CODEWITHSUNDEEP
java
neverendingqs
neverendingqs

Reputation: 4276

Are arithmetic operations on literals calculated at compile time or run time?

I have the following:

double timeInMinutes = (double) timeInMilliseconds / (1000 * 60);

Is the operation (1000 * 60) done at compile time or at run time? In other words, are there performance differences during run time between the code snippet above and:

double timeInMinutes = (double) timeInMilliseconds / 60000;

EDIT: my question is different from Will the Java compiler precalculate sums of literals?, as I'm mixing the use of variables and literals in the arithmetic operations. It's a small difference, but as @TagirValeev noted in the comments (Are arithmetic operations on literals calculated at compile time or run time?), there are instances where some literals aren't pre-compiled even though they could be.

Upvotes: 30

Views: 2882

Answers (3)

hagrawal7777
hagrawal7777

Reputation: 14658

As per JLS §15.2 - Forms of Expressions

Some expressions have a value that can be determined at compile time. These are constant expressions (§15.28).

Multiplicative operators like *, /, and % falls under constant expressions, so it would be determined at compile time.

@SergeyMorozov was faster than me to write and get byte code proof (#2 = Integer 60000) but here is the practical proof and above is theoretical/official statement:

Try generating byte code at your end as well using 1000 * 60 and 60000, and you will see same byte code instructions, and hence there would be same runtime performance.

Java class:

public class Test {
    public static void main(String[] args) {
        int compileTest = 1000 * 60;
    }
}

Byte code:

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
}

Upvotes: 21

Sergey Morozov
Sergey Morozov

Reputation: 4608

Just create class Test

public class Test {
    public static void main(String [] args) {
        long timeInMilliseconds = System.currentTimeMillis();
        double timeInMinutes = (double) timeInMilliseconds / (1000 * 60);
        System.out.println(timeInMinutes);
    }
}

and decompile it using command: javap -v Test

You can see output of decompiled class:

  public static void main(java.lang.String[]);
flags: ACC_PUBLIC, ACC_STATIC
Code:
  stack=4, locals=5, args_size=1
     0: invokestatic  #2                  // Method java/lang/System.currentTimeMillis:()J
     3: lstore_1
     4: lload_1
     5: l2d
     6: ldc2_w        #3                  // double 60000.0d
     9: ddiv
    10: dstore_3
    11: getstatic     #5                  // Field java/lang/System.out:Ljava/io/PrintStream;
    14: dload_3
    15: invokevirtual #6                  // Method java/io/PrintStream.println:(D)V
    18: return
  LineNumberTable:
    line 3: 0
    line 4: 4
    line 5: 11
    line 6: 18

Take a look on the line 6: ldc2_w #3 // double 60000.0d

Upvotes: 8

sh0rug0ru
sh0rug0ru

Reputation: 1626

At compile time. This is one of those most basic compiler optimizations, known as Constant Folding.

Upvotes: 10

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