Collection Framework in Java

In above program,

//Instead of
  HashMap hm=new HashMap ();
  //Use below
  // Creation of LinkedHashMap 
  LinkedHashMap hm = new LinkedHashMap()

    Output:
     null 
     C++
     ====== By using Iterator ====== 
     Key :101
     Value :java 
     Key :102
     Value :.Net 
     Key :103
     Value :C
     ====== By using Map.Entry ====== 
     101: java
     102: .Net
     103: C
  

Example:

package com.javabykiran.Collection; 
import java.util.Iterator;
import java.util.Set; 
import java.util.TreeMap;
/*
* @author Java By Kiran
*/
public class TreeMapTest {
   public static void main(String[] args) {
    // Creation of TreeMap 
    TreeMap tm = new TreeMap();
    // adding elements with key 
    tm.put("103", "java");
    tm.put("102", ".Net");
    Object o = tm.put("101", "C++");
      // below will print null 
      System.out.println (o);
      Object o1 = tm.put("101", "C"); 
      System.out.println (o1);
      // retrieving elements from TreeMap 
      Set s = tm.keySet();
     // set s contains all keys 
     Iterator itr = s.iterator(); 
      while (itr.hasNext()) {
          String key = (String) itr.next();     
          System.out.println("Key :" + key); 
          System.out.println("Value :" + tm.get(key));
    }
    // Try putting null value
    // will not throw compile time error
    // but gives runtime error uncomment below
    // tm.put(null, null);
  }	
}

    Output:
     null
     C++
     Key :101
     Value :C 
     Key   :102
     Value :.Net
     Key   :103
     Value :java
  

Example:

package com.javabykiran.Collection; 
import java.util.Hashtable;
/*
* @author Java By Kiran
*/
public class HashTableTest {
    public static void main(String[] args){ Hashtable ht = new Hashtable(); ht.put("ind", "India");
      ht.put("bhu", "Bhutan");
      ht.put("ind", "India");
    //the below will print size of ht by
    //ignoring the duplicate key
 System.out.println("size of hashTable> "+ht.size());	// 2
    //OK - compile time
    //NOT OK AT RUNTIME
    //ht.put(null, "India");
    //OK - compile time
    //NOT OK AT RUNTIME
    //ht.put(null, null);
    //OK - compile time
    //NOT OK AT RUNTIME
    //ht.put("ind", null);
   System.out.println(ht.size()); // 2
  }
}

    Output:
     size of hashTable > 2
     Exception in thread "main" java.lang.NullPointerException 
     at java.util.Hashtable.put(Unknown Source)
     at com.javabykiran.Collection.HashTableTest.main(HashTableTest.java:23)
  

Example:

package com.javabykiran.Collection; 
 import java.io.FileOutputStream; 
 import java.util.Iterator;
 import java.util.Properties; 
 import java.util.Set;
 /*
 * @author Java By Kiran
 */
public class PropertiesTest {
    public static void main(String[] args) { 
        Properties properties = new Properties();
        // Same way we can put elements 
        properties.put("101", "java");
        properties.put("102", "C");
        properties.put("103", "C++");
        properties.put("104", ".Net");
        // to retrieve the elements. 
        Set s = properties.keySet(); 
        Iterator itr = s.iterator();
     while (itr.hasNext()) {     
        System.out.println(properties.get((String)itr.next()));
      }
      // to store in xml file
     try {
        FileOutputStream fout = new FileOutputStream("D:\\prop.xml"); 
        properties.storeToXML(four,"key-value pair");
     }
     catch (Exception e) {    
        e.printStackTrace();
    }
  }
}

    Output:
     .Net 
     C++ 
     C
     java
  

An XML file is created as shown below in the D drive that we have just printed in above Program
Example:

<!DOCTYPE properties SYSTEM "http://java.sun.com/dtd/properties.dtd">
<properties>
    <comment>key-value pair</comment>
    <entry key="104">.Net</entry>
    <entry key="103">C++</entry>
    <entry key="102">C</entry>
    <entry key="101">java</entry>
</properties>

Example:

package com.javabykiran.Collection; 
import java.util.ArrayList;
import java.util.Collections;
/*
* @author Java By Kiran
*/
public class CollectionsTest {
  public static void main(String[] args) {
    ArrayList al = new ArrayList(); 
    al.add("Java");
    al.add("By");
    al.add("Kiran");
  System.out.println("before sorting " + al); 
  Collections.sort(al); 
  System.out.println("after sorting" + al);
  //To make arraylist object synchronised 
    Collections.synchronizedList(al);
 }
}

    Output:
     before sorting [Java, By, Kiran] 
     after sorting [By, Java, Kiran]
  

Example:
Consider the following program of sorting students on the basis of age:

package com.javabykiran.Collection;
/*
* @author Java By Kiran
*/
class Student implements Comparable{ 
    String name;
    int age;
  public int compareTo(Object obj){ 
    Student s = (Student)obj; 
    if(age==s.age)
      return 0;
    else if(age>s.age) 
      return -1;
    return age;
    }
}

package com.javabykiran.Collection; 
import java.util.*;
/*
* @author Java By Kiran
*/
public class CollectionLab9 {
  public static void main(String args[]){ 
     ArrayList al = new ArrayList (); 
     Student s1 = new Student ();
       s1.name = "ABC"; 
       s1.age = 24;
     Student s2 = new Student (); 
       s2.name = "XYZ";
       s2.age = 21;
     Student s3 = new Student (); 
       s3.name = "PQR"; 
       s3.age = 19;
      al.add(s1);
      al.add(s2);
      al.add(s3); 
    Collections.sort(al); 
      Iterator itr=al.iterator();          
      while(itr.hasNext()){
        Student st= (Student) itr.next ();   
        System.out.println(st.name+"--"+st.age);
      }
   }
}

    Output:
     ABC--24
     XYZ--21
     PQR--19

  

package com.javabykiran.Collection;
/*
* @author Java By Kiran
*/
class Student { 
    String name; 
    int age;
}

package com.javabykiran.Collection;
/*
* @author Java By Kiran
*/
import java.util.*;
public class AgeComparator implements Comparator{ 
   public int compare(Object o1, Object o2) {
     Student s1=(Student)o1; 
     Student s2=(Student)o2;
     if(s1.age==s2.age) 
       return 0;
     else if(s1.age>s2.age) 
       return 1;
     else return -1;
   }
}

package com.javabykiran.Collection;
/*
* @author Java By Kiran
*/ import java.util.Comparator;
public class NameComparator implements Comparator {  
public int compare(Object o1, Object o2) {
     Student s1=(Student)o1; 
     Student s2=(Student)o2;
   return s1.name.compareTo(s2.name);
  }
}

package com.javabykiran.Collection;
/*
* @author Java By Kiran
*/
import java.util.*;
public class CollectionLab10 {
   public static void main(String args[]){ 
   ArrayList al = new ArrayList(); 
     Student s1 = new Student();
       s1.name = "ABC"; 
       s1.age = 24;
     Student s2 = new Student(); 
       s2.name = "XYZ";
       s2.age = 21;
     Student s3 = new Student(); 
       s3.name = "PQR";
       s3.age = 19; 
     al.add(s1);
     al.add(s2);
     al.add(s3); 
   System.out.println("Sorting by Name...");
   Collections.sort(al,new NameComparator());
        Iterator itr=al.iterator(); 
        while(itr.hasNext()){
            Student st=(Student)itr.next(); 
            System.out.println(st.name+" "+st.age);
        }
   System.out.println("sorting by age..."); 
    Collections.sort (al,new AgeComparator()); 
    Iterator itr2=al.iterator();
     while (itr2.hasNext()){
        Student st= (Student)itr2.next();
        System.out.print(st.name); 
        System.out.println(" "+st.age);
      }
   }
}

    Output:
     Sorting by Name... 
     ABC 24
     PQR 19
     XYZ 21
     Sorting by age... 
     PQR 19
     XYZ 21
     ABC 24