java - 性能差异 : initialize and override in an if-else block, 还是额外的 "else"？

Question

我很好奇以下哪种方法更有效:用默认值初始化变量，并且仅在 if-else block 中需要时才覆盖它，或者在根本没有值的情况下初始化变量并设置if-else block 中的值？

这是前者和后者的示例:

前:

    String weightStatus = "Underweight";

    if (bMI > 29.9)
    {
    weightStatus = "Obese";
    }

    else if (bMI >= 25.0)
    {
    weightStatus = "Overweight";
    }
    
    else if (bMI >= 18.5)
    {
    weightStatus = "Healthy Weight";
    }

后者:

    String weightStatus;

    if (bMI > 29.9)
    {
    weightStatus = "Obese";
    }
    
    else if (bMI >= 25.0)
    {
    weightStatus = "Overweight";
    }
    
    else if (bMI >= 18.5)
    {
    weightStatus = "Healthy Weight";
    }
    else
    {
    weightStatus = "Underweight";
    }
    

差异很小，但我忍不住想知道根据变量赋值的工作原理，哪一个在技术上更快。

Answer 1

如你所知，前一种情况，字节码总是会设置变量，然后根据if/then，它可能会再次重置它。

但这也取决于在运行时传递哪些值，如果它们大部分不在 then 分支中，那么我想这不会有太大区别，但如果它们经常进入 then 分支，然后大部分是双重设置执行。

您可以使用以下内容进行测试:

public class MyTest {

    private static double START = 0;
    private static double END = 100;
    private static double INCREMENT = 0.0001;

    @Test
    public void testFirst() throws Exception {
        long time = System.nanoTime();
        for (double bMI = START; bMI < END; bMI += INCREMENT) {
            first(bMI);
        }
        System.out.println("First  " + (System.nanoTime() - time));
    }

    @Test
    public void testSecond() throws Exception {
        long time = System.nanoTime();
        for (double bMI = START; bMI < END; bMI += INCREMENT) {
            second(bMI);
        }
        System.out.println("Second " + (System.nanoTime() - time));
    }

    private String first(double bMI) {
        String weightStatus = "Underweight";
        if (bMI > 29.9) {
            weightStatus = "Obese";
        } else if (bMI >= 25.0) {
            weightStatus = "Overweight";
        } else if (bMI >= 18.5) {
            weightStatus = "Healthy Weight";
        }
        return weightStatus;
    }

    private String second(double bMI) {
        String weightStatus;
        if (bMI > 29.9) {
            weightStatus = "Obese";
        } else if (bMI >= 25.0) {
            weightStatus = "Overweight";
        } else if (bMI >= 18.5) {
            weightStatus = "Healthy Weight";
        } else {
            weightStatus = "Underweight";
        }
        return weightStatus;
    }
}