Java Tutorial/Collections/Collections

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A combination of two collections into a collection

   <source lang="java">

/* DualCollection.java {{IS_NOTE 

 Purpose:
   
 Description:
   
 History:
   Sun Sep  2 21:29:38     2007, Created by tomyeh

}}IS_NOTE Copyright (C) 2007 Potix Corporation. All Rights Reserved. {{IS_RIGHT 

 This program is distributed under GPL Version 3.0 in the hope that
 it will be useful, but WITHOUT ANY WARRANTY.

}}IS_RIGHT

  • /

import java.util.Collection; import java.util.Collections; import java.util.Iterator; import java.util.AbstractCollection; /** 

* A combination of two collections into a collection.
*
* @author tomyeh
* @since 3.0.0
*/

public class DualCollection extends AbstractCollection implements java.io.Serializable { 

 private final Collection _first, _second;
 /** Returns a collection by combining two collections.
  * It checks whether any of them is null, or equals. And, returns
  * the non-null one if another is null.
  * If both null, it returns null.
  */
 public static final
 Collection combine(Collection first, Collection second) {
   if (first == second) //we don"t use equals to have better performance
     return first;
   if (first != null)
     if (second != null)
       return new DualCollection(first, second);
     else
       return first;
   else
     return second;
 }
 /** Constructor.
  * It is better to use {@link #combine} instead of this method
  * since it checks whether any of them is null or equals.
  */
 public DualCollection(Collection first, Collection second) {
   _first = first != null ? first: Collections.EMPTY_LIST;
   _second = second != null ? second: Collections.EMPTY_LIST;
 }
 //Collection//
 public int size() {
   return _first.size() + _second.size();
 }
 public Iterator iterator() {
   return new Iter();
 }
 private class Iter implements Iterator {
   private Iterator _it;
   private boolean _bSecond;
   private Iter() {
     _it = _first.iterator();
   }
   public boolean hasNext() {
     return _it.hasNext() || (!_bSecond && !_second.isEmpty());
   }
   public Object next() {
     if (!_bSecond && !_it.hasNext()) {
       _it = _second.iterator();
       _bSecond = true;
     }
     return _it.next();
   }
   public void remove() {
     _it.remove();
   }
 }

}</source>





Collections.reverseOrder()

   <source lang="java">

import java.util.*;

class AlgorithmsDemo { 

 public static void main(String args[]) {
  
   LinkedList<Integer> ll = new LinkedList<Integer>();
   ll.add(-8);
   ll.add(20);
   ll.add(-20);
   ll.add(8);
  
   Comparator<Integer> r = Collections.reverseOrder();
  
   Collections.sort(ll, r);
  
   for(int i : ll)
     System.out.print(i+ " ");
  
  
   Collections.shuffle(ll);
  
   System.out.print("List shuffled: ");
   for(int i : ll)
     System.out.print(i + " ");
  
   System.out.println("Minimum: " + Collections.min(ll));
   System.out.println("Maximum: " + Collections.max(ll));
 }

}</source>





Copy Elements of ArrayList to Java Vector

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; import java.util.Vector; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("1");
   arrayList.add("2");
   arrayList.add("3");
   arrayList.add("4");
   arrayList.add("5");
   Vector<String> v = new Vector<String>();
   v.add("A");
   v.add("B");
   v.add("D");
   v.add("E");
   v.add("F");
   v.add("G");
   v.add("H");
   System.out.println(v);
   Collections.copy(v, arrayList);
   System.out.println(v);
 }

} /* [A, B, D, E, F, G, H] [1, 2, 3, 4, 5, G, H]

  • /</source>





Copy Elements of One ArrayList to Another ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList1 = new ArrayList<String>();
   arrayList1.add("1");
   arrayList1.add("2");
   arrayList1.add("3");
   ArrayList<String> arrayList2 = new ArrayList<String>();
   arrayList2.add("One");
   arrayList2.add("Two");
   arrayList2.add("Three");
   arrayList2.add("Four");
   arrayList2.add("Five");
   System.out.println(arrayList2);
   Collections.copy(arrayList2, arrayList1);
   System.out.println(arrayList2);
 }

} /* [One, Two, Three, Four, Five] [1, 2, 3, Four, Five]

  • /</source>





Create and demonstrate an immutable collection.

   <source lang="java">

import java.util.ArrayList; import java.util.Collection; import java.util.Collections; import java.util.List; public class Main { 

 public static void main(String args[]) {
   List<Character> list = new ArrayList<Character>();
   list.add("X");
   System.out.println("Element added to list: " + list.get(0));
   Collection<Character> immutableCol = Collections.unmodifiableCollection(list);
   immutableCol.add("Y");
 }

}</source>





Create an empty collection object

   <source lang="java">

import java.util.Collections; import java.util.Date; import java.util.List; import java.util.Map; import java.util.Set; public class Main { 

 public static void main(String args[]) {
   List list = Collections.EMPTY_LIST;
   Set set = Collections.EMPTY_SET;
   Map map = Collections.EMPTY_MAP;
   List<String> s = Collections.emptyList();
   Set<Long> l = Collections.emptySet();
   Map<Date, String> d = Collections.emptyMap();
 }

}</source>





Find maximum element of ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<Integer> arrayList = new ArrayList<Integer>();
   arrayList.add(new Integer("3"));
   arrayList.add(new Integer("1"));
   arrayList.add(new Integer("8"));
   arrayList.add(new Integer("3"));
   arrayList.add(new Integer("5"));
   Object obj = Collections.max(arrayList);
   System.out.println(obj);
 }

} //8</source>





Find Minimum element of ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<Integer> arrayList = new ArrayList<Integer>();
   arrayList.add(new Integer("1"));
   arrayList.add(new Integer("2"));
   arrayList.add(new Integer("3"));
   arrayList.add(new Integer("4"));
   arrayList.add(new Integer("5"));
   Object obj = Collections.min(arrayList);
   System.out.println(obj);
 }

}</source>





Get Enumeration over ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; import java.util.Enumeration; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("A");
   arrayList.add("B");
   arrayList.add("D");
   arrayList.add("E");
   arrayList.add("F");
   Enumeration e = Collections.enumeration(arrayList);
   while (e.hasMoreElements())
     System.out.println(e.nextElement());
 }

}</source>





Get Synchronized List from ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; import java.util.List; public class Main { 

 public static void main(String[] args) {
   ArrayList arrayList = new ArrayList();
   List list = Collections.synchronizedList(arrayList);
 }

}</source>





Perform Binary Search on ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("1");
   arrayList.add("4");
   arrayList.add("2");
   arrayList.add("5");
   arrayList.add("3");
   Collections.sort(arrayList);
   System.out.println("Sorted ArrayList contains : " + arrayList);
   int index = Collections.binarySearch(arrayList, "4");
   System.out.println("Element found at : " + index);
 }

}</source>





Replace All Elements Of ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("A");
   arrayList.add("B");
   arrayList.add("D");
   System.out.println(arrayList);
   Collections.fill(arrayList, "REPLACED");
   System.out.println(arrayList);
 }

} /* [A, B, D] [REPLACED, REPLACED, REPLACED]

  • /</source>





Replace all occurrences of specified element of ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("A");
   arrayList.add("B");
   arrayList.add("A");
   arrayList.add("C");
   arrayList.add("D");
   System.out.println(arrayList);
   Collections.replaceAll(arrayList, "A", "Replace All");
   System.out.println(arrayList);
 }

} /* [A, B, A, C, D] [Replace All, B, Replace All, C, D]

  • /</source>





Reverse order of all elements of ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("A");
   arrayList.add("B");
   arrayList.add("C");
   arrayList.add("D");
   arrayList.add("E");
   System.out.println(arrayList);
   Collections.reverse(arrayList);
   System.out.println(arrayList);
 }

}</source>





Rotate elements of a collection

   <source lang="java">

import java.util.ArrayList; import java.util.Arrays; import java.util.Collections; import java.util.List; public class Main { 

 public static void main(String[] args) {
   List numbers = new ArrayList();
   for (int i = 0; i < 25; i++) {
     numbers.add(i);
   }
   System.out.println(Arrays.toString(numbers.toArray()));
   Collections.rotate(numbers, 10);
   System.out.println(Arrays.toString(numbers.toArray()));
 }

}</source>





Search collection element

   <source lang="java">

import java.text.DateFormatSymbols; import java.util.Collections; import java.util.LinkedList; import java.util.List; public class Main { 

 public static void main(String[] args) {
   List list = new LinkedList();
   DateFormatSymbols dfs = new DateFormatSymbols();
   String[] months = dfs.getMonths();
   for (int i = 0; i < months.length; i++) {
     String month = months[i];
     list.add(month);
   }
   Collections.sort(list);
   System.out.println("Month Names = " + list);
   int index = Collections.binarySearch(list, "October");
   if (index > 0) {
     System.out.println("Found at index = " + index);
     String month = (String) list.get(index);
     System.out.println("Month = " + month);
   }
 }

}</source>





Shuffle elements of ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("A");
   arrayList.add("B");
   arrayList.add("C");
   arrayList.add("D");
   arrayList.add("E");
   System.out.println(arrayList);
   Collections.shuffle(arrayList);
   System.out.println(arrayList);
 }

} /* [A, B, C, D, E] [E, D, A, C, B]

  • /</source>





Sort ArrayList in descending order using comparator

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; import java.util.ruparator; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("A");
   arrayList.add("B");
   arrayList.add("C");
   arrayList.add("D");
   arrayList.add("E");
   
   Comparator comparator = Collections.reverseOrder();
   System.out.println(arrayList);
   Collections.sort(arrayList, comparator);
   System.out.println(arrayList);
 }

}</source>





Sort elements of ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("1");
   arrayList.add("3");
   arrayList.add("5");
   arrayList.add("2");
   arrayList.add("4");
   Collections.sort(arrayList);
   for (String str: arrayList)
     System.out.println(str);
 }

} /* 1 2 3 4 5

  • /</source>





Sort items of an ArrayList with Collections.reverseOrder()

   <source lang="java">

import java.util.ArrayList; import java.util.Arrays; import java.util.Collections; import java.util.List; public class Main { 

 public static void main(String[] args) {
   List<String> colours = new ArrayList<String>();
   colours.add("red");
   colours.add("green");
   colours.add("blue");
   colours.add("yellow");
   colours.add("cyan");
   colours.add("white");
   colours.add("black");
   Collections.sort(colours);
   System.out.println(Arrays.toString(colours.toArray()));
   Collections.sort(colours, Collections.reverseOrder());
   System.out.println(Arrays.toString(colours.toArray()));
 }

}</source>





Swap elements of ArrayList

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; public class Main { 

 public static void main(String[] args) {
   ArrayList<String> arrayList = new ArrayList<String>();
   arrayList.add("A");
   arrayList.add("B");
   arrayList.add("C");
   arrayList.add("D");
   arrayList.add("E");
   System.out.println(arrayList);
   Collections.swap(arrayList, 0, 4);
   System.out.println(arrayList);
 }

} /* [A, B, C, D, E] [E, B, C, D, A]

  • /</source>





The Collections.fill() method

   <source lang="java">

import java.util.ArrayList; import java.util.Collections; import java.util.List; public class MainClass { 

 public static void main(String[] args) {
   List list = new ArrayList();
   for (int i = 0; i < 10; i++)
     list.add("");
   Collections.fill(list, "Hello");
   System.out.println(list);
 }

} //</source> 



[Hello, Hello, Hello, Hello, Hello, Hello, Hello, Hello, Hello, Hello]


Using the Collections.synchronized methods

   <source lang="java">

import java.util.ArrayList; import java.util.Collection; import java.util.Collections; import java.util.HashMap; import java.util.HashSet; import java.util.List; import java.util.Map; import java.util.Set; public class MainClass { 

 public static void main(String[] args) {
   Collection c = Collections.synchronizedCollection(new ArrayList());
   List list = Collections.synchronizedList(new ArrayList());
   Set s = Collections.synchronizedSet(new HashSet());
   Map m = Collections.synchronizedMap(new HashMap());
 }

}</source>


Источник — «http://www.jexp.ru/index.php?title=Java_Tutorial/Collections/Collections&oldid=4728»