for循环优化 [英] for loop optimization

问题描述

List<String> flowers = new ArrayList<String>();

我的for循环目前看起来像这样...

My for loop currently looks like this...

for (int i = 0; i < flowers.size(); i++) {
...
}

或者我应该改变它看起来像下面给出的代码

OR should I change this to look like the code given below

int size = flowers.size();
for (int i = 0; i < size; i++) {
...
}

这是更高效的（假设我有一个大阵列的花），我猜它应该是后者。

Which is more performant (assuming I have a large array of flowers), I am guessing it should be the latter.

推荐答案

最好使用for-each循环可读]

for (Flower flower :flowers){
    //...
}

---

我使用 javap 代表以下代码：

public void forLoop1() {
    List<String> lst = new ArrayList<String>();
    for (int i = 0; i < lst.size(); i++) {
        System.out.println("hi");
    }
}

public void forLoop2() {
    List<String> lst = new ArrayList<String>();
    int size = lst.size();
    for (int i = 0; i < size; i++) {
        System.out.println("hi");
    }
}

---

---

public void forLoop1();
  Code:
   0:   new     #2; //class java/util/ArrayList
   3:   dup
   4:   invokespecial   #3; //Method java/util/ArrayList."<init>":()V
   7:   astore_1
   8:   iconst_0
   9:   istore_2
   10:  iload_2
   11:  aload_1
   12:  invokeinterface #4,  1; //InterfaceMethod java/util/List.size:()I
   17:  if_icmpge       34
   20:  getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   23:  ldc     #6; //String hi
   25:  invokevirtual   #7; //Method java/io/PrintStream.println:(Ljava/lang/Str
ing;)V
   28:  iinc    2, 1
   31:  goto    10
   34:  return

public void forLoop2();
  Code:
   0:   new     #2; //class java/util/ArrayList
   3:   dup
   4:   invokespecial   #3; //Method java/util/ArrayList."<init>":()V
   7:   astore_1
   8:   aload_1
   9:   invokeinterface #4,  1; //InterfaceMethod java/util/List.size:()I
   14:  istore_2
   15:  iconst_0
   16:  istore_3
   17:  iload_3
   18:  iload_2
   19:  if_icmpge       36
   22:  getstatic       #5; //Field java/lang/System.out:Ljava/io/PrintStream;
   25:  ldc     #6; //String hi
   27:  invokevirtual   #7; //Method java/io/PrintStream.println:(Ljava/lang/Str
ing;)V
   30:  iinc    3, 1
   33:  goto    17
   36:  return

它不会为我优化。

Java版本1.6.0_22Java（TM）SE
运行时环境（build
1.6.0_22-b04）Java HotSpot b03，混合模式，
共享）

java version "1.6.0_22" Java(TM) SE Runtime Environment (build 1.6.0_22-b04) Java HotSpot(TM) Client VM (build 17.1-b03, mixed mode, sharing)

所以如果你需要从上述两个选择，

So if you need to choose from mentioned two, go for second, but I personally would go for for-each.

来自Joshua的有效Java 中的第46项Bloch：

From Item 46 in Effective Java by Joshua Bloch :

release 1.5中引入的for-each循环消除了杂乱的
和错误的机会
完全隐藏迭代器或索引变量
。生成的成语
同样适用于集合和
数组：

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);
}

当看到冒号（:)时，读为
in。因此，上面的循环读为
为元素中的每个元素e。注意
没有性能惩罚
使用for-each循环，即使是
数组。事实上，在某些情况下，它可能比普通的
的循环提供轻微的
的性能优势，因为它
只计算数组索引
的限制一次。虽然你可以通过
手（Item 45）来实现，但程序员不会总是这样做。

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.

---

另请参阅

• Is-there-a-performance-difference-between-a-for-loop- and-a-for-each-loop



---

See Also

• Is-there-a-performance-difference-between-a-for-loop-and-a-for-each-loop

这篇关于for循环优化的文章就介绍到这了，希望我们推荐的答案对大家有所帮助，也希望大家多多支持IT屋！