Java 哪个代码段执行得更快？

我可以通过两种类型的代码片段做同样的事情

第一种方式：

String makeDate = Integer.toString(now.year) + Integer.toString(now.month) + Integer.toString(now.monthDay);

或者第二种方式：

String makeDate = now.year + "" + now.month + "" + now.monthDay;

我的问题是:

哪种方法更可取[第一种方法还是第二种方法]

哪个代码段执行得更快

---

你展示的两个鼻涕虫几乎一模一样

Java中的

字符串是不可变的；这是无法改变的。当使用串联运算符（
+
）时，编译器实际上使用
StringBuilder

例如，您的第二个snippit变为：

String makeDate = new StringBuilder()
                   .append(now.year)
                   .append("")
                   .append(now.month)
                   .append("")
                   .append(now.monthDay)
                   .toString();

您可以查看生成的字节码来了解这一点。Java附带了一个程序
javap
，允许您查看编译的
.class

我创建了一个简单的
main（）
，以提供最少的字节码：

public static void main(String[] args) 
{
    String makeDate = Integer.toString(1) + Integer.toString(1) + Integer.toString(1);
    System.out.println(makeDate);
}

产生：

public static void main(java.lang.String[]);
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=2, args_size=1
         0: new           #2                  // class java/lang/StringBuilder
         3: dup           
         4: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
         7: iconst_1      
         8: invokestatic  #4                  // Method java/lang/Integer.toString:(I)Ljava/lang/String;
        11: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        14: iconst_1      
        15: invokestatic  #4                  // Method java/lang/Integer.toString:(I)Ljava/lang/String;
        18: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        21: iconst_1      
        22: invokestatic  #4                  // Method java/lang/Integer.toString:(I)Ljava/lang/String;
        25: invokevirtual #5                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        28: invokevirtual #6                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        31: astore_1      
        32: getstatic     #7                  // Field java/lang/System.out:Ljava/io/PrintStream;
        35: aload_1       
        36: invokevirtual #8                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        39: return     

产生：

public static void main(java.lang.String[]);
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1      
         1: istore_1      
         2: new           #2                  // class java/lang/StringBuilder
         5: dup           
         6: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
         9: iload_1       
        10: invokevirtual #4                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        13: ldc           #5                  // String 
        15: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        18: iload_1       
        19: invokevirtual #4                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        22: ldc           #5                  // String 
        24: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        27: iload_1       
        28: invokevirtual #4                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        31: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        34: astore_2      
        35: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
        38: aload_2       
        39: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        42: return        

publicstaticvoidmain（java.lang.String[]）；
标志：ACC_公共，ACC_静态
代码：
堆栈=2，局部变量=3，参数大小=1
0:iconst_1
1:istore_1
2:new#2//类java/lang/StringBuilder
5:dup
6:invokespecial#3//方法java/lang/StringBuilder。”“：（）V
9:iload_1
10:invokevirtual#4//方法java/lang/StringBuilder.append：（I）Ljava/lang/StringBuilder；
13:ldc#5//String
15:invokevirtual#6//方法java/lang/StringBuilder.append:（Ljava/lang/String；）Ljava/lang/StringBuilder；
18:iload_1
19:invokevirtual#4//方法java/lang/StringBuilder.append：（I）Ljava/lang/StringBuilder；
22:ldc#5//String
24:invokevirtual#6//方法java/lang/StringBuilder.append:（Ljava/lang/String；）Ljava/lang/StringBuilder；
27:iload_1
28:invokevirtual#4//方法java/lang/StringBuilder.append：（I）Ljava/lang/StringBuilder；
31:invokevirtual#7//方法java/lang/StringBuilder.toString:（）Ljava/lang/String；
34:astore_2
35:getstatic#8//fieldjava/lang/System.out:Ljava/io/PrintStream；
38:aload_2
39:invokevirtual#9//方法java/io/PrintStream.println:（Ljava/lang/String；）V
42:返回

从技术上讲，后者可能在一些几乎无法测量（<1ns）的规模上更快，但对于所有实际目的来说，这并不重要；用你喜欢的任何东西。
用你觉得最可读的东西。您需要执行数十亿次才能注意到差异。它们都将在固定时间内执行，因此执行时间肯定不会成为问题。。。。。在我看来，第二种方式更优雅，没有必要包装任何东西。
public static void main(java.lang.String[]);
    flags: ACC_PUBLIC, ACC_STATIC
    Code:
      stack=2, locals=3, args_size=1
         0: iconst_1      
         1: istore_1      
         2: new           #2                  // class java/lang/StringBuilder
         5: dup           
         6: invokespecial #3                  // Method java/lang/StringBuilder."<init>":()V
         9: iload_1       
        10: invokevirtual #4                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        13: ldc           #5                  // String 
        15: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        18: iload_1       
        19: invokevirtual #4                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        22: ldc           #5                  // String 
        24: invokevirtual #6                  // Method java/lang/StringBuilder.append:(Ljava/lang/String;)Ljava/lang/StringBuilder;
        27: iload_1       
        28: invokevirtual #4                  // Method java/lang/StringBuilder.append:(I)Ljava/lang/StringBuilder;
        31: invokevirtual #7                  // Method java/lang/StringBuilder.toString:()Ljava/lang/String;
        34: astore_2      
        35: getstatic     #8                  // Field java/lang/System.out:Ljava/io/PrintStream;
        38: aload_2       
        39: invokevirtual #9                  // Method java/io/PrintStream.println:(Ljava/lang/String;)V
        42: return