Deep understanding of the for and foreach loops in Java _java

A variable in a cyclic condition in a for loop is only a one-time value! Look at the final picture in detail

The foreach statement is Java5 new, and foreach has good performance when traversing arrays and collections.

foreach is a simplification of the for statement, but foreach does not replace the for loop. It can be said that any foreach can be rewritten as a for loop, but vice versa.

foreach is not a keyword in java. A Foreach Loop object is generally a collection, List, ArrayList, LinkedList, Vector, array, and so on.

foreach Format:

for (element type T Each loop element's name O: Loop object) {

Operate on O

}

One, the common use way.

1. Foreach traversal array.

/**
 * Description:
 * Created by Ascend on 2016/7/8.
 * * Public
class Client {public
  static void Main (string[] args) {
    string[] names = {"Beibei", "Jingjing"};
   for (String name:names) {
      System.out.println (name);}}}

2.foreach Traverse list.

/**
 * Description:
 * Created by Ascend on 2016/7/8.
 * * Public
class Client {public

  static void Main (string[] args) {
    list<string> List = new ArrayList (); 
   list.add ("a");
    List.add ("B");
    List.add ("C");
    for (String str:list) {
      System.out.println (str);}}}

Second, limitations.

foreach can traverse an array or a collection, but it can only be traversed, unable to modify the array or collection during traversal, and the for loop can modify the source arrays or collections during traversal.

1. Array

/**
 * Description:
 * Created by Ascend on 2016/7/8.
 * * Public
class Client {public

  static void Main (string[] args) {
    string[] names = {"Beibei", "Jingjing"};
   
    for (String name:names) {
      name = ' Huanhuan ';
    }
    foreach
    System.out.println ("foreach:" +arrays.tostring (names));
    For for
    (int i = 0; i < names.length i++) {
      names[i] = "Huanhuan";
    }
    System.out.println ("For:" +arrays.tostring (names));
  }

Output:
Foreach:[beibei, Jingjing]
For:[huanhuan, Huanhuan]
   

2. Collection

/**
 * Description:
 * Created by Ascend on 2016/7/8.
 * * Public
class Client {public

  static void Main (string[] args) {
    list<string> names = new arraylist< String> ();
    Names.add ("Beibei");
    Names.add ("Jingjing");
    foreach for
    (String name:names) {
      name = ' Huanhuan ';
    }
    System.out.println (Arrays.tostring (Names.toarray ()));
    for for (int i = 0; i < names.size (); i++) {
      Names.set (i, "Huanhuan");
    }
    System.out.println (Arrays.tostring (Names.toarray ()));
  }

Output:
[Beibei, Jingjing]
[Huanhuan, Huanhuan]

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