ZOOKEEPER-2817: Using `Collections.singletonList` instead of `Arrays.asList(oneElement)`

[asdf2014]

Using Collections.singletonList instead of Arrays.asList(oneElement) for reusing a immutable object instead of creating a new object.

[afine]

This seems like a very small optimization for a part of the code that is not performance sensitive. The change creates inconsistencies in Commands.java between the commands without much explanation as to why it is necessary. @asdf2014 would you mind explaining why this change is necessary?

[asdf2014]

Hi, @afine. Thank you for code review. It maybe due to...

// -ea -Xmx512M -Xms512M -Xmn256M -XX:+AlwaysPreTouch
@Test
public void testSingletonListPerformance() {
    String site = "yuzhouwan.com";
    int count = 1_0000_0000;
    long spendTime = 0;
    while (count > 0) {
        long startTime = System.nanoTime();
        Collections.singleton(site);
        long endTime = System.nanoTime();
        count--;
        spendTime += endTime - startTime;
    }
    Runtime runtime = Runtime.getRuntime();
    long totalMemory = runtime.totalMemory() / 1024 / 1024;
    long freeMemory = runtime.freeMemory() / 1024 / 1024;
    // [Collections.singleton] Spend Time: 1370786852ns = 1370.786852ms, Mem: 39/441/480 MB (diff/free/total)
    _log.info("[Collections.singleton] Spend Time: {}ns = {}ms, Mem: {}/{}/{} MB (diff/free/total)",
            spendTime, spendTime / Math.pow(10, 6), totalMemory - freeMemory, freeMemory, totalMemory);
}

// -ea -Xmx512M -Xms512M -Xmn256M -XX:+AlwaysPreTouch
@Test
public void testArraysAsListPerformance() {
    String site = "yuzhouwan.com";
    int count = 1_0000_0000;
    long spendTime = 0;
    while (count > 0) {
        long startTime = System.nanoTime();
        Arrays.asList(site);
        long endTime = System.nanoTime();
        count--;
        spendTime += endTime - startTime;
    }
    Runtime runtime = Runtime.getRuntime();
    long totalMemory = runtime.totalMemory() / 1024 / 1024;
    long freeMemory = runtime.freeMemory() / 1024 / 1024;
    // [Arrays.asList] Spend Time: 1549508768ns = 1549.508768ms, Mem: 195/312/507 MB (diff/free/total)
    _log.info("[Arrays.asList] Spend Time: {}ns = {}ms, Mem: {}/{}/{} MB (diff/free/total)",
            spendTime, spendTime / Math.pow(10, 6), totalMemory - freeMemory, freeMemory, totalMemory);
}

Full code is here.