当前位置：首页 -> AI编程基础 -> JAVA

java编译器对于循环进行了真正的编译优化吗？

2014-11-23 23:19:08 · 作者: · 浏览: 0
标签: java 编译器 对于 循环 进行 真正 编译 优化
此文章只是用来在java编译器进行循环化方面的验证，本文是有感而发，并希望求得大家的分析结果，这里引用了一位网友的话（没有人身攻击的色彩）:
///////////////////////////
对于Vector v=new Vector()若v中包含了10000个对象,我们进行一个loop如下
for(int I=0; I大家一定会用如下的写法吧
int size = v.size(); for(int I=0; I///////////////////////////
但有些网友说(只是为了说明问题而已，无攻击色彩）：
不能用完全解释语言过程来理解Java，v.size()在循环中并没有被调用10000次，因为，编译的循环优化会对其进行优化，
因此，实际运行时只运行一次，和用初始化v.size()的程序效率一样。这是很早的编译优化机制。
大家肯定这种说法正确吗？我不太赞同,很简单地可以用一个test作难证:
一个类Vector的类TetstObj.java
  2. public class TestObj {
  3. int count=0;
  4. public TestObj() {
  5. count+=5;
  6. }
  7. public int size(){
  8. System.out.println("size="+count);
  9. return count;
  10. }
  11. }

测试类:test.java
  2. public class test{
  3. public test(){}
  4. public static void main(String[] args){
  5. TestObj similarVector=new TestObj();
  6. for(int i=1;i
  7. //do something
  8. }
  9. }
  10. }

结果还是5个size=1的打印结果.
size=5
size=5
size=5
size=5
size=5
大家也可以分析一下java.util.Vector这个类(下面摘录Vector的部分源码)
例如只以一个insert对象而言:
  2. /**
  3. * Returns the number of components in this vector.
  4. *
  5. * @return the number of components in this vector.
  6. */
  7. protected int elementCount;
  8. /**
  9. * Inserts the specified object as a component in this vector at the
  10. * specified index. Each component in this vector with
  11. * an index greater or equal to the specified index is
  12. * shifted upward to have an index one greater than the value it had
  13. * previously.

  14. *
  15. * The index must be a value greater than or equal to 0
  16. * and less than or equal to the current size of the vector. (If the
  17. * index is equal to the current size of the vector, the new element
  18. * is appended to the Vector.)

  19. *
  20. * This method is identical in functionality to the add(Object, int) method
  21. * (which is part of the List interface). Note that the add method reverses
  22. * the order of the parameters, to more closely match array usage.
  23. *
  24. * @param obj the component to insert.
  25. * @param index where to insert the new component.
  26. * @exception ArrayIndexOutOfBoundsException if the index was invalid.
  27. * @see #size()
  28. * @see #add(int, Object)
  29. * @see List
  30. */
  31. public synchronized void insertElementAt(Object obj, int index) {
  32. modCount++;
  33. if (index >= elementCount + 1) {
  34. throw new ArrayIndexOutOfBoundsException(