我有以下几点:
double timeInMinutes = (double) timeInMilliseconds / (1000 * 60);
操作 (1000 * 60)
是在编译时还是在运行时完成的?换句话说,上面的代码片段在运行时是否存在性能差异:
double timeInMinutes = (double) timeInMilliseconds / 60000;
编辑:我的问题与 Will the Java compiler precalculate sums of literals? 不同,因为我在算术运算中混合使用变量和文字。这是一个很小的区别,但正如@TagirValeev 在评论中指出的那样( Are arithmetic operations on literals calculated at compile time or run time? ),有些文字即使可以预编译也没有预编译。
最佳答案
根据 JLS §15.2 - Forms of Expressions
Some expressions have a value that can be determined at compile time. These are constant expressions (§15.28).
*、/和 %
等乘法运算符属于常量表达式,因此将在编译时确定。
@SergeyMorozov 比我更快地编写和获得字节码证明(#2 = Integer 60000
),但这是实际证明,以上是理论/官方声明:
尝试使用 1000 * 60
和 60000
在您的最后生成字节码,您将看到相同的字节码指令,因此会有相同的运行时性能.
Java 类:
public class Test {
public static void main(String[] args) {
int compileTest = 1000 * 60;
}
}
字节码:
Classfile /E:/Test.class
Last modified Oct 9, 2015; size 265 bytes
MD5 checksum fd115be769ec6ef7995e4c84f7597d67
Compiled from "Test.java"
public class Test
SourceFile: "Test.java"
minor version: 0
major version: 51
flags: ACC_PUBLIC, ACC_SUPER
Constant pool:
#1 = Methodref #4.#13 // java/lang/Object."<init>":()V
#2 = Integer 60000
#3 = Class #14 // Test
#4 = Class #15 // java/lang/Object
#5 = Utf8 <init>
#6 = Utf8 ()V
#7 = Utf8 Code
#8 = Utf8 LineNumberTable
#9 = Utf8 main
#10 = Utf8 ([Ljava/lang/String;)V
#11 = Utf8 SourceFile
#12 = Utf8 Test.java
#13 = NameAndType #5:#6 // "<init>":()V
#14 = Utf8 Test
#15 = Utf8 java/lang/Object
{
public Test();
flags: ACC_PUBLIC
Code:
stack=1, locals=1, args_size=1
0: aload_0
1: invokespecial #1 // Method java/lang/Object."<init>":()V
4: return
LineNumberTable:
line 1: 0
public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
stack=1, locals=2, args_size=1
0: ldc #2 // int 60000
2: istore_1
3: return
LineNumberTable:
line 3: 0
line 4: 3
}
关于java - 文字的算术运算是在编译时还是运行时计算的?,我们在Stack Overflow上找到一个类似的问题: https://stackoverflow.com/questions/33040680/