当我们在Java中将Integer分配给int时，为什么编译器不会出错

Question

Why compiler doesn't give error when we assign Integer(object) to int(primitive)?

int i;
Integer ii = new Integer(5);
i = ii;//no compilation error.

And this is the case with all other types(byte-Byte, float-Float)..

What is the reason? Am i missing something here?

Answer 1

It's called autoboxing/unboxing.

As of Java 1.5, the compiler automatically "boxes" primitives into their corresponding class (eg int and Integer , double and Double etc), and un-boxes as required.

See this page in the documentation for more details.

Answer 2

Java SE 5.0 introduced autoboxing as a new feature. You can find more information in the Java documentation. http://docs.oracle.com/javase/1.5.0/docs/guide/language/autoboxing.html

Answer 3

Java 5 and newer are able to perform autoboxing . The compiler will implicitly transform your code into:

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

Answer 4

i = ii;//no compilation error.

Because this is called autounboxing. When you assign object to primitive variable , value from object is taken out and assigned to primitive. this process is called autounboxing . Vice versa is Autoboxing .

Answer 5

This is called "autoboxing/unboxing". The primitive types like int are automatically converted to classes like Integer and vice-versa when it is needed.