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In the first of three lessons on the different collection types within the Collection hierarchy we look at Sets which are probably the easiest collection type to understand, and as such, a good starting point. So what type of collection is a Set? A Set doesn't allow duplicates so every entry within a Set must be unique and as the name implies the interface models the mathematical set abstraction. In practical terms this means that we can never have more than one element within the Set referencing the same object or more than one element within the Set referencing two objects that are considered equal. See the Checking Object Equality section from the OO Concepts - The Object Superclass section of the site for a reminder of what constitutes object equality. We can also only have a maximum of one entry within the set with a value of null.

Set Hierarchy Diagram Top

The diagram below is a representation of the Set hierarchy and covers the interfaces and classes we will study in this lesson. The diagram has several interfaces missing and also the java.util.EnumSet<E> concrete implemetation which is not covered on the site, but should help in visualisation:

Set Interfaces & Classes Top

The table below gives a description of each interface and class within the above diagram. Click a link to go to detailed information about a particular class:

Set Types and Ordering Top

The table below extrapolates the commented information about ordering from the diagram above into a more readable tabular format:

HashSets Top

A HashSet is an unordered and unsorted implementation of the Set<E> interface, backed by a hash table using a HashMap instance. This type of set makes no guarantees over iteration order and in particular no guarantees that the iteration order will remain constant over time.

The hashcode of the object being inserted into the collection is used so the more efficient your override of the hashCode() method of the Object class is, the more efficient the HashSet. This also means that if you want to find out if an object exists within the set, or you want to delete an object from the set, then you need a reference to the object in question.

This type of set is a good choice when you want a collection with no duplicates and fast access and the order doesn't matter when iterating over it.

HashSet<E> Method Overview Top

The table below shows the declarations of all the methods for the HashSet<E> class implemented from the Set<E> interface:

Usage for the the first six methods is shown in the example below. For more information on the other methods in the HashSet<E> class the following link will take you to the online version of documentation for the JavaTM 2 Platform Standard Edition 5.0 API Specification. Take a look at the documentation for the HashSet<E> class which you can find by scrolling down the lower left pane and clicking on HashSet.

HashSet<E> Example Top

Lets write a simple class that creates a HashSet and uses some of the methods from the HashSet<E> class:

package info.java8;
/*
  Simple class to create a HashSet and use several methods from the class 
*/
import java.util.*; // Import the java.util package

public class HashSetClass {

    public static void main (String[] args) {
        Set<String> hs = new HashSet<String>();
        String s = "hello";
        hs.add(s);
        hs.add("goodbye");
        hs.add(s+s);
        System.out.println("hash set size = " + hs.size());
        // Use enhanced for loop to iterate over the collection
        for (String str : hs) { 
            System.out.println(str);  // Print element
        }        
        hs.remove(s);  
        System.out.println(hs.contains(s));  
        System.out.println("hash set size = " + hs.size());
        // Use enhanced for loop to iterate over the collection
        for (String str : hs) { 
            System.out.println(str);  // Print element
        }        
        // Check for empty set, clear set and then check again
        System.out.println(hs.isEmpty());
        hs.clear();  
        System.out.println(hs.isEmpty());
    }
}

Save, compile and run the HashSetClass class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our HashSetClass class and using several of the methods contained within the HashSet<E> class.

LinkedHashSets Top

A LinkedHashSet is a Hash table and linked list implementation of the Set interface with predictable iteration order and differs from a HashSet by maintaintaining a doubly-linked list running through all of its elements. The linked list defines the iteration order of a LinkedHashSet by the order in which elements are inserted into the set.

This type of set is a good choice when you want a collection with no duplicates and fast access and you want to maintain an insertion order when iterating over it.

LinkedHashSet<E> Method Overview Top

The table below shows the declarations of all the methods for the LinkedHashSet<E> class that are inherited directly from the HashSet<E> class:

Usage for the the first six methods is shown in the example below.

LinkedHashSet<E> Example Top

Lets write a simple class that creates a HashSet and uses some of the methods from the HashSet<E> class:

package info.java8;
/*
  Simple class to create a LinkedHashSet and use several methods inherited from the HashSet class 
*/
import java.util.*; // Import the java.util package

public class LinkedHashSetClass {

    public static void main (String[] args) {
        Set<String> lhs = new LinkedHashSet<String>();
        String s = "one";
        lhs.add(s);
        lhs.add("two");
        lhs.add(s+s);
        System.out.println("linked hash set size = " + lhs.size());
        // Use enhanced for loop to iterate over the collection
        for (String str : lhs) { 
            System.out.println(str);  // Print element
        }        
        lhs.remove(s);  
        System.out.println(lhs.contains(s));  
        System.out.println("linked hash set size = " + lhs.size());
        // Use enhanced for loop to iterate over the collection
        for (String str : lhs) { 
            System.out.println(str);  // Print element
        }        
        // Check for empty set, clear set and then check again
        System.out.println(lhs.isEmpty());
        lhs.clear();  
        System.out.println(lhs.isEmpty());
    }
}

Save, compile and run the LinkedHashSetClass class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our LinkedHashSetClass class and using several of the methods contained within the HashSet<E> class.

TreeSets Top

A TreeSet is a sorted implementation of the Set<E> interface, backed by a TreeMap instance. This type of set guarantees that the set will be sorted in ascending order, according to the natural order of the elements contained within it, or by a custom comparator/comparable, dependant upon the type of constructor used on creation.

The ordering maintained by a set whether through natural order of the elements contained within it or by a custom comparator/comparable, must be consistent with the equals() method override rules of the Object class if it is to correctly implement the Set<E> interface and obey the rules of equality.

This type of set is a good choice when you want a sorted collection with no duplicates with the option of custom sorting.

TreeSet<E> Method Overview Top

The table below shows the declarations of all the methods for the TreeSet<E> class implemented from the Set<E> and SortedSet<E> interfaces:

We will go use the first five methods in in the example below. For more information on the other methods in the TreeSet<E> class the following link will take you to the online version of documentation for the JavaTM 2 Platform Standard Edition 5.0 API Specification. Take a look at the documentation for the TreeSet<E> class which you can find by scrolling down the lower left pane and clicking on TreeSet.

TreeSet<E> Example Top

Lets write a simple class that creates a TreeSet and uses some of the methods from the TreeSet<E> class:

package info.java8;
/*
  Simple class to create a TreeSet and use several methods fron from the class 
*/
import java.util.*; // Import the java.util package

public class TreeSetClass {

    public static void main (String[] args) {
        Set<Object> ts = new TreeSet<Object>();
        String s = "one";
        ts.add(s);
        ts.add(123);  // Autoboxed to Integer
        ts.add("two");
        ts.add(s+s);
        System.out.println("tree set size = " + ts.size());
        // Use enhanced for loop to iterate over the collection
        for (Object o : ts) { 
            System.out.println(o);  // Print element
        }
        ts.remove(s);
        // Use enhanced for loop to iterate over the collection
        for (Object o : ts) { 
            System.out.println(o);  // Print element
        }        
        // Check for empty set, clear set and then check again
        System.out.println(ts.isEmpty());
        ts.clear();  
        System.out.println(ts.isEmpty());
    }
}

Save, compile and run the TreeSetClass class in directory   c:\_Collections in the usual way.

The above screenshot shows the results of creating and running our TreeSetClass class. We get a ClassCastException and the reason for this is that if a collection is to be sorted, then the objects within it need to be mutually comparable and the String and Integer objects are not. We will talk more about sorting collection, comparables, comparators and natural order in much more detail in the Sorting Collections lesson. For now remove the add() to the TreeSet for the Integer object and save, compile and run the TreeSetClass class again.

The above screenshot shows the results of creating and running our TreeSetClass class and it now works.

Lesson 3 Complete

In this lesson we looked at Sets.

What's Next?

In the second of three lessons on the different collection types within the Collection hierarchy we look at three of the concrete implementations of the List<E> interface.

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