Java中for的幾種常見形式

1. For loop using index.

for (int i = 0; i < arr.length; i++) { 
    type var = arr[i];
    body-of-loop
}

2. Loop using explicit iterator.

for (Iterator<type> iter = coll.iterator(); iter.hasNext(); ) {
    type var = iter.next();
    body-of-loop
}

3. 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)

1. 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);
}

2. 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)，所以它就與普通for循環(huán)有一些差異

1. 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；
2. 增強for循環(huán)不能使用迭代器中的方法，例如remove()方法刪除元素；
3. 與普通for循環(huán)的區(qū)別：增強For循環(huán)有遍歷對象，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，推薦使用普通for，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.

參考資料

• The For-Each Loop
• The RandomAccess Interface
• 增強for循環(huán)
• For-each Loop