Experience 18--JDK new Features-static import, automatic encapsulation and disassembly, enhanced for loop

1. An important theme of "JDK 5.0" is to simplify development by adding a few features, including:

• Static Import

• Automatic packaging/Unpacking

For-each Cycle

• Variable Parameters

• Enumerating

• generic type

• Meta data

static import Syntax is used to import a static property value (method) or all static property values (methods) of a specified class. syntax: import static package name. Class name. static Properties | static method |*; For example: Import the specified class all static properties and methods import staticjava.lang.math.*

Automatic Boxing (autoboxing): assigns a basic data type directly to the corresponding wrapper class variable, or assigns to the Object variable

Auto-unpacking: Assigning a wrapper class object directly to a corresponding base type variable

2. Enhanced for Loop

the reason to introduce an enhanced for loop: In previous versions of JDK5, traversing an array or set of elements, it was cumbersome to get the length of the array or the iterator of the collection . To simplify this type of operation, a new enhanced for loop is added to the JDK5. An enhanced for loop can only be used on an array , or on a collection class that implements the iterator interface

When traversing an array and a collection element using a foreach loop, you do not need to obtain an array and a collection length without accessing the array elements and the collection elements based on the index, and theforeach loop automatically iterates through the array and each element of the collection

L Grammar Format:

• For (type variable name: Collection variable name) {...}

Variable name: Automatic iterative access to each element

Specific case Analysis:

Package com.java.test;

Importjava.util.ArrayList;

Import Java.util.Iterator;

Importjava.util.LinkedHashMap;

Import java.util.List;

Import Java.util.Map;

Import Java.util.Set;

Importjava.util.Map.Entry;

public class Fortest {

public void Test () {

Do not use the generic traversal list:

List List = new ArrayList ();

List.add ("AAA");

List.add ("BBB");

List.add ("CCC");

For (object str:list) {//Because the list is a list object type, it is used to declare variable str with object type

System.out.println (str);

}

To iterate through a list using a generic type:

List<string> List1 = new arraylist<string> ();

List1.add ("AAA");

List1.add ("BBB");

List1.add ("CCC");

for (string str:list1) {///here the generic type of the list is string, so this is the time to iterate over the string declaration

System.out.println (str);

}

}

by Map.keyset () The key value is taken out, traversing

public void Test1 () {

Map map = new Linkedhashmap ();

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

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

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

Set keys = Map.keyset ();

Iterator it = Keys.iterator ();

while (It.hasnext ()) {

String key = (string) it.next ();

String value = (string) map.get (key);

SYSTEM.OUT.PRINTLN (key + "=" + value);

}

}

Through Map.entryset () The key value pair is taken out, a traversal

public void Test2 () {

Map map = new Linkedhashmap ();

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

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

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

Set keys = Map.entryset ();

Iterator it = Keys.iterator ();

while (It.hasnext ()) {

Map.entry me = (Entry) it.next ();

String key = (string) me.getkey ();

String value = (string) me.getvalue ();

SYSTEM.OUT.PRINTLN (key + "=" + value);

}

}

Enhancing the For loop approach

public void Test3 () {

Map map = new Linkedhashmap ();

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

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

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

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

Map.entry me = (Entry) obj;

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

}

}

Using JavaBean traversal

public void Test4 () {

list<person> list = new arraylist<person> ();

List.add (New person ("Jine", 20));

List.add (New person ("Hello", 24);

List.add (New person ("calcium carbide method", 22));

for (person P:list) {

System.out.println (P.getname () + "..." +p.getage ());

}

}

}

Precautions :

An enhanced for loop can only traverse an array or inherit a collection of iterator, cannot assign a value to it, or modify its value

Case One: (Assignment failure case)

intarr[] = new INT[5];

for (Intnum:arr) {

num = 1;

}

System.out.println (Arr[0]); The output is 0, not 1.

Case Two: (Modify value failure case)

List<string>list = new arraylist<string> ();

List.add ("xxx");

for (stringstr:list) {

str = "YYY";

}

System.out.println (list.get (0)); //output result is xxx, not yyy