Java中for的幾種常見形式
- For loop using index.
for (int i = 0; i < arr.length; i++) {
type var = arr[i];
body-of-loop
}
- Loop using explicit iterator.
for (Iterator<type> iter = coll.iterator(); iter.hasNext(); ) {
type var = iter.next();
body-of-loop
}
- Foreach loop over all elements in arr.
for (type var : coll) {
body-of-loop
}
For循環(huán)用來處理哪些數(shù)據(jù)結(jié)構(gòu)
- 數(shù)組
int[] a = {1,2,3,4,5,6};
int[] b = new int[]{1,2,3,4,5,6};
int[] c = new int[6];
for (int i = 0; i < a.length; i++) {
System.out.println(i);
}
for (int i : a) {
System.out.println(i);
}
- 實現(xiàn)了java.util.Iterator的類
````aidl
import java.util.Iterator;
/**
* Created by MoXingwang on 2017/6/30.
*/
public class IterableTest<E> implements Iterable<E> {
public static void main(String[] args) {
IterableTest<Integer> integers = new IterableTest<Integer>();
for (Integer integer : integers) {
System.out.println(integer);
}
}
@Override
public Iterator<E> iterator() {
return new Iterator() {
@Override
public boolean hasNext() {
//...
return false;
}
@Override
public Object next() {
//...
return null;
}
@Override
public void remove() {
//...
}
};
}
}
普通for遍歷和增強for的一些區(qū)別
增強的for循環(huán)的底層使用迭代器來實現(xiàn)藕甩，所以它就與普通for循環(huán)有一些差異
- 增強for使用增強for循環(huán)的時候不能使用集合刪除集合中的元素瘾境；
- 增強for循環(huán)不能使用迭代器中的方法嘿悬，例如remove()方法刪除元素；
- 與普通for循環(huán)的區(qū)別：增強For循環(huán)有遍歷對象设捐，普通for循環(huán)沒有遍歷對象;
對于實現(xiàn)了RandomAccess接口的集合類淮悼，推薦使用普通for挑胸，這種方式faster than Iterator.next
The RandomAccess interface identifies that a particular java.util.List implementation has fast random access. (A more accurate name for the interface would have been "FastRandomAccess.") This interface tries to define an imprecise concept: what exactly is fast? The documentation provides a simple guide: if repeated access using the List.get( ) method is faster than repeated access using the Iterator.next( ) method, then the List has fast random access. The two types of access are shown in the following code examples.
