Chapter 6 ■ Generics and Collections

Now, how to get the characters from a String? Remember that array indexing doesn’t work for Strings. For example, to get the first character "t", if you use pangram[0] in the program, you’ll get a compiler error. Fortunately, String has a method called toCharArray() that returns a char[]. So, you use this method for traversing over the string and get all the characters. As you add the characters into the TreeSet, the characters are stored in a sorted order. So, you get all the lowercase letters when you print the set.

Note in the output that there is one leading comma. Why? The pangram string has many whitespace characters. One whitespace also gets stored in the set, so it also gets printed!

The Map Interface

A Map stores key and value pairs. The Map interface does not extend the Collection interface. However, there are methods in the Map interface that you can use to get classes in the Collection to work around this problem. Also, the method names in Map are very similar to the methods in Collection, so it is easy to understand and use Map. There are two important concrete classes of Map that we’ll cover: HashMap and TreeMap.

A HashMap uses a hash table data structure internally. In HashMap, searching (or looking up elements) is a fast operation. However, HashMap neither remembers the order in which you inserted elements nor keeps elements in any sorted order.

A TreeMap uses a red-black tree data structure internally. Unlike HashMap, TreeMap keeps the elements in sorted order (i.e., sorted by its keys). So, searching or inserting is somewhat slower than the HashMap.

The HashMap Class

Assume that you are implementing a simple spell checker. Given an input string, the spell checker looks for words that are usually misspelled; if there is a match, it prints the correct spelling. So, the spell checker should maintain a list of frequently misspelled words and their correct spellings. How can you implement this?

Given a key, you can look out for a value using a Map. Now, which map to use, HashMap or a TreeMap? There is no need (though you are able) to keep the misspelled words in sorted order, and the lookup for misspelled words should be very fast. So, HashMap is suitable for solving this problem.

Listing 6-18 is a simple program showing how to implement a spell checker.

Listing 6-18.  SpellChecker.java

// This program shows the usage of HashMap class public class SpellChecker {

public static void main(String []args) {

Map<String, String> misspeltWords = new HashMap<String, String>(); misspeltWords.put("calender", "calendar"); misspeltWords.put("tomatos", "tomatoes"); misspeltWords.put("existance", "existence"); misspeltWords.put("aquaintance", "acquaintance");

String sentence = "Buy a calender for the year 2013"; System.out.println("The given sentence is: " + sentence); for(String word : sentence.split("\\W+")) {

if(misspeltWords.containsKey(word)) { System.out.println("The correct spelling for " + word

+ " is: " + misspeltWords.get(word));

}

}

}

}

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It prints the following:

The given sentence is: Buy a calender for the year 2013 The correct spelling for calender is: calendar

First, you need to create a table of misspelled words and their correct spellings. Since both key and value are Strings, you create a HashMap<String, String> object. You insert four misspelled words and their correct spellings in the HashMap. The misspelled word is the key and the correct spelling is the value. You use the put() method (instead of the add() method you use in Container) for inserting a pair (a key and its value) into the Map.

You use the simple approach of separating the words in a sentence—you use String’s split() method. For each word, you check if the word is an exact match for the misspelled word; if so, you print the value matching that key. You use the containsKey() method for checking if the key exists in the map; it returns a Boolean value. You use the get() method to return the value from the map given the key as argument. Since the given sentence has one word misspelled (“calender”), you print the correct spelling for that word.

Now, let’s look at the keys in the misspeltWords HashMap. You can get all the keys in the HashMap using the keySet() method. Since you have HashMap<String, String>, the returned set is of type Set<String>.

Set<String> keys = misspeltWords.keySet(); System.out.print("Misspelt words in spellchecker are: "); System.out.println(keys);

It prints the following:

Mispelled words in spellcheker are: [calender, existance, aquaintance, tomatos]

Similarly, you can use valueSet() method to get the values available in the map.

Overriding the hashCode() Method

Overriding the equals and hashCode methods correctly is important for using the classes with containers (particularly, HashMap and HashSet). Listing 6-19 is a simple Circle class example so you can understand what can go wrong.

Listing 6-19.  TestCircle.java

// This program shows the importance of equals() and hashCode() methods

import java.util.*;

class Circle {

private int xPos, yPos, radius; public Circle(int x, int y, int r) {

xPos = x; yPos = y; radius = r;

}

public boolean equals(Object arg) { if(arg == null) return false; if(this == arg) return true; if(arg instanceof Circle) {

Circle that = (Circle) arg;

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if( (this.xPos == that.xPos) && (this.yPos == that.yPos) && (this.radius == that.radius )) {

return true;

}

}

return false;

}

}

class TestCircle {

public static void main(String []args) {

Set<Circle> circleList = new HashSet<Circle>(); circleList.add(new Circle(10, 20, 5)); System.out.println(circleList.contains(new Circle(10, 20, 5)));

}

}

It prints false (not true)! Why? The Circle class overrides the equals() method, but it doesn’t override the hashCode() method. When you use objects of Circle in standard containers, it becomes a problem. For fast lookup, the containers compare hashcode of the objects. If the hashCode() method is not overridden, then—even if an object with same contents is passed—the container will not find that object! So you need to override the hashCode() method.

Okay, how do you override the hashCode() method? In the ideal case, the hashCode() method should return unique hash codes for different objects.

The hashCode() method should return the same hash value if the equals() method returns true. What if the objects are different (so that the equals() method returns false)? It is better (although not required) for the

hashCode() to return different values if the objects are different. The reason is that it is difficult to write a hashCode() method that gives unique value for every different object.

 The methods hashCode() and equals() need to be consistent for a class. For practical purposes, ensure that you follow this one rule: the hashCode() method should return the same hash value for two objects if the equals() method returns true for them.

When implementing the hashCode() method, you can use the values of the instance members of the class to create a hash value. Here is a simple implementation of the hashCode() method of the Circle class:

public int hashCode() {

//use bit-manipuation operators such as ^ to generate close to unique hash codes

//here we are using the magic numbers 7, 11 and 53, but you can use any numbers,

Now if you run the main() method, it prints “true”. In this implementation of the hashCode() method, you multiply the values by a prime number as well as bit-wise operation. You can write complex code for hashCode() if you want a better hashing function, but this implementation is sufficient for practical purposes.

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You can use bitwise operators for int values. What about other types, like floating-point values or reference types? To give you an example, here is hashCode() implementation of java.awt.Point2D, which has floating point values x and y. The methods getX() and getY() return the x and y values respectively:

public int hashCode() {

long bits = java.lang.Double.doubleToLongBits(getX()); bits ^= java.lang.Double.doubleToLongBits(getY()) * 31; return (((int) bits) ^ ((int) (bits >> 32)));

}

This method uses the doubleToLongBits() method, which takes a double value and returns a long value. For floating-point values x and y (returned by the getX and getY methods), you get long values in bits and you use bit-manipulation to get hashCode().

Now, how do you implement the hashCode method if the class has reference type members? For example, consider using an instance of Point class as a member instead of xPos and yPos, which are primitive type fields:

class Circle {

private int radius; private Point center; // other members...

}

In this case, you can use call the hashCode() method of Point to implement Circle’s hashCode method:

public int hashCode() {

return center.hashCode() ^ radius;

}

 If you’re using an object in containers like HashSet or HashMap, make sure you override the hashCode()­ and equals() methods correctly. If you don’t, you’ll get nasty surprises (bugs) while using these ­containers!

The NavigableMap Interface

The NavigableMap interface extends the SortedMap interface. In the Collection hierarchy, the TreeMap class is the widely used class that implements NavigableMap. As the name indicates, with NavigableMap, you can navigate the Map easily. It has many methods that make Map navigation easy. You can get the nearest value matching the given key, all values less than the given key, all values greater than the given key, etc. Let’s look at an example: Lennon, McCartney, Harrison, and Starr have taken an online exam. In that exam, the maximum they can score is 100, with a passing score of 40. If you want to find details such as who passed the exam, and sort the exam scores in ascending or descending order, NavigableMap is very convenient (see Listing 6-20).

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