Performance impact of unnecessary casting

A simple test to see how unnecessary casting will cost in Java.

// direct cast to double
methodWithDoubleAsParam((double)Integer.parseInt(anInteger))

vs

// explicitly cast to float and implicitly to double
methodWithDoubleAsParam((float)Integer.parseInt(anInteger))

Quick start

git clone https://github.com/kitsook/casting-unnecessary.git
./gradlew build
./gradlew bootRun --args="1"

Output:

> Task :bootRun

  .   ____          _            __ _ _
 /\\ / ___'_ __ _ _(_)_ __  __ _ \ \ \ \
( ( )\___ | '_ | '_| | '_ \/ _` | \ \ \ \
 \\/  ___)| |_)| | | | | || (_| |  ) ) ) )
  '  |____| .__|_| |_|_| |_\__, | / / / /
 =========|_|==============|___/=/_/_/_/

......

Extra time spent for each extra casting: 2.00ns

What is going on

Looking at the generated bytecode (tested with openjdk 11.0.11 2021-04-20) by running

javap -c -p build/classes/java/main/net/clarenceho/test/castingunnecessary/UnnecessaryCastingApplication.class

Output:

......
  private void lambda$run$2(java.lang.String[], int);
    Code:
       0: aload_0
       1: aload_1
       2: iconst_0
       3: aaload
       4: invokestatic  #20                 // Method java/lang/Integer.parseInt:(Ljava/lang/String;)I
       7: i2f
       8: f2d
       9: invokevirtual #21                 // Method dummy:(D)V
      12: return

  private void lambda$run$1(java.lang.String[], int);
    Code:
       0: aload_0
       1: aload_1
       2: iconst_0
       3: aaload
       4: invokestatic  #20                 // Method java/lang/Integer.parseInt:(Ljava/lang/String;)I
       7: i2d
       8: invokevirtual #21                 // Method dummy:(D)V
      11: return
......

For direct casting, a single i2d (integer to double) instruction is used. With extra casting, it used i2f to convert integer to float and then f2d to convert the float to double.