不幸的是，两者之间似乎存在差异。

如果查看这两种循环生成的字节代码，就会发现它们是不同的。

下面是来自Log4j源代码的一个示例。

在/log4j-api/src/main/java/org/apache/ loggging/ log4j/ markermanager .java中，我们有一个名为Log4jMarker的静态内部类，它定义了:

    /*
     * Called from add while synchronized.
     */
    private static boolean contains(final Marker parent, final Marker... localParents) {
        //noinspection ForLoopReplaceableByForEach
        for (final Marker marker : localParents) {
            if (marker == parent) {
                return true;
            }
        }
        return false;
    }

使用标准循环:

  private static boolean contains(org.apache.logging.log4j.Marker, org.apache.logging.log4j.Marker...);
    Code:
       0: iconst_0
       1: istore_2
       2: aload_1
       3: arraylength
       4: istore_3
       5: iload_2
       6: iload_3
       7: if_icmpge     29
      10: aload_1
      11: iload_2
      12: aaload
      13: astore        4
      15: aload         4
      17: aload_0
      18: if_acmpne     23
      21: iconst_1
      22: ireturn
      23: iinc          2, 1
      26: goto          5
      29: iconst_0
      30: ireturn

for - each:

  private static boolean contains(org.apache.logging.log4j.Marker, org.apache.logging.log4j.Marker...);
    Code:
       0: aload_1
       1: astore_2
       2: aload_2
       3: arraylength
       4: istore_3
       5: iconst_0
       6: istore        4
       8: iload         4
      10: iload_3
      11: if_icmpge     34
      14: aload_2
      15: iload         4
      17: aaload
      18: astore        5
      20: aload         5
      22: aload_0
      23: if_acmpne     28
      26: iconst_1
      27: ireturn
      28: iinc          4, 1
      31: goto          8
      34: iconst_0
      35: ireturn

那个神谕是怎么回事?

我在Windows 7上用Java 7和Java 8尝试过。

摘自Joshua Bloch的《Effective Java》第46条:

The for-each loop, introduced in release 1.5, gets rid of the clutter and the opportunity for error by hiding the iterator or index variable completely. The resulting idiom applies equally to collections and arrays: // The preferred idiom for iterating over collections and arrays for (Element e : elements) { doSomething(e); } When you see the colon (:), read it as “in.” Thus, the loop above reads as “for each element e in elements.” Note that there is no performance penalty for using the for-each loop, even for arrays. In fact, it may offer a slight performance advantage over an ordinary for loop in some circumstances, as it computes the limit of the array index only once. While you can do this by hand (Item 45), programmers don’t always do so.

即使使用像ArrayList或Vector这样的东西，其中“get”是一个简单的数组查找，第二个循环仍然有第一个循环没有的额外开销。我预计它会比第一次慢一点。

不幸的是，两者之间似乎存在差异。

如果查看这两种循环生成的字节代码，就会发现它们是不同的。

下面是来自Log4j源代码的一个示例。

在/log4j-api/src/main/java/org/apache/ loggging/ log4j/ markermanager .java中，我们有一个名为Log4jMarker的静态内部类，它定义了:

    /*
     * Called from add while synchronized.
     */
    private static boolean contains(final Marker parent, final Marker... localParents) {
        //noinspection ForLoopReplaceableByForEach
        for (final Marker marker : localParents) {
            if (marker == parent) {
                return true;
            }
        }
        return false;
    }

使用标准循环:

  private static boolean contains(org.apache.logging.log4j.Marker, org.apache.logging.log4j.Marker...);
    Code:
       0: iconst_0
       1: istore_2
       2: aload_1
       3: arraylength
       4: istore_3
       5: iload_2
       6: iload_3
       7: if_icmpge     29
      10: aload_1
      11: iload_2
      12: aaload
      13: astore        4
      15: aload         4
      17: aload_0
      18: if_acmpne     23
      21: iconst_1
      22: ireturn
      23: iinc          2, 1
      26: goto          5
      29: iconst_0
      30: ireturn

for - each:

  private static boolean contains(org.apache.logging.log4j.Marker, org.apache.logging.log4j.Marker...);
    Code:
       0: aload_1
       1: astore_2
       2: aload_2
       3: arraylength
       4: istore_3
       5: iconst_0
       6: istore        4
       8: iload         4
      10: iload_3
      11: if_icmpge     34
      14: aload_2
      15: iload         4
      17: aaload
      18: astore        5
      20: aload         5
      22: aload_0
      23: if_acmpne     28
      26: iconst_1
      27: ireturn
      28: iinc          4, 1
      31: goto          8
      34: iconst_0
      35: ireturn

那个神谕是怎么回事?

我在Windows 7上用Java 7和Java 8尝试过。

通过变量名称objectArrayList，我假设它是java.util.ArrayList的一个实例。在这种情况下，性能差异是不明显的。

另一方面，如果它是java.util的实例。LinkedList，第二种方法会慢得多，因为list# get(int)是一个O(n)操作。

因此，第一种方法总是首选的，除非循环中的逻辑需要索引。

for-each循环通常是首选的。如果您使用的List实现不支持随机访问，那么“get”方法可能会慢一些。例如，如果使用LinkedList，则会产生遍历代价，而For -each方法使用迭代器跟踪其在列表中的位置。关于for-each循环的细微差别的更多信息。

我想文章现在在这里:新的位置

这里显示的链接已经失效。