Toán tử số học và so sánh — phép chia nguyên, modulo âm và bẫy ==

int a = 10;
int b = 3;

System.out.println(a + b);   // 13
System.out.println(a - b);   // 7
System.out.println(a * b);   // 30
System.out.println(a / b);   // 3  -- integer division: phan nguyen, bo phan du
System.out.println(a % b);   // 1  -- phan du cua phep chia

String ten = "An";
int tuoi = 25;
System.out.println("Xin chao, " + ten + ", " + tuoi + " tuoi.");
// Xin chao, An, 25 tuoi.

// Canh giac: + tinh tu trai sang phai
System.out.println("Tong: " + 1 + 2);  // "Tong: 12" -- khong phai "Tong: 3"!
System.out.println("Tong: " + (1 + 2)); // "Tong: 3" -- dung ngoac

System.out.println(5 / 2);    //  2  -- KHONG phai 2.5!
System.out.println(-5 / 2);   // -2  -- truncate ve phia 0 (khong phai -3)
System.out.println(7 / 10);   //  0  -- nho hon 1, bo het

System.out.println(5.0 / 2);    // 2.5
System.out.println(5 / 2.0);    // 2.5
System.out.println((double) 5 / 2); // 2.5 -- ep kieu truoc khi chia

System.out.println( 7 %  3);  //  1  (7 = 2*3 + 1)
System.out.println(-7 %  3);  // -1  (KHONG phai 2 nhu toan hoc!)
System.out.println( 7 % -3);  //  1  (dau la dau cua 7 -- duong)
System.out.println(-7 % -3);  // -1

int n = -7, m = 3;
int modDuong = ((n % m) + m) % m;  // 2 -- modulo duong

String s1 = new String("hello");
String s2 = new String("hello");
System.out.println(s1 == s2);      // false -- khac object
System.out.println(s1.equals(s2)); // true  -- cung noi dung

Integer a = 200;
Integer b = 200;
System.out.println(a == b);      // false -- ngoai Integer Cache
System.out.println(a.equals(b)); // true

int diem = 75;
boolean dat = diem >= 50;       // true
boolean xuatSac = diem >= 90;   // false

if (diem >= 50 && diem < 90) {
    System.out.println("Dat -- chua xuat sac");
}

int x = 10;
x += 5;   // x = 15
x -= 3;   // x = 12
x *= 2;   // x = 24
x /= 4;   // x = 6
x %= 4;   // x = 2

int i = 5;
System.out.println(i++);  // in 5 (gia tri cu), sau do i = 6
System.out.println(i);    // 6
System.out.println(++i);  // tang truoc: i = 7, in 7
System.out.println(i);    // 7

byte b = 10;

// SAI -- COMPILE ERROR
// b = b + 1;
// Ly do: b + 1 la int (byte duoc promote len int theo integer promotion)
// int khong tu dong thu hep xuong byte

// DUNG -- compound assignment ngam cast ket qua ve kieu goc
b += 1;  // OK -- tuong duong (byte)(b + 1)
b++;     // OK -- tuong tu

byte b = 100;
b += 100;  // 100 + 100 = 200, nhung byte chi chua den 127!
System.out.println(b);  // -56 -- overflow ngam, khong bao loi!

// Tuong tu voi short
short s = 30_000;
s += 10_000;  // 40_000 vuot short range (32_767) -- overflow ngam
System.out.println(s);  // -25536

public class ArithmeticDemo {
    public static void main(String[] args) {

        // --- Integer division pitfall ---
        System.out.println("5 / 2 = " + (5 / 2));       // 2
        System.out.println("5.0 / 2 = " + (5.0 / 2));   // 2.5

        // --- Modulo am ---
        System.out.println("-7 % 3 = " + (-7 % 3));      // -1 (dau cua dividend)

        // --- String concat tinh trai sang phai ---
        System.out.println("Tong: " + 1 + 2);            // Tong: 12
        System.out.println("Tong: " + (1 + 2));          // Tong: 3

        // --- Comparison tra boolean ---
        int score = 75;
        boolean passed = score >= 50;
        System.out.println("Passed: " + passed);         // true

        // --- Increment: post vs pre ---
        int i = 5;
        System.out.println(i++);  // 5 -- tra gia tri cu roi moi tang
        System.out.println(++i);  // 7 -- tang truoc roi tra gia tri moi

        // --- Compound assignment va byte overflow ngam ---
        byte b = 100;
        b += 100;  // (byte)(200) = -56
        System.out.println("byte b = " + b);  // -56
    }
}