Automatic packing, automatic unpacking

Before jdk1.5, you need to store basic data types in wrapper classes like this:

Integer iObj = new Integer(5);
int i = iObj.intValue();

After jdk1.5, java introduced new features, automatic boxing and automatic unboxing.

So it can be written like this:

Integer i = 5 ;

Java has a total of eight basic data types, each of which has a corresponding wrapper class.

basic data type	Packaging class
byte	Byte
short	Short
int	Integer
long	Long
char	Character
float	Float
double	Double
boolean	Boolean

Autoboxing:

Integer i = 10;

In fact, the compiler does some extra operations for us. The decompiled code should be as follows:

Integer i = Integer.valueOf(10);

Automatic unboxing:

int x = i;

After decompiling:

int x = i.intValue();

Integer a = 100;
Integer b = 100;
Integer c = 200;
Integer d = 200;
syso(a == b);
syso(c == d);

The output is

true

false

When Integer creates an Integer object through the valueOf method, when the value range is between [-128, 127], it will point to the application of an existing object, otherwise a new Integer object will be created.

If the type of the above question is changed to double type, both results are false

Note that the implementation of the valueOf method of the Integer, Short, Byte, Character, and Long classes is similar.

The implementation of the valueOf method of Double and Float is similar.

When both operands of the "==" operator are references of the wrapper type, the comparison is made to see if they point to the same object, and if one of the operands is an expression (that is, contains an arithmetic operation), the comparison is made is a numeric value (i.e. triggers the process of automatic unboxing). Also, for wrapper types, the equals method does not perform type conversions.