Java Example - Collection Traversal

The following example demonstrates how to traverse collections derived from the Collection interface, such as List, Set, and Map (which stores data in key-value pairs). We use regular for loops, enhanced for loops, and iterators to traverse these collections:

Traversal of List and Set Collections

Main.java File

import java.util.ArrayList;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;

public class Main {

   public static void main(String[] args) {
      // Traversal of List collection
      listTest();
      // Traversal of Set collection
      setTest();
   }

   private static void setTest() {
      Set<String> set = new HashSet<String>();
      set.add("JAVA");
      set.add("C");
      set.add("C++");
      // Duplicate data addition fails
      set.add("JAVA");
      set.add("JAVASCRIPT");

      // Traverse set collection using iterator
      Iterator<String> it = set.iterator();
      while (it.hasNext()) {
         String value = it.next();
         System.out.println(value);
      }

      // Traverse set collection using enhanced for loop
      for(String s: set){
         System.out.println(s);
      }
   }

   // Traverse list collection
   private static void listTest() {
      List<String> list = new ArrayList<String>();
      list.add("菜");
      list.add("鸟");
      list.add("教");
      list.add("程");
      list.add("www.tutorialpro.org");

      // Traverse using iterator
      Iterator<String> it = list.iterator();
      while (it.hasNext()) {
         String value = it.next();
         System.out.println(value);
      }

      // Traverse using traditional for loop
      for (int i = 0, size = list.size(); i < size; i++) {
         String value = list.get(i);
         System.out.println(value);
      }

      // Traverse using enhanced for loop
      for (String value : list) {
         System.out.println(value);
      }
   }
}

The above code outputs the following results:

菜
鸟
教
程
www.tutorialpro.org
菜
鸟
教
程
www.tutorialpro.org
菜
鸟
教
程
www.tutorialpro.org
JAVA
JAVASCRIPT
C++
C
JAVA
JAVASCRIPT
C++
C

Traversal of Map Collections

The following example uses the keySet() and entrySet() methods of HashMap to traverse the collection:

/*
 author by tutorialpro.org
 Main.java
 */

import java.util.Map;
import java.util.HashMap;
import java.util.HashSet;
import java.util.Iterator;
import java.util.List;
import java.util.Set;
import java.util.Map.Entry;

// Enhanced For Loop
public class Main {

public static void main(String[] args) {
      // Create a HashMap object and add some key-value pairs.
      Map&lt;String, String> maps = new HashMap&lt;String, String>();
      maps.put("1", "PHP");
      maps.put("2", "Java");
      maps.put("3", "C");
      maps.put("4", "C++");
      maps.put("5", "HTML");

      // Traditional method to traverse the map collection using keySet()
      //traditionalMethod1(maps);
      // Traditional method to traverse the map collection using entrySet()
      //traditionalMethod2(maps);
      // Enhanced For loop to traverse the map collection using keySet()
      //strongForMethod1(maps);
      // Enhanced For loop to traverse the map collection using entrySet()
      strongForMethod2(maps);
   }

   private static void strongForMethod2(Map&lt;String, String> maps) {
      Set&lt;Entry&lt;String, String>> set = maps.entrySet();
      for (Entry&lt;String, String> entry : set) {
         String key = entry.getKey();
         String value = entry.getValue();
         System.out.println(key + " : " + value);
      }
   }

   private static void strongForMethod1(Map&lt;String, String> maps) {
      Set<String> set = maps.keySet();
      for (String s : set) {
         String key = s;
         String value = maps.get(s);
         System.out.println(key + " : " + value);
      }
   }

   // Using entrySet() method to get each key-value pair in the maps collection
   private static void traditionalMethod2(Map&lt;String, String> maps) {
      Set&lt;Map.Entry&lt;String, String>> sets = maps.entrySet();
      // Get the iterator and traverse to get the corresponding values.
      Iterator&lt;Entry&lt;String, String>> it = sets.iterator();
      while (it.hasNext()) {
         Map.Entry&lt;String, String> entry = (Entry&lt;String, String>) it.next();
         String key = entry.getKey();
         String value = entry.getValue();
         System.out.println(key + " : " + value);
      }
   }

   // Using keySet() method to get all keys in the maps collection and traverse to get the corresponding values.
   private static void traditionalMethod1(Map&lt;String, String> maps) {
      Set<String> sets = maps.keySet();
      // Get the iterator and traverse to get the corresponding values
      Iterator<String> it = sets.iterator();
      while (it.hasNext()) {
         String key = it.next();
         String value = maps.get(key);
         System.out.println(key + " : " + value);
      }
   }
}

The output of the above code is:

1 : PHP
2 : Java
3 : C
4 : C++
5 : HTML

1 : PHP 2 : Java 3 : C 4 : C++ 5 : HTML

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