.
c. character
character
3.
3. int
int

d.

d. boolean
boolean

Size:
Size:
Range:
Range:

4.
4. long
long

Size:
Size:
Range:
Range:

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44 bytes
bytes
31
31
31
31
-2
-2  22 -- 11

88 bytes
bytes

63
63
63
63
-2
-2  22 -- 11

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Primitives: Floating Points
 "General" numbers
 Can have fractional parts

 Initialized to zero

Categories:
Categories:
a.
a. integer
integer
b.
b. floating
floating

1.
1. float
float

Size:
Size: 44 bytes
bytes
-45
38
-45
38
Range:
Range: ±1.4
±1.4 xx 10
10  ±3.4
±3.4 xx 10
10

c.
c. character
character
d.
d. boolean
boolean

2.
2. double
double

Size:
Size: 88 bytes
bytes
-324

-324  ±1.8 x 10308
308
Range:
Range: ±4.9
±4.9 xx 10
10
 ±1.8 x 10

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Primitives: Characters

 Char is any unsigned Unicode character
 Initialized to zero (\u0000)

Categories:
Categories:
a.
a. integer
integer
b.
b. floating
floating
char
char

c.
c. character
character

Size:
Size: 22 bytes
bytes
Range:
Range: \u0000
\u0000  \uFFFF
\uFFFF

d.
d. boolean
boolean

Java Basic

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Primitives: Booleans
 boolean values are distinct in Java
 Can only have a true or false value
 An int value can NOT be used in place of a boolean

 Initialized to false

Categories:
Categories:
a.
a. integer
integer
b.
b. floating
floating
c.
c. character
character
d.
d. boolean
boolean

boolean
boolean

Size:
Size: 11 byte
byte
Range:
Range: true
true || false
false

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Primitive Data Types (summary)

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The Sign bit for a byte

All six number types in Java are signed
Integer literals three way store present integer numbers in the Java language:
decimal(base10), octal(base8), hexadecimal(base16)

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Variables and Data Types (cont.)
 Decimal Literals: default
 int length = 343;

 Octal Literals: represent an integer in octal form by placing a zero in front of the
number

 int nine = 011;

 Hexadecimal Literals: including the prefix 0x or the optional suffix extension L
 int z = 0xDeadCafe;

 Note: Java will accept capital or lowercase letters for the extra digits in
hexadecimal
 0XCAFE and 0xcafe are both legal

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Variables and Data Types (cont.)
 Floating-Point Literals:
 Default are defined as double (64 bits)
 Attach the suffix F or f to the number if want using floating-point (32

bits)
 Example:
 float f = 23.467890; // Error
 float g = 49837849.029847F;

 Boolean Literals
 true, false

 Character Literals: 16-bit unsigned integer
 char letterN = '\u004E'; // The letter 'N‘

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Variables and Data Types (cont.)
 Some special characters
 \n: Used to denote new line
 \r: Used to denote a return
 \t: Used to denote a tab
 \b: Used to denote a backspace
 \f: Used to denote a form feed
 \': Used to denote a single quote
 \": Used to denote a double quote
 \\: Used to denote a backslash

 Literal Values for Strings: A string literal is a source code representation of a value
of a String object
 String s = "Bill Joy";

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Casting Primitive Types
 Casting creates a new value and allows it to be treated as a different type
than its source
 Java is a strictly typed language
 Assigning the wrong type of value to a variable could result in a compile

error or a JVM exception

 The JVM can implicitly promote from a narrower type to a wider type
 To change to a narrower type, you must cast explicitly

double
double f;
f;
int
int a,
a, b;
b;

int
int d;
d;

long
long g;
g;

short
short c;
c;

short
short e;
e;

ff == g;
g;

aa == bb ++ c;
c;

ee == (short)d;
(short)d;

gg == f;
f; //error
//error

Java Basic

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Implicit vs. Explicit Casting
 Implicit casting is automatic when no loss of information is possible.
 byte → short → int → long → float → double

 An explicit cast required when there is a "potential" loss of accuracy:

long p = (long) 12345.56;

// p == 12345

int g = p; // illegal even though an int
// can hold 12345
char c = 't';
int j = c;

// automatic promotion

short k = c;

// why is this an error?

short k = (short) c;
float f = 12.35;

// explicit cast

// what’s wrong with this?
Java Basic

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