Bài giảng cấu trúc dữ liệu chương 1 nguyễn xuân vinh

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CẤU TRÚC DỮ LIỆU
DATA STRUCTURES

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[214441]

JAVA BASIC

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Nguyễn Xuân Vinh



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1. Basic Language Elements

 Identifiers
 A name in a program is called an identifier

 An identifier in java is composed of a sequence of characters (include: letter, digits or connecting symbols ($, _).
 The first character in an identifier can’t be a digit.
 Identifiers in java are case sensitive.
 Keywords
 Key words are reserved identifiers.
 In java, all keywords are in lower case.
 Literals
 A literal denotes a constant value of a particular data type
 The value a literal represents remains unchanged in the program.


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2. Data Types


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2.1 Primitive Data Types

Types

Length

Values

Default value

byte

8-bit

8
8
-2  2 – 1

0

short

16-bit

-2

int

32-bit

-2

long

64-bit

-2

float

32-bit IEEE 754 floating point

+/-3.40282347E+38  +/- 1.40239846E-45

0.0f

double

64-bit IEEE 754 floating point

+/-1.79769313486231570E  +/-

0.0d

16
31
63

2
2
2

16

31

63

–1

0

–1

0

–1

0

4.9065645841246544E-324
boolean

1-bit

true, false

char

16-bit Unicode

\u0000 (0)  \uffff (65.535)

false


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2.1 Primitive Data Types


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2.2 Reference Data Types

 Reference to Tham chiếu tới một giá trị hay là tập hợp các giá trị mà biến khai báo.

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 Các kiểu dữ liệu dẫn xuất:


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3. Literal

 The new keyword isn't used when initializing a variable of a primitive type.
 A literal is the source code representation of a fixed value.
 Literals are represented directly in your code without requiring computation
boolean result = true;

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char capitalC = 'C';
byte b = 100;
short s = 10000;
int i = 100000;


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4. Variables

 Parameters
» The variables that are listed as part of a method declaration. Each parameter must have a unique name and a defined data
type.

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public void method(int a, double b, boolean c) {
...
}


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4. Variables

 Instance variables
»

Every object of the class will have its own copies of these variables, which are local to object

» The values of these variables at any given time constitute the state of the object
» Instance variables exist as long as object they belong to exist

 Static variables
» Belong only to the class, but not created for only object of the class
» All objects of the class share the same copy of this variable
» Class variables exist as long as class exist

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 Local Variables
» Variables declared in methods including parameters or blocks are called Local variables
» After execution of the method or block completes local variables are no longer accessible


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Instance variables and local variables



Instance variables are declared inside a class but not inside a method
public class Student{
int num; // num is instance variable
public void method(){}
}



Local variables are declared inside a method including method arguments.

public class Student {

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public void sum(int a) {
int x = a +

3;

// a , x are local variables</strong>
}
}


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5. Constant

 Từ khóa final chỉ dẫn đến 1 biến không thể thay đổi giá trị.
 Các hàm và lớp cũng có thể được khai báo final
 Hàm final không thể viết chồng
 Lớp final không thể là lớp con

static final int VAR = 1;


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6. JVM Memory
The JVM divided the memory into following sections.

1.
2.
3.
4.

Heap: contains Objects (may also contain reference variables).
Stack: contains methods, local variables and reference variables.
Code: contains your bytecode
Static: contains Static data/methods.

This division of memory is required for its effective management.

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6. JVM Memory


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6. JVM Memory Example 1
class A {
B child = new B();
int e;
// more code
}
class B {

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int c;
int d;
// more code
}
public static void main(String args[]) {
A parent = new A();

// more code

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}


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6. JVM Memory: Example 2

Heap

public void m1() {
int x = 20;
m2(10);
}
public void m2(int b) {

Account
[int]

p=0

20

q=0

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boolean c;
//more code
m3();
}
Stack

public void m3() {
Account ref = new Account();
//more code

}
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m3

ref

int p;

m2

b c

int q;

m1

x

public class Account {

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}


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6. JVM Memory: Example 3 (Array)


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6. JVM Memory: (Exp 4) Primitive vs Reference



Primitive Data Types (byte,short,int,long,float,double,Boolean,chat)

int a = 1;
int b = 1;
int[] arrayInt = new int[2];
arrayInt[0] = a;

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arrayInt[1] = b;



Reference Data Type: Student (int sid, String name)

Student s1 = new Student(1, “A”);
Student s2 = new Student(2, “B”);
Student s3 = new Student(1, “A”);
Student[] arrayStudent = new Student[3];
arrayStudent[0] = s1;
arrayStudent[1] = s2;

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arrayStudent[2] = s3;

String string1 = "abcd";
String string2 = "abcd";

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6. JVM Memory (Exp 5) - String Pool

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returning its reference.

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already exists in the pool, the reference of the existing string will be returned, instead of creating a new object and

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String pool (String intern pool) is a special storage area in Java heap. When a string is created and if the string


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6. JVM Memory: (Exp 7) String Literal vs. String Object

String
String
String
String
String

s1
s2
s3
s4
s5

=
=
=
=
=

"Hello"; // String literal
"Hello"; // String literal
s1; // same reference
new String("Hello"); //String Object
new String("Hello"); // String object


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7. Mutable vs Immutable


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7. Why String is immutable

 Allow String to Cache its Hashcode
The hashcode of string is frequently used in Java. For example, in a HashMap. Being immutable guarantees that
hashcode will always the same, so that it can be cashed without worrying the changes.That means, there is no need to
calculate hashcode every time it is used. This is more efficient.

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 Security
String is widely used as parameter for many java classes, e.g. network connection, opening files, etc. Were String not
immutable, a connection or file would be changed and lead to serious security threat.


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7. Object Immutable & Mutable
class Mutable{
private int value;
public Mutable(int value) {
this.value = value;
}
}
class Immutable {
private final int value;
public Immutable(int value) {
this.value = value;
}
}


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