Use of foreach in Java

Use of foreach in Java

 

After Java 5, the foreach syntax is added, which can be seen as an extension of the for loop. Its main use is to make it easier to traverse arrays or other collections. The new syntax is named enhanced for or foreeach (refer to the naming method in other languages ).

Foreach can be used to traverse the array and each successive value in collections. Of course, it can also be used to traverse those that have implemented iterable <E>
(Iterator () function must be defined), and many collections classes have implemented iterable <E>, which makes
The foreach syntax is useful.

Common Format

The following lists the use of foreach and for, and provides two basic equivalence types, the difference between arrays and other iteratable objects.

Example-calculate the elements and

Foreach syntax implementation:

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double [] ar = { 1.2 , 3.0 , 0.8 };int sum = 0 ;for ( double d : ar) {  // d gets successively each value in ar.     sum += d;}

For syntax implementation:

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double [] ar = { 1.2 , 3.0 , 0.8 };int sum = 0 ;for ( int i = 0 ; i < ar.length; i++) {  // i indexes each element successively.     sum += ar[i];}

When to use foreach

From the example above, we can see that foreach can clean the code sometimes, but note that it is not suitable for writing in some cases:

1. Read-Only: The accessed element cannot be assigned a value, such as the auto-increment of the element.
2. Single structure: it is impossible to traverse two structures at the same time, such as comparing two Arrays
3. Single element: Only applicable to reading a single element
4. Unidirectional: Only iterations of the previous element can be performed.
5. Java 5 supported: Do not use this syntax before Java 5

Use of foreach in two-dimensional array

Example:

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public class test02 {     public static void main(String args[])     {         int [][] a = new int [][] {             { 1 , 2 , 3 , 4 , 5 },             { 10 , 11 , 12 }         };           for ( int [] row : a) {             for ( int element : row)                 System.out.print(element + " " );             System.out.println();         }     }}

More use of foreach

As mentioned above, foreach applies to objects that implement the iterable <t> interface, such as traversing arrays or collections. For example, traverse list or set:

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//Listpublic void go(List<String> list) {     for (String element : list) {         System.out.println(element);     }} //setpublic void go(Set<String> set) {     for (String element : set) {         System.out.println(element);     }}

The traversal of collections is as follows:

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public void go(Collection<String> collection) {     for (String element : collection) {         System.out.println(element);     }}

In fact, during compilation, the compiler will expand foreach. For example, the preceding collections is expanded as follows:

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public void go(Collection collection) {     String s;     for (Iterator iterator = collection.iterator();         iterator.hasNext(); System.out.println(s))         s = (String)iterator.next();}

 

Reprinted from: http://www.leyond.info/usage-of-foreach-in-java/#more-131521

 

For more mysterious information about foreach,

 

Reference: http://caterpillar.onlyfun.net/Gossip/JavaEssence/Foreach.html
, This article also references:

1. Http://yczhuang.blogspot.com/2009/02/javaforeach.html
   
2. Http://leepoint.net/notes-java/flow/loops/foreach.html