Java: The scary thing about auto-boxing in the unconscious

Preface : "Effective Java" is really a classic book, it is really not a little early to buy this book and feel sorry. Again to learn a little knowledge, before, I just know that integer is the boxed class of int, and int is the basic type, never realize that there is such a big difference in automatic boxing, " to take precedence over the basic type, instead of boxing the basic type, beware of the unconscious automatic boxing." "

As written in the book, I tested the following simple examples.

 PackageCom.mwq.number; Public  class Test {     Public Static void Main(string[] args) {Longtime1 = System.currenttimemillis ();Longsum =0L for(inti =0; I < Integer.max_value/2;        i++) {sum + = i; } System.out.println (sum);LongTime2 = System.currenttimemillis ();        System.out.println (TIME2-TIME1); Long sum1 =0L for(inti =0; I < Integer.max_value/2;        i++) {sum1 + = i; } System.out.println (sum);LongTime3 = System.currenttimemillis ();    System.out.println (time3-time2); }}

The example is simple enough to see how long and long are different in +i, and how much time is spent.

See the result , because my computer calculates 0 to integer.max_value the operation speed is too slow, therefore only takes half of the quantity.

576460750692810753190357646075069281075310939

An order of magnitude difference!!!!!!!!!

Sum is declared as a long instead of a long, it will create n (how many n, I really do not know, the book is said to be 2 of the 31 square, obviously I am not) multiple long instances!

Summary : The project has a large number of boxing and basic types, it seems to be a good optimization.

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Java: The scary thing about auto-boxing in the unconscious