This blog is written more clearly: Java automatic boxing and unboxing and their pitfalls

Look at this piece of code first

Integer a = 1; int b = 2; int c = a + b; Integer d = a + b;

This is the debugging result

In Java, only the same type can be directly calculated, but no transformation is performed here. When Integer and int are calculated in the third line, they will be automatically unboxed and converted to int type for calculation, and the fourth line will be converted to int calculation first. After the result is obtained, it is automatically packaged into Integer

Take integer as an example, Integer.intValue() and Integer.valueOf(int x)