Bài giảng cấu trúc dữ liệu chương 1 nguyễn xuân vinh

» The variables that are listed as part of a method declaration.. » Belong only to the class, but not created for only object of the class» All objects of the class share the same copy o

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 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

char 16-bit Unicode \u0000 (0)  \uffff (65.535)

• Reference to Tham chiếu tới một giá trị hay là tập hợp các giá trị

mà biến khai báo

• Các kiểu dữ liệu dẫn xuất:

 A literal is the source code representation of a fixed value.

 Literals are represented directly in your code without requiring computation

boolean result = true;

char capitalC = 'C';

byte b = 100;

int i = 100000;

» The variables that are listed as part of a method declaration Each

parameter must have a unique name and a defined data type.

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

}

» 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

 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 {

public void sum( int a) {

int x = a + 3;

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

}

}

• 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

1;

The JVM divided the memory into following sections.

1 Heap: contains Objects (may also contain reference

variables)

2 Stack: contains methods, local variables and reference

variables.

3 Code: contains your bytecode

4 Static: contains Static data/methods.

This division of memory is required for its effective management

Account

p=0

q=0 [int]

20

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

 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;

arrayStudent[2] = s3;

String string1 = "abcd";

String string2 = "abcd";

String pool (String intern pool) is a special storage area in Java

String s1 = "Hello"; // String literal

String s2 = "Hello"; // String literal

String s3 = s1; // same reference

String s4 = new String("Hello"); //String Object String s5 = new String("Hello"); // String object

 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

 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

private int value ;

public Mutable(int value) {

this value = value;

}

}

class Immutable {

private final int value ;

public Immutable(int value) {

this value = value;

}

}