Question

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[第一种方式或第二种方式]？
哪个代码段执行得更快？

Answer 1

你展示的两个snippits几乎相同。

Java中的String是不可变的;它无法改变。使用连接运算符（+）时，编译器实际使用StringBuilder生成代码

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以查看此信息。 Java附带了一个程序javap，允许您查看已编译的.class。

我创建了一个简单的main()来提供最小字节码：

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

对战：

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

产地：

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

技术上后者可能在某种程度上更快，几乎无法估量（<1ns）但是对于所有实际目的而言并不重要;使用你喜欢的任何一个。

哪个代码段执行得更快？

1 个答案: