Kiểu tham chiếu — null, NPE, wrapper class và autoboxing

String ten = "An";           // class type
int[] so   = {1, 2, 3};     // array type (int[] is reference)
List<String> ds = new ArrayList<>();  // interface type

String s = null;  // variable s exists but points to no object

String s = null;
int n = null;   // COMPILE ERROR -- primitive cannot be null

String s = null;
int len = s.length();  // NullPointerException at runtime!

Exception in thread "main" java.lang.NullPointerException:
Cannot invoke "String.length()" because "s" is null

String s = null;
s.length();              // (A) goi method qua null
s.charAt(0);             // (B) goi method qua null
int[] arr = null;
int x = arr[0];          // (C) truy cap phan tu mang null
int len = arr.length;    // (D) doc .length cua mang null
throw null;              // (E) throw null reference

if (s != null) {
    System.out.println(s.length());
}

import java.util.Objects;

void xuLy(String ten) {
    Objects.requireNonNull(ten, "ten must not be null");
    // an toan dung ten tu day tro di
    System.out.println(ten.toUpperCase());
}

String a = null;
String b = "hello";

// Nguy hiem: a.equals(b) nem NPE vi a la null
// An toan:
boolean bang = Objects.equals(a, b);  // false, khong throw NPE

import java.util.Optional;

Optional<String> maybeName = Optional.ofNullable(getUserName());
maybeName.ifPresent(name -> System.out.println("Xin chao, " + name));
String name = maybeName.orElse("Khach");

List<Integer> soNguyen = new ArrayList<>();
soNguyen.add(1);    // OK: autoboxing int -> Integer
soNguyen.add(2);
soNguyen.add(3);

Map<String, Integer> diem = new HashMap<>();
diem.put("An", 95);

// DUNG: valueOf -- su dung cache khi co the
Integer a = Integer.valueOf(42);
Integer b = Integer.parseInt("42");  // tu String

// TRANH: new Integer() deprecated tu Java 9, danh dau forRemoval tu Java 16
// (tinh den Java 21 van chua bi xoa, nhung dung dung)
Integer c = new Integer(42);  // @Deprecated

// Autoboxing: int -> Integer (compiler tao Integer.valueOf(42))
Integer x = 42;   // tuong duong: Integer x = Integer.valueOf(42);

// Unboxing: Integer -> int (compiler tao x.intValue())
int y = x;        // tuong duong: int y = x.intValue();

Integer x = null;
int y = x;  // NullPointerException! unbox null khong the lay intValue()

// Vi du thuc te: diem co the null (hoc sinh chua thi)
Integer diem = layDiemTuDB(hocSinhId);  // co the tra null
if (diem != null && diem > 50) {  // unbox o day neu diem != null
    System.out.println("Dat");
}
// DUNG -- khong unbox null

// CHAM: moi lan += tao Integer moi (unbox + tinh + autobox)
Integer sum = 0;
for (int i = 0; i < 1_000_000; i++) {
    sum += i;  // unbox sum, cong i, autobox lai
}

// NHANH: dung int thuan
int sum2 = 0;
for (int i = 0; i < 1_000_000; i++) {
    sum2 += i;
}

Integer a = 100;
Integer b = 100;
System.out.println(a == b);  // true -- bat ngo?

Integer c = 200;
Integer d = 200;
System.out.println(c == d);  // false -- tai sao khac?

Integer a = 100;  // Integer.valueOf(100) -- tra cached instance
Integer b = 100;  // Integer.valueOf(100) -- tra CUNG cached instance
// a va b tro cung 1 object -> a == b la true

Integer c = 200;  // Integer.valueOf(200) -- tao object moi
Integer d = 200;  // Integer.valueOf(200) -- tao object moi KHAC
// c va d la 2 object khac nhau -> c == d la false

String s1 = "hello";           // literal -> String Pool
String s2 = "hello";           // cung literal -> cung object trong Pool
String s3 = new String("hello"); // buoc pool, tao object rieng
String s4 = s3.intern();       // dua vao pool, tra reference cua pooled object

System.out.println(s1 == s2);  // true  (cung object trong pool)
System.out.println(s1 == s3);  // false (khac object)
System.out.println(s1 == s4);  // true  (s4 = pooled reference)
System.out.println(s1.equals(s3)); // true (cung noi dung)

String s = "hello";
s.toUpperCase();         // khong co tac dung -- tra String moi bi bo di
s = s.toUpperCase();     // dung: gan ket qua lai
System.out.println(s);   // HELLO

import java.util.*;

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

        // --- Integer Cache demo ---
        Integer a = 100, b = 100;
        Integer c = 200, d = 200;
        System.out.println("a == b (100): " + (a == b));   // true (cached)
        System.out.println("c == d (200): " + (c == d));   // false (new objects)
        System.out.println("c.equals(d): " + c.equals(d)); // true (value equal)

        // --- NPE khi unbox null ---
        Integer diemNull = null;
        try {
            int x = diemNull;  // unbox null -> NPE
        } catch (NullPointerException e) {
            System.out.println("NPE khi unbox null: " + e.getMessage());
        }

        // --- Objects.requireNonNull ---
        try {
            String ten = null;
            Objects.requireNonNull(ten, "ten khong duoc null");
        } catch (NullPointerException e) {
            System.out.println("requireNonNull: " + e.getMessage());
        }

        // --- Objects.equals null-safe ---
        String s1 = null;
        String s2 = "hello";
        System.out.println("Objects.equals: " + Objects.equals(s1, s2)); // false, no NPE
    }
}

a == b (100): true
c == d (200): false
c.equals(d): true
NPE khi unbox null: Cannot invoke "java.lang.Integer.intValue()" because "diemNull" is null
requireNonNull: ten khong duoc null
Objects.equals: false