Java-Collection-Essays

One, the experience of the collection (Collection, list, set, map)

1, the way to traverse the list:

First method: For-each method

public class listtest{

public static void Main (string[] args) {

list<string> list = new arraylist<string> ();
Long t1,t2;
for (int j = 0; J < 10000000; J + +)
{
List.add ("aaaaaa" + j);
}

for (String tmp:list)
{
SYSTEM.OUT.PRINTLN (TMP);
}

}

The second method: The method of for-variable
public class listtest{

public static void Main (string[] args) {

list<string> list = new arraylist<string> ();
Long t1,t2;
for (int j = 0; J < 10000000; J + +)
{
List.add ("aaaaaa" + j);
}

for (int i = 0; i < list.size (); i++)
{
List.get (i);
System.out.println (List.get (i));
}

}

T2=system.currenttimemillis ();
}

The third method: Iterative Traversal method
public class listtest{

public static void Main (string[] args) {

list<string> List = new arraylist<string> ();
long t1,t2; 　　　　　　
for (int j = 0; J < 10000000; J + +)

List.add ("aaaaaa" + j);

Iterator<string> iter = List.iterator ();
T1=system.currenttimemillis ();
while (Iter.hasnext ())
{
Iter.next ();
System.out.println (Iter.next ());
}
T2=system.currenttimemillis ();

for (Iterator i = List.iterator (); I.hasnext ();) {
System.out.println (I.next ());
//}

}

}
2, the method of traversing set:

1. Iterative traversal:

set<string> set = new hashset<string> ();
Iterator<string> it = Set.iterator ();
while (It.hasnext ()) {
String str = It.next ();
System.out.println (str);
}

2.for Loop Traversal:
for (String Str:set) {
System.out.println (str);
}

　　　　The advantages are also reflected in generics if the set is stored in the Object

set<object> set = new hashset<object> ();
For loop traversal:
for (Object Obj:set) {
if (obj instanceof Integer) {
int aa= (Integer) obj;
}else if (obj instanceof String) {
String AA = (string) obj
}
}

Traversal of 3.Map　　　　

A method of looping through a map
public class Circlemap {
public static void Main (string[] args) {
map<string, integer> tempmap = new hashmap<string, integer> ();
Tempmap.put ("A", 1);
Tempmap.put ("B", 2);
Tempmap.put ("C", 3);
Traversal method one HashMap entryset () traversal
System.out.println ("Method One");
Iterator it = Tempmap.entryset (). Iterator ();
while (It.hasnext ()) {
Map.entry Entry = (map.entry) it.next ();
Object key = Entry.getkey ();
Object value = Entry.getvalue ();
System.out.println ("key=" + key + "value=" + value);
}
System.out.println ("");
JDK1.5, apply new features For-each loop
Traversal method Two
System.out.println ("Method II");
For (map.entry<string, integer> entry:tempMap.entrySet ()) {
String key = Entry.getkey (). toString ();
String value = Entry.getvalue (). toString ();
System.out.println ("key=" + key + "value=" + value);
}
System.out.println ("");

Traversal method three HashMap KeySet () traversal
System.out.println ("Method three");
for (Iterator i = Tempmap.keyset (). Iterator (); I.hasnext ();) {
Object obj = I.next ();
System.out.println (obj);//Loop Output key
System.out.println ("key=" + obj + "value=" + tempmap.get (obj));
}
for (Iterator i = tempmap.values (). Iterator (); I.hasnext ();) {
Object obj = I.next ();
System.out.println (obj);//Loop output value
}
System.out.println ("");

Traversal method four TreeMap KeySet () traversal
System.out.println ("Method four");
For (Object O:tempmap.keyset ()) {
System.out.println ("key=" + O + "value=" + tempmap.get (o));
}
System.out.println ("11111");

How Java traverses map <string, arraylist> map = new HashMap <string,
Arraylist> ();
SYSTEM.OUT.PRINTLN ("Java traversal map <string, arraylist> map = new HashMap <string, arraylist> ();");
map<string, arraylist> map = new hashmap<string, arraylist> ();
set<string> keys = Map.keyset ();
iterator<string> Iterator = Keys.iterator ();
while (Iterator.hasnext ()) {
String key = Iterator.next ();
ArrayList ArrayList = Map.get (key);
for (Object o:arraylist) {
SYSTEM.OUT.PRINTLN (o + "traversal process");
}
}
System.out.println ("2222");
map<string, list> maplist = new hashmap<string, list> ();
For (Map.entry Entry:mapList.entrySet ()) {
String key = Entry.getkey (). toString ();
list<string> values = (List) entry.getvalue ();
for (String value:values) {
SYSTEM.OUT.PRINTLN (key +--+ value);
}
}
}
}

Q:comparable and Comparator differences
A: When you call the Java.util.Collections.sort list method to sort, the object in the list must implement the comparable interface.
Java.util.Collections.sort (List list,comparator c), you can temporarily declare a Comparator to implement sorting.
Collections.sort (ImageList, New Comparator () {
public int Compare (object A, object B) {
int ordera = Integer.parseint ((Image) a). Getsequence ());
int orderb = Integer.parseint ((Image) b). Getsequence ());
return ordera-orderb;
}
});
If you need to change the sorting order
Change to return Orderb-ordera.

　　

Java-Collection-Essays