深入理解Java的自动装箱与拆箱机制及其潜在问题

int

Integer

Integer i = 10; // 自动装箱
int j = i;     // 自动拆箱

Integer i = 10;

Integer i = Integer.valueOf(10);

int j = i;

int j = i.intValue();

Integer.valueOf()

long sum = 0L;
for (int i = 0; i < Integer.MAX_VALUE; i++) {
    sum += i; // i 会被自动装箱成Long，然后拆箱成long
}
System.out.println(sum);

i

Long

Long

Long

long sum = 0L;
for (long i = 0; i < Integer.MAX_VALUE; i++) {
    sum += i;
}
System.out.println(sum);

NullPointerException

null

Integer i = null;
int j = i; // 抛出NullPointerException

i

null

int j = i;

null

NullPointerException

null

Integer i = null;
int j = (i != null) ? i : 0; // 安全的拆箱

Optional

Integer i = null;
int j = Optional.ofNullable(i).orElse(0);

Integer

Integer

==

Integer

true

false

Integer a = 100;
Integer b = 100;
System.out.println(a == b); // true

Integer c = 200;
Integer d = 200;
System.out.println(c == d); // false

a == b

true

a

b

c == d

false

c

d

Integer

equals()

==

Integer a = 200;
Integer b = 200;
System.out.println(a.equals(b)); // true