Java：將long轉換為int

Question

假設變量start和stop都是長型變量。

int diff = (int) start-stop;

這段代碼產生錯誤Type mismatch: cannot convert from long to int ，而

int diff = (int) (start-stop);

運行正常。 為什么？

Answer 1

因為首先， (int)適用於start ，而不適用於表達式，如下所示：

int diff = ((int) start) - stop; // <== Your first expression clarified

然后，從結果int減去long ，該表達式的結果為long 。 （如果它的工作，它可能將不得不給予你一個不正確的結果，如果副作用start曾經有過大在存儲值int ）。

通過首先進行減法並獲得long結果， 然后將其強制轉換為int ，可以將其存儲在int變量中。 （大概可以從您的應用程序邏輯中了解它的適用性。）

Answer 2

因為Java不允許隱式縮小轉換。 在第一種情況下將需要一個，因為它等效於：

int diff = ((long)(int)start) - stop;

Answer 3

int diff = (int) start - stop;

等於

int diff = (int) (long) start - (long) stop; // we substract long types

可以簡化為

int diff = (int) start - (long) stop; // (int)(long) x means (int) x

等於

int diff = ((int) start) - (long) stop; // casting is applied only to start

等於

int diff = ((long)(int) start) - (long) stop; // compiler make types equal

可以簡化為

int diff = (long) start - (long) stop; // we can substract now

等於

int diff = (long) startMinusStop; // because long - long => long

這是我們可讀的錯誤Type mismatch: cannot convert from long to int 。

Answer 4

在第一個語句中，您只會將變量start轉換為整數。 所以結果是int - long ，這是一個long，不適合整數。

它就像int = (int - long)而Java則不允許。