A brief discussion on java enhanced for loop for each-javaTutorial-php.cn

A brief discussion on java enhanced for loop for each

高洛峰

Release： 2017-01-21 16:47:21

Original

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For-Each loop

The For-Each loop is also called an enhanced for loop, or a foreach loop.

For-Each loop is a new feature of JDK5.0 (other new features such as generics, autoboxing, etc.).

The addition of For-Each loop simplifies collection traversal.

The syntax is as follows:

for(type element: array)
 
{
 
   System.out.println(element);
 
}

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Example

You can directly look at the code for its basic use:

The code first compares the two A for loop; then implemented an enhanced for loop to traverse a two-dimensional array; and finally used three methods to traverse a List collection.

import java.util.ArrayList;
import java.util.Iterator;
import java.util.List;
 
public class ForeachTest
{
  public static void main(String[] args)
  {
    int[] arr = {1, 2, 3, 4, 5};
     
    System.out.println("----------旧方式遍历------------");
    //旧式方式   
    for(int i=0; i<arr.length; i++)
    {
      System.out.println(arr[i]);
    }
     
    System.out.println("---------新方式遍历-------------");
     
    //新式写法,增强的for循环
    for(int element:arr)
    {
      System.out.println(element);
    }
     
    System.out.println("---------遍历二维数组-------------");
     
    //遍历二维数组
     
    int[][] arr2 = {{1, 2, 3}, {4, 5, 6}, {7, 8, 9}} ;
     
    for(int[] row : arr2)
    {
      for(int element : row)
      {
        System.out.println(element);
      }
    }
     
    //以三种方式遍历集合List
     
    List<String> list = new ArrayList<String>();
     
    list.add("a");
    list.add("b");
    list.add("c");
     
    System.out.println("----------方式1-----------");
    //第一种方式，普通for循环
    for(int i = 0; i < list.size(); i++)
    {
      System.out.println(list.get(i));
       
    }
     
    System.out.println("----------方式2-----------");
    //第二种方式，使用迭代器
    for(Iterator<String> iter = list.iterator(); iter.hasNext();)
    {
      System.out.println(iter.next());
    }
    System.out.println("----------方式3-----------");
    //第三种方式，使用增强型的for循环
    for(String str: list)
    {
      System.out.println(str);
       
    }
  }
 
}

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Disadvantages of For-Each loop: index information is lost.

When traversing a collection or array, if you need to access the subscript of the collection or array, it is better to use the old-style way to implement the loop or traversal instead of using the enhanced for loop, because it loses the subscript information.

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