Java Enhanced for Loop

3. Enhanced for Loop1) Function:

Iterate over the container of the storage object

2) How Jdk5 used to iterate 3) enhance the For loop iterative algebra Group

String [] arr = {"A", "B", "C"}; Static definition of an array, only when the array is first defined

Traditional way

for (int i=0; i<arr.length; i++) {

I, in turn, represent the array's Corner label

Strings = Arr[i];

System.out.println (s);

}

System.out.println ("-------------------------------------");

In Jdk5 we can use the enhanced for loop iteration

Enhanced for loop two arguments are written in parentheses, the first is declaring a variable, and the variable type must be the type of the array element

The second is a container that needs to be iterated

The For loop loops the length of the container, assigning the n-1 element of the container to the declared variable each time

for (String S:arr) {

Loop body, perform arr.length

Each time the n-1 element in Arr is given s

System.out.println (s); //

}

4) Single-row collection collection

List List = new ArrayList ();

List.add ("AAA");

List.add ("BBB");

List.add ("CCC");

Traditional Way 1

/* 1. Get iterators

Iterator iter = List.iterator ();

2. Loop to determine if the iterator has the next

while (Iter.hasnext ()) {

Stringstr = (String) iter.next (); Moves the pointer of the iterator to the next and returns the element that is currently pointing to the iteration

System.out.println (str);

}

*/

Traditional Way 2

For (Iterator iter=list.iterator (); Iter.hasnext ();) {

Strings = (String) iter.next ();

System.out.println (s);

}

System.out.println ("--------------------------------");

Enhanced for loop, does not use a generic collection can I use an enhanced for loop iteration? Yes

for (Object obj:list) {

Strings = (String) obj;

System.out.println (s);

}

5) Double Row set map

Map map = new HashMap ();

Map.put ("A", "AAA");

Map.put ("B", "BBB");

Map.put ("C", "CCC");

Traditional way Iteration 1

1. Get all the keys

Set keys = Map.keyset ();

2. Iterate keys to get all keys

Iterator iter = Keys.iterator ();

while (Iter.hasnext ()) {

Stringkey = (String) iter.next (); A b C

3. Obtain the corresponding value according to key

StringValue = (String) map.get (key);

System.out.println (key+ "=" + value);

}

System.out.println ("---------------------------------");

Traditional way 2, must master this way

1. Get all key-value pairs entry objects

Set Entrys = Map.entryset ();

2. Iterate through all entry

iter = Entrys.iterator ();

while (Iter.hasnext ()) {

Map.entryentry = (Entry) iter.next ();

Get key and value separately

Stringkey = (String) entry.getkey ();

StringValue = (String) entry.getvalue ();

System.out.println (key+ "=" + value);

}

System.out.println ("-------------------------------------");

SYSTEM.OUT.PRINTLN ("Enhanced for loop iteration,");

Enhanced for loop iterations,

In principle, the map collection cannot be iterated using the enhanced for loop,

Because the enhanced for loop can only iterate over a collection that implements the Iterable interface

Iterable is a newly defined interface in Jdk5, a method iterator method

Only the class that implements the Iterable interface can guarantee that there must be a iterator method

Java has such a qualification because it is enhanced internally by the For loop or by an iterator.

In fact, we can use the enhanced for loop in some way

For (Object Obj:map.entrySet ()) {

Obj in turn represents the entry

Map.entryentry = (Entry) obj;

System.out.println (Entry.getkey () + "=" + Entry.getvalue ());

}

6) Collection Iteration Attention issues

The collection cannot be deleted or removed during the iteration of the collection using iterators

@Test

public void Test4 () {

Listlist = new ArrayList ();

List.add ("Wangwu");

List.add ("Zhangsan");

List.add ("Lisi");

Iteratoriter = List.iterator ();

while (Iter.hasnext ()) {

Stringname = (String) iter.next ();

if ("Wangwu". Equals (name)) {

Delete from the collection

List.remove (name);

Methods to call iterators for deleting elements during an iteration

Iter.remove (); Delete the collection of my iterations the last element of my iteration

}

}

1 2 4

System.out.println (List.size ());

}

@Test

public void Test5 () {

Listlist = new ArrayList ();

List.add ("AA");

List.add ("BB");

Using Listiterator iterators

Listiteratorlistiterator = List.listiterator ();

while (Listiterator.hasnext ()) {

Listiterator.next ();

Adding elements during an iteration

Listiterator.add ("CC");

}

System. out. println (List.size ());

}

7) Enhanced for loop attention issues

element cannot be assigned when using enhanced for loop

Int[] arr = {A-i};

for (int num:arr) {

num= 0;

}

System.out.println (arr[1]);

Publicclass Demo3 {

@Test

public void Test3 () {

Map map=new HashMap ();

Map map2=newlinkedhashmap<k, v> ();

Map.put ("1", "AAA");

Map.put ("2", "BBB");

Map.put ("3", "CCC");

Traditional Way 1

Set Set=map.keyset ();

Iterator It=set.iterator ();

while (It.hasnext ()) {

stringkey= (String) it.next ();

Stringvalue= (String) map.get (key);

System.out.println ("key=" +key+ ", value=" +value);

}

Traditional Way 2

Set Set2=map.entryset ();

Iterator It2=set2.iterator ();

while (It2.hasnext ()) {

Map.entry entry= (Entry) it2.next ();

System.out.println ("key=" +entry.getkey () + ", value=" +entry.getvalue ());

}

1 ways to enhance a For loop

For (Object Obj:map.keySet ()) {

stringkey2= (String) obj;

Stringvalue2= (String) map.get (Key2);

System.out.println ("key2=" +key2+ ", value2=" +value2);

}

2 ways to enhance A For loop

For (Objectobj:map.entrySet ()) {

map.entryentry3= (Entry) obj;

String key3= (String) Entry3.getkey ();

stringvalue3= (String) entry3.getvalue ();

System.out.println ("key3=" +key3+ ", value3=" +value3);

}

Issues to be aware of when enhancing for loops: data only

int arr[]={1,2,3};

for (int i:arr) {

i=10;

}

System.out.println (arr[0]);//1

List li=new ArrayList ();

Li.add ("1");

for (Object Obj:li) {

obj= "888";

}

System.out.println (li.get (0));//1

}

}

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Java Enhanced for Loop