java - 移位问题

Question

给定以下代码:

public class Something {
public static void main(String[] args) {
    int num = 1;

    num <<= 32;
    System.out.println(num); 

    num = 1;
    for (int i = 0 ; i < 32; i++)
        num <<= 1;
    System.out.println(num);
}
}

第一个输出(从 num <<= 32)是 1。

第二个输出(来自 for 循环)是 0。

我不明白..我看起来是一样的..两种方式都将“1”位 (lsb) 移位 32 次，结果不同。

谁能解释一下？

提前致谢。

Answer 1

Can anyone explain?

当然。基本上，int 上的移位操作会屏蔽正确的操作数以获取 [0, 31] 范围内的值。 long 的移位操作将其屏蔽以获取 [0, 63] 范围内的值。

所以:

num <<= 32;

相当于:

num <<= 0;

来自 section 15.19 of the JLS :

If the promoted type of the left-hand operand is int, only the five lowest-order bits of the right-hand operand are used as the shift distance. It is as if the right-hand operand were subjected to a bitwise logical AND operator & (§15.22.1) with the mask value 0x1f (0b11111). The shift distance actually used is therefore always in the range 0 to 31, inclusive.