Java asoslari 4-qism. Massivlar va Kolleksiyalarga kirish

Bu bo‘limda Java’da bir nechta qiymatlarni saqlashni o‘rganamiz.

Oldingi mavzularda bitta qiymat saqladik:

String name = "Ali";
int age = 25;

Lekin real dasturda ko‘pincha bitta emas, ko‘p qiymatlar bilan ishlaymiz:

10 ta user
100 ta product
1000 ta order
telefon raqamlar ro‘yxati
baholar ro‘yxati

Shuning uchun Java’da array va collection ishlatiladi.

Bu qism quyidagilarni o‘z ichiga oladi: 1D & 2D arrays, ArrayList basics, HashMap basics, iterating collections.

1. Array nima?

Array - bir xil turdagi bir nechta qiymatlarni bitta joyda saqlaydigan tuzilma.

Masalan, 5 ta son saqlash:

int[] numbers = {10, 20, 30, 40, 50};

Bu yerda:

int[]    → int array
numbers  → array nomi
{...}    → qiymatlar

Oddiy qilib:

numbers degan qutida 5 ta int qiymat turibdi.

2. Array index

Array elementlari index orqali olinadi.

Muhim qoida:

Java’da index 0 dan boshlanadi.

int[] numbers = {10, 20, 30, 40, 50};

System.out.println(numbers[0]);
System.out.println(numbers[1]);
System.out.println(numbers[2]);

Natija:

10
20
30

Array ko‘rinishi:

index:   0   1   2   3   4
value:  10  20  30  40  50

Demak:

numbers[0] // 10
numbers[1] // 20
numbers[4] // 50

3. Array elementini o‘zgartirish

Array ichidagi qiymatni index orqali o‘zgartirish mumkin:

int[] numbers = {10, 20, 30};

numbers[1] = 99;

System.out.println(numbers[1]);

Natija:

99

Oldin:

numbers[1] = 20

Keyin:

numbers[1] = 99

4. Array uzunligi

Array uzunligini olish uchun .length ishlatiladi.

int[] numbers = {10, 20, 30, 40};

System.out.println(numbers.length);

Natija:

4

E’tibor ber:

numbers.length

Bu method emas, field. Shuning uchun qavs yozilmaydi.

Noto‘g‘ri:

numbers.length();

To‘g‘ri:

numbers.length;

5. Array yaratishning boshqa usuli

Agar qiymatlarni keyinroq beradigan bo‘lsak:

int[] numbers = new int[3];

numbers[0] = 10;
numbers[1] = 20;
numbers[2] = 30;

Bu yerda:

new int[3]

3 ta int saqlaydigan array yaratadi.

Default qiymatlar:

int      → 0
double   → 0.0
boolean  → false
String   → null
object   → null

Misol:

int[] numbers = new int[3];

System.out.println(numbers[0]);
System.out.println(numbers[1]);
System.out.println(numbers[2]);

Natija:

0
0
0

6. Array chegarasidan chiqib ketish

Array’da mavjud bo‘lmagan indexga murojaat qilsak, xato bo‘ladi.

int[] numbers = {10, 20, 30};

System.out.println(numbers[3]);

Bu xato:

ArrayIndexOutOfBoundsException

Nega?

numbers length = 3
indexlar: 0, 1, 2
numbers[3] mavjud emas

Boshlang'ichlar ko‘p qiladigan xatolar:

int[] numbers = {10, 20, 30};

for (int i = 0; i <= numbers.length; i++) {
    System.out.println(numbers[i]);
}

Bu noto‘g‘ri. Chunki oxirida i = 3 bo‘ladi.

To‘g‘ri:

int[] numbers = {10, 20, 30};

for (int i = 0; i < numbers.length; i++) {
    System.out.println(numbers[i]);
}

Muhim qoida:

i < array.length

<= emas.

7. Array bo‘ylab yurish: for

String[] names = {"Ali", "Vali", "Sami"};

for (int i = 0; i < names.length; i++) {
    System.out.println(names[i]);
}

Natija:

Ali
Vali
Sami

Bu usul index kerak bo‘lganda yaxshi.

Masalan:

String[] names = {"Ali", "Vali", "Sami"};

for (int i = 0; i < names.length; i++) {
    System.out.println(i + ": " + names[i]);
}

Natija:

0: Ali
1: Vali
2: Sami

8. Array bo‘ylab yurish: for-each

Agar index kerak bo‘lmasa, for-each osonroq.

String[] names = {"Ali", "Vali", "Sami"};

for (String name : names) {
    System.out.println(name);
}

Natija:

Ali
Vali
Sami

Tushunish:

names ichidagi har bir elementni olib, name degan variable’ga ber.

9. for va for-each farqi

Misol: elementni o‘zgartirish kerak bo‘lsa:

int[] numbers = {1, 2, 3};

for (int i = 0; i < numbers.length; i++) {
    numbers[i] = numbers[i] * 2;
}

Natija array ichida:

2, 4, 6

10. 2D array nima?

2D array - array ichida array. Jadvalga o‘xshaydi.

Masalan:

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

Ko‘rinishi:

row 0:  1  2  3
row 1:  4  5  6
row 2:  7  8  9

Element olish:

System.out.println(matrix[0][0]);
System.out.println(matrix[1][2]);
System.out.println(matrix[2][1]);

Natija:

1
6
8

Tushunish:

matrix[row][column]

11. 2D array bo‘ylab yurish

int[][] matrix = {
    {1, 2, 3},
    {4, 5, 6},
    {7, 8, 9}
};

for (int row = 0; row < matrix.length; row++) {
    for (int col = 0; col < matrix[row].length; col++) {
        System.out.print(matrix[row][col] + " ");
    }
    System.out.println();
}

Natija:

1 2 3
4 5 6
7 8 9

Bu yerda:

matrix.length

row soni.

matrix[row].length

shu row ichidagi column soni.

12. Array muammosi

Array yaxshi, lekin bitta katta cheklovi bor:

Array uzunligi yaratilgandan keyin o‘zgarmaydi.

Masalan:

int[] numbers = new int[3];

Bu array faqat 3 ta element saqlaydi. Keyin 4-elementni qo‘shib bo‘lmaydi.

numbers[3] = 40; // xato

Shuning uchun real loyihalarda ko‘pincha ArrayList ishlatiladi.

13. ArrayList nima?

ArrayList - o‘lchami o‘zgaradigan ro‘yxat.

Array:

fixed size

ArrayList:

dynamic size

Ishlatish uchun import kerak:

import java.util.ArrayList;

Misol:

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> names = new ArrayList<>();

        names.add("Ali");
        names.add("Vali");
        names.add("Sami");

        System.out.println(names);
    }
}

Natija:

[Ali, Vali, Sami]

14. ArrayList generic type

ArrayList<String> names = new ArrayList<>();

Bu degani:

Bu ro‘yxatda faqat String saqlanadi.

Sonlar uchun:

ArrayList<Integer> numbers = new ArrayList<>();

E’tibor ber:

ArrayList<int> numbers = new ArrayList<>(); // xato

To‘g‘ri:

ArrayList<Integer> numbers = new ArrayList<>();

Nega?

Chunki collection’lar primitive type bilan emas, object type bilan ishlaydi.

15. ArrayList asosiy methodlari

add()

Element qo‘shadi.

ArrayList<String> names = new ArrayList<>();

names.add("Ali");
names.add("Vali");

get()

Index bo‘yicha element oladi.

System.out.println(names.get(0));

Natija:

Ali

set()

Indexdagi elementni o‘zgartiradi.

names.set(1, "Sami");

remove()

Elementni o‘chiradi.

names.remove("Ali");

Yoki index bo‘yicha:

names.remove(0);

size()

Elementlar sonini qaytaradi.

System.out.println(names.size());

E’tibor ber:

array.length
arrayList.size()

Array’da length, ArrayList’da size().

contains()

Element bor-yo‘qligini tekshiradi.

boolean exists = names.contains("Ali");
System.out.println(exists);

16. ArrayList bilan to‘liq misol

import java.util.ArrayList;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> users = new ArrayList<>();

        users.add("Ali");
        users.add("Vali");
        users.add("Sami");

        System.out.println("Users count: " + users.size());

        System.out.println("First user: " + users.get(0));

        users.set(1, "Hasan");

        users.remove("Sami");

        for (String user : users) {
            System.out.println(user);
        }
    }
}

Natija:

Users count: 3
First user: Ali
Ali
Hasan

17. ArrayList bo‘ylab yurish

Index bilan:

ArrayList<String> names = new ArrayList<>();

names.add("Ali");
names.add("Vali");
names.add("Sami");

for (int i = 0; i < names.size(); i++) {
    System.out.println(names.get(i));
}

for-each bilan:

for (String name : names) {
    System.out.println(name);
}

Ko‘p hollarda for-each o‘qilishi osonroq.

18. HashMap nima?

HashMap - key-value ko‘rinishida ma’lumot saqlaydi.

Ya’ni har bir qiymatning kaliti bo‘ladi.

Masalan:

phoneBook:
Ali  → +998901111111
Vali → +998902222222
Sami → +998903333333

Java’da:

import java.util.HashMap;

HashMap<String, String> phoneBook = new HashMap<>();

phoneBook.put("Ali", "+998901111111");
phoneBook.put("Vali", "+998902222222");
phoneBook.put("Sami", "+998903333333");

Bu yerda:

HashMap<String, String>

Birinchi String - key type.
Ikkinchi String - value type.

key   → name
value → phone number

19. HashMap asosiy methodlari

put()

Key-value qo‘shadi yoki mavjud key qiymatini yangilaydi.

phoneBook.put("Ali", "+998901111111");

Agar Ali oldin bor bo‘lsa, qiymati yangilanadi.

phoneBook.put("Ali", "+998909999999");

get()

Key bo‘yicha value oladi.

String phone = phoneBook.get("Ali");

System.out.println(phone);

Natija:

+998901111111

containsKey()

Key bor-yo‘qligini tekshiradi.

if (phoneBook.containsKey("Ali")) {
    System.out.println("Ali bor");
}

remove()

Key bo‘yicha elementni o‘chiradi.

phoneBook.remove("Ali");

size()

Elementlar sonini qaytaradi.

System.out.println(phoneBook.size());

20. HashMap bilan to‘liq misol

import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        HashMap<String, Integer> scores = new HashMap<>();

        scores.put("Ali", 90);
        scores.put("Vali", 75);
        scores.put("Sami", 88);

        System.out.println("Ali score: " + scores.get("Ali"));

        if (scores.containsKey("Vali")) {
            System.out.println("Vali bor");
        }

        scores.put("Vali", 80);

        scores.remove("Sami");

        System.out.println(scores);
    }
}

Natija:

Ali score: 90
Vali bor
{Vali=80, Ali=90}

HashMap’da tartib kafolatlanmaydi. Shuning uchun natija tartibi boshqacha chiqishi mumkin.

21. HashMap bo‘ylab yurish

Faqat key’lar bo‘yicha

for (String name : scores.keySet()) {
    System.out.println(name);
}

Faqat value’lar bo‘yicha

for (Integer score : scores.values()) {
    System.out.println(score);
}

Key va value birga

for (String name : scores.keySet()) {
    Integer score = scores.get(name);
    System.out.println(name + ": " + score);
}

Yaxshiroq usul:

for (var entry : scores.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

Agar var ishlatmasak:

for (Map.Entry<String, Integer> entry : scores.entrySet()) {
    System.out.println(entry.getKey() + ": " + entry.getValue());
}

Bunda import kerak:

import java.util.Map;

22. Array, ArrayList, HashMap farqi

Oddiy misol:

Array      → 5 ta baho
ArrayList  → buyurtmalar ro‘yxati
HashMap    → productId bo‘yicha product topish

23. Real hayotga yaqin misol

Tasavvur qil: mini shop dasturi bor.

Mahsulot nomlari ro‘yxati:

ArrayList<String> products = new ArrayList<>();

products.add("Laptop");
products.add("Mouse");
products.add("Keyboard");

Mahsulot narxlari:

HashMap<String, Double> prices = new HashMap<>();

prices.put("Laptop", 1000.0);
prices.put("Mouse", 20.0);
prices.put("Keyboard", 50.0);

Chiqarish:

for (String product : products) {
    Double price = prices.get(product);
    System.out.println(product + " = $" + price);
}

Natija:

Laptop = $1000.0
Mouse = $20.0
Keyboard = $50.0

Bu yerda:

ArrayList → productlar tartibli ro‘yxati
HashMap   → product nomi orqali narx olish

24. null bilan ehtiyot bo‘lish

HashMap’da mavjud bo‘lmagan key bo‘yicha get() qilsak, null qaytadi.

HashMap<String, Integer> scores = new HashMap<>();

scores.put("Ali", 90);

System.out.println(scores.get("Sami"));

Natija:

null

Shuning uchun tekshirish yaxshi:

if (scores.containsKey("Sami")) {
    System.out.println(scores.get("Sami"));
} else {
    System.out.println("Sami topilmadi");
}

Yoki default qiymat berish:

Integer score = scores.getOrDefault("Sami", 0);

System.out.println(score);

Natija:

0

25. Boshlang'ichlar xatolari

Xato 1: Array index 1 dan boshlanadi deb o‘ylash

Noto‘g‘ri:

int[] numbers = {10, 20, 30};

System.out.println(numbers[1]); // birinchi element deb o‘ylash

Aslida bu ikkinchi element.

To‘g‘ri:

System.out.println(numbers[0]); // birinchi element

Xato 2: length va size()ni adashtirish

Array:

numbers.length

ArrayList:

names.size()

String:

text.length()

E’tibor ber:

array.length       → field
arrayList.size()   → method
string.length()    → method

Xato 3: ArrayList’da primitive type ishlatish

Noto‘g‘ri:

ArrayList<int> numbers = new ArrayList<>();

To‘g‘ri:

ArrayList<Integer> numbers = new ArrayList<>();

Xato 4: HashMap’da mavjud bo‘lmagan key

Integer score = scores.get("Sami");
System.out.println(score + 10);

Agar "Sami" yo‘q bo‘lsa, score = null. Keyin score + 10 xato berishi mumkin.

Yaxshi variant:

Integer score = scores.getOrDefault("Sami", 0);
System.out.println(score + 10);

26. Kichik mashq

Vazifa

Quyidagi ishni bajaring:

1. ArrayList ichida 5 ta product nomi saqla
2. HashMap ichida product narxlarini saqla
3. Har bir product nomi va narxini chiqar
4. Narxi 100 dan katta productlarni alohida chiqar

Yechim:

import java.util.ArrayList;
import java.util.HashMap;

public class Main {
    public static void main(String[] args) {
        ArrayList<String> products = new ArrayList<>();

        products.add("Laptop");
        products.add("Mouse");
        products.add("Keyboard");
        products.add("Monitor");
        products.add("USB Cable");

        HashMap<String, Double> prices = new HashMap<>();

        prices.put("Laptop", 1000.0);
        prices.put("Mouse", 20.0);
        prices.put("Keyboard", 50.0);
        prices.put("Monitor", 200.0);
        prices.put("USB Cable", 5.0);

        System.out.println("All products:");

        for (String product : products) {
            double price = prices.get(product);
            System.out.println(product + " = $" + price);
        }

        System.out.println("Expensive products:");

        for (String product : products) {
            double price = prices.get(product);

            if (price > 100) {
                System.out.println(product + " = $" + price);
            }
        }
    }
}

Natija:

All products:
Laptop = $1000.0
Mouse = $20.0
Keyboard = $50.0
Monitor = $200.0
USB Cable = $5.0
Expensive products:
Laptop = $1000.0
Monitor = $200.0

27. Qisqa xulosa

Bu bo‘limda Java’da bir nechta qiymat bilan ishlashni ko‘rdik:

Array      → fixed size ro‘yxat
2D array   → jadval ko‘rinishidagi array
ArrayList  → dynamic size ro‘yxat
HashMap    → key-value saqlash
for         → index bilan yurish
for-each    → elementlar bo‘ylab oson yurish

Eng muhim eslab qoladigan joy:

Array index 0 dan boshlanadi.
Array uzunligi o‘zgarmaydi.
ArrayList uzunligi o‘zgaradi.
HashMap key orqali value topadi.