[SPARK-39056][CORE][SQL] Use Collections.singletonList instead of Arrays.asList when only one argument

[LuciferYang]

What changes were proposed in this pull request?

This is pr use Collections.singletonList instead of Arrays.asList when there is only one argument.

Before

List<String> one = Arrays.asList("one"); 

After

List<String> one = Collections.singletonList("one"); 

Why are the changes needed?

A minor optimization will save a little memory due to Arrays.asList will create an E[] but Collections.SingletonList not.

Does this PR introduce any user-facing change?

No

How was this patch tested?

Pass GA

[srowen]

I'm not sure this is worth doing, it just doesn't save any memory

[LuciferYang]

OK~ close this

[LuciferYang]

I'm not sure this is worth doing, it just doesn't save any memory

Do a simple size test

    println(s"Size of Arrays.asList(1) is ${SizeEstimator.estimate(java.util.Arrays.asList(1))}")
    println(s"Size of Collections.singletonList(1) is " +
      s"${SizeEstimator.estimate(java.util.Collections.singletonList(1))}")

The results as follows:

Size of Arrays.asList(1) is 64
Size of Collections.singletonList(1) is 40

And for performance

val valuesPerIteration = 1000000
val benchmark = new Benchmark(
      s"Test singletonList",
      valuesPerIteration,
      output = output)

    benchmark.addCase("Arrays.asList") { _: Int =>
      for (i <- 0L until valuesPerIteration) {
        java.util.Arrays.asList(i)
      }
    }

    benchmark.addCase("Collections.singletonList") { _: Int =>
      for (i <- 0L until valuesPerIteration) {
        java.util.Collections.singletonList(i)
      }
    }
    benchmark.run()

The results as follows:

OpenJDK 64-Bit Server VM 1.8.0_322-b06 on Mac OS X 11.4
Apple M1
Test singletonList:                       Best Time(ms)   Avg Time(ms)   Stdev(ms)    Rate(M/s)   Per Row(ns)   Relative
------------------------------------------------------------------------------------------------------------------------
Arrays.asList                                         5              5           0        213.4           4.7       1.0X
Collections.singletonList                             4              4           0        252.7           4.0       1.2X

Although the advantage is very small, it is good for performance and memory

[srowen]

As used, 24 bytes in 1 allocation doesn't matter. These aren't in tight loops

[LuciferYang]

Got it