LAB 7. Objectives: Navin Sridhar D 8 54

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1 LAB 7 Objectives: 1. Learn to create and define constructors. 2. Understand the use and application of constructors & instance variables. 3. Experiment with the various properties of arrays. 4. Learn to create complex programs dealing with arrays and integrate all previously learnt knowledge with it. Navin Sridhar D 8 54

2 Chapter 11 Create your own objects Section 11.3: Constructors Constructors initialize instance variables. Rules to define a constructor: The name of the constructor is the same as the name of the class. Constructors have no return type and no return value. The keyword static is omitted. public Time() { this.hour = 0; this.minute = 0; this.second = 0.0; Section 11.5: Creating a new object class Time { int hour, minute; double second; public Time() { this.hour = 0; this.minute = 0; this.second = 0.0; public Time(int hour, int minute, double second) { this.hour = hour; this.minute = minute; this.second = second; public static void main(string[] args) { // one way to create and initialize a Time object Time t1 = new Time(); t1.hour = 11; t1.minute = 8; t1.second = ; System.out.println(t1); // another way to do the same thing Time t2 = new Time(11, 8, ); System.out.println(t2); OUTPUT:

3 Section 11.8: Pure Functions Result depends only on the arguments, and it has no side effects like modifying an argument or printing something. public static boolean isafter(time time1, Time time2) { if (time1.hour > time2.hour) return true; if (time1.hour < time2.hour) return false; if (time1.minute > time2.minute) return true; if (time1.minute < time2.minute) return false; if (time1.second > time2.second) return true; return false; public static Time addtime(time t1, Time t2) { Time sum = new Time(); sum.hour = t1.hour + t2.hour; sum.minute = t1.minute + t2.minute; sum.second = t1.second + t2.second; return sum; The problem is that this method does not deal with cases where the number of seconds or minutes adds up to more than 60. In that case, we have to carry the extra seconds into the minutes column, or extra minutes into the hours column. Let s do that using the conditional statement if : public static Time addtime(time t1, Time t2) { Time sum = new Time(); sum.hour = t1.hour + t2.hour; sum.minute = t1.minute + t2.minute; sum.second = t1.second + t2.second; if (sum.second >= 60.0) { sum.second -= 60.0; sum.minute += 1; if (sum.minute >= 60) { sum.minute -= 60; sum.hour += 1; return sum;

4 Section 11.9: Modifiers As an example of a modifier, consider increment, which adds a given number of seconds to a Time object. public static void increment(time time, double secs) { time.second += secs; if (time.second >= 60.0) { time.second -= 60.0; time.minute += 1; if (time.minute >= 60) { time.minute -= 60; time.hour += 1; What happens if the argument secs is much greater than 60? Corrected example: public static void increment(time time, double secs) { time.second += secs; while (time.second >= 60.0) { time.second -= 60.0; time.minute += 1; while (time.minute >= 60) { time.minute -= 60; time.hour += 1; Replacing the if statements with while statements solves this problem.

5 Section 11.10: Fill-in methods public static void addtimefill(time t1, Time t2, Time sum) { sum.hour = t1.hour + t2.hour; sum.minute = t1.minute + t2.minute; sum.second = t1.second + t2.second; if (sum.second >= 60.0) { sum.second -= 60.0; sum.minute += 1; if (sum.minute >= 60) { sum.minute -= 60; sum.hour += 1; Modifiers and fill-in methods are efficient because they don t have to create new objects. Pure functions help manage the complexity of large projects, in part by making certain kinds of errors impossible. Section 11.11: Incremental development and planning Here is a method that converts Time into seconds (with datatype double). public static double converttoseconds(time t) { int minutes = t.hour * 60 + t.minute; double seconds = minutes * 60 + t.second; return seconds; Here is a constructor that converts back seconds into Time. public Time(double secs) { this.hour =(int)(secs / ); secs -= this.hour * ; this.minute =(int)(secs / 60.0); secs -= this.minute * 60; this.second = secs;

6 Now, to add two times, public static Time addtime(time t1, Time t2) { double seconds = converttoseconds(t1) + converttoseconds(t2); return new Time(seconds); Exercises Exercise 11.1: Q. Write a definition for a class named Tile that represents Scrabble tiles. The instance variables should be a character named letter and an integer named value. Write a method named printtile that takes a Tile object as a parameter and prints the instance variables in a reader-friendly format. Write a method named testtile that creates a Tile object with the letter Z and the value 10, and then uses printtile to print the state of the object. public class Tile { char letter; int value; // Constructor public Tile(char letter, int value) { this.letter = letter; this.value = value; // Prints a tile public static void printtile(tile tile) { System.out.println("Tile['" + tile.letter + "' " + tile.value + "]"); // Tests the constructor and printtile functions public static void testtile() { Tile tile = new Tile('Z', 10); printtile(tile); public static void main(string[] args) { testtile();

7 Exercise 11.2: Q. Write a class definition for Date, an object type that contains three integers, year, month and day. This class should provide two constructors. The first should take no parameters. The second should take parameters named year, month and day, and use them to initialize the instance variables. Write a main method that creates a new Date object named birthday. The new object should contain your birthdate. You can use either constructor. class Date { int year, month, day; public Date() { this.year = 0; this.month = 0; this.day = 0; public Date(int year, int month, int day) { this.year = year; this.month = month; this.day = day; public static void main(string[] args) { Date date = new Date(1965, 01, 11); Exercise 11.3: Q. Create a new program called Rational.java that defines a class named Rational. A Rational object should have two integer instance variables to store the numerator and denominator. Write a constructor that takes no arguments and that sets the numerator to 0 and denominator to 1. Write a method called printrational that takes a Rational object as an argument and prints it in some reasonable format. Write a main method that creates a new object with type Rational, sets its instance variables to some values, and prints the object. Write a second constructor for your class that takes two arguments and that uses them to initalize the instance variables. Write a method called negate that reverses the sign of a rational number. This method should be a modifier, so it should return void. Add lines to main to test the new method. Write a method called invert that inverts the number by swapping the numerator and denominator. Add lines to main to test the new method. Write a method called todouble that converts the rational number to a double (floating-point number) and returns the result. This method is a pure function; it does not modify the object. As always, test the new method.

8 Write a modifier named reduce that reduces a rational number to its lowest terms by finding the greatest common divisor (GCD) of the numerator and denominator and dividing through. This method should be a pure function; it should not modify the instance variables of the object on which it is invoked. Write a method called add that takes two Rational numbers as arguments and returns a new Rational object. The return object should contain the sum of the arguments. public class Rational { int numerator, denominator; // simple constructor that sets the numerator to 0 // and denominator to 1 public Rational() { this.numerator = 0; this.denominator = 1; public static void printrational(rational rational) { System.out.println(rational.numerator + "/" + rational.denominator); public static void main(string[] args) { // Create a new rational number -1/3 Rational rational1 = new Rational(); rational1.numerator = -1; rational1.denominator = 3; printrational(rational1); constructor // Create a rational number using the second Rational rational2 = new Rational(8, 2); printrational(rational2); // Negate the rational number negate(rational1); printrational(rational1); // Invert the numerator and denominator invert(rational1); printrational(rational1); // Convert the rational number to a double double floatingpoint = todouble(rational1); System.out.println(floatingpoint);

9 // Reduce a rational number to its lowest terms Rational rational3 = new Rational(8, 64); printrational(rational3); reduce(rational3); printrational(rational3); // Sum two rationals printrational(add(rational2, rational3)); public Rational(int numerator, int denominator) { this.numerator = numerator; this.denominator = denominator; // negate is a modifier that reverses the sign of the rational number public static void negate(rational rational) { int numerator = rational.numerator; int negatednumerator = -1 * numerator; rational.numerator = negatednumerator; // invert is a modifier that swaps the numerator and denominator // of the rational number public static void invert(rational rational) { int oldnumerator = rational.numerator; int olddenominator = rational.denominator; rational.numerator = olddenominator; rational.denominator = oldnumerator; // todouble is a pure function that returns the rational number // expressed as a decimal public static double todouble(rational rational) { double numerator = rational.numerator; double denominator = rational.denominator; return (numerator / denominator); // returning the gcd of two integers public static int gcd(int a, int b) { if (b == 0) { return a; int rem = a % b; return gcd(b, rem);

10 // reduce is a modifier that reduces a rational number to its // lowest terms public static void reduce(rational rational) { int gcd = gcd(rational.numerator, rational.denominator); rational.numerator = rational.numerator / gcd; rational.denominator = rational.denominator / gcd; // returns the sum of the two rational numbers (lcm) public static Rational add(rational rational1, Rational rational2) { int newnumerator = rational1.numerator * rational2.denominator + rational1.denominator * rational2.numerator; int newdenominator = rational1.denominator * rational2.denominator; Rational newrational = new Rational(newNumerator, newdenominator); reduce(newrational); return newrational;

11 Chapter 12 Arrays An array is a set of values where each value is identified by an index. One of the most common ways to index an array is with a loop variable. Section 12.1: Accessing elements public static void count (int i) { int i = 0; while (i < 4) { System.out.println(count[i]); i++; The body of this loop is only executed when i is 0, 1, 2 and 3. Section 12.2: Copying arrays public static void main (String[] args) { double[] a = new double [3]; double[] b = new double [3]; int i = 0; while (i < 4) { b[i] = a[i]; i++; Section 12.4: for loops The general syntax looks like this: for (INITIALIZER; CONDITION; INCREMENTOR) { BODY It is more concise and, since it puts all the loop-related statements in one place, it is easier to read. for (int i = 0; i < 4; i++) { System.out.println(count[i]);

12 Section 12.5: Array length All arrays have one named instance variable: length. for (int i = 0; i < a.length; i++) { b[i] = a[i]; This code assumes that the array b contains at least as many elements as a. Section 12.6: Random numbers Java provides a method called random() that generates pseudorandom numbers from 0.0 to 1.0 (not including 1.0). for (int i = 0; i < 10; i++) { double x = Math.random(); System.out.println(x); NOTE: The method random() was also used in GridWorld to make the bug perform random walk. Section 12.7: Array of random numbers public static int[] randomarray(int n) { int[] a = new int[n]; for (int i = 0; i<a.length; i++) { a[i] = randomint(0, 100); return a; The above method takes a single argument, the size of the array. It allocates a new array of integers, fills it with random values, and returns a reference to the new array. To print this array, public static void printarray(int[] a) { for (int i = 0; i<a.length; i++) { System.out.println(a[i]);

13 Section 12.8: Counting The following is a method called inrange that counts the number of elements in an array that fall in a given range. public static int inrange(int[] a, int low, int high) { int count = 0; for (int i = 0; i < a.length; i++) { if (a[i] >= low && a[i] < high) count++; return count; Section 12.9: The histogram The counting pattern is the same whether we use a single counter or an array of counters. In this case, we initialize the array outside the loop; then, inside the loop, we invoke inrange and store the result. int[] counts = new int[100]; for (int i = 0; i < counts.length; i++) { counts[i] = inrange(scores, i, i+1); Section 12.10: A single-pass solution As the number of ranges increases, that gets to be a lot of traversals. It would be better to make a single pass through the array, and for each value, compute which range it falls in. Then we could increment the appropriate counter. int[] counts = new int[100]; for (int i = 0; i < scores.length; i++) { int index = scores[i]; //changes with each execution counts[index]++;

14 Exercises Exercise 12.1: Q. Write a method called clonearray that takes an array of integers as a parameter, creates a new array that is the same size, copies the elements from the first array into the new one, and then returns a reference to the new array. class clonearray { public static void main (String[] args) { int[] a = {2,4,4,4,2,22; System.out.println("Array A is: "); printarray(a); System.out.println(); int[] b = clonearray(a); System.out.println("Array B is: "); System.out.println(); printarray(b); public static int[] clonearray (int[] a) { int len = a.length; double[] b = new double [len]; int i = 0; while (i < len) { b[i] = a[i]; i++; return a; public static void printarray(int[] a) { for (int i = 0; i<a.length; i++) { System.out.println(a[i]);

15 Exercise 12.2: Q. Write a method called randomdouble that takes two doubles, low and high, and that returns a random double x so that low x < high. public static double randomdouble(double low, double high) { double difference = high - low; return Math.random()*difference + low; Exercise 12.3: Q. Write a method called randomint that takes two arguments, low and high, and that returns a random integer between low and high, not including high. public static double randomint(double low, double high) { double difference = high - low; return (int)(math.random()*difference + low); Exercise 12.4: Q. Write a method called makehist that takes an array of scores and returns a histogram of the values in the array. public static int[] makehist(int[] scores) { int[] count = new int[10]; Exercise 12.5: for (int i = 0; i < scores.length; i++) { int index = scores[i]; count[index]++; return count; Q. Write a method named arefactors that takes an integer n and an array of integers, and that returns true if the numbers in the array are all factors of n. public static boolean arefactors(int n, int[] a) { for (int i = 0; i < a.length; i++) { if (a [i] % n == 0) {continue; else {return false; return true;

16 Exercise 12.6: Q. Write a method that takes an array of integers and an integer named target as arguments, and that returns the first index where target appears in the array, if it does, and -1 otherwise. public static int find(int[] arr, int target) { for (int i = 0; i < arr.length; i++) { if (arr[i] == target) { return i; return -1; Exercise 12.10: Q. Write a method called indexofmaxinrange that takes an array of integers, finds the largest element in the given range, and returns its index. public static int indexofmaxinrange(int[] arr) { int max = 0; for (int i = 0; i < arr.length; i++) { if (arr[i] >= max) { max = arr[i]; return max; Q. Write a method called swapelement that takes an array of integers and two indices, and that swaps the elements at the given indices. public static int[] swapelement(int[] arr, int x, int y) { int z = arr[y-1]; //temp variable arr[y-1] = arr[x-1]; arr[x-1] = z; System.out.println("Element at position " + x + " was swapped with element at position " + y); return arr;

17 Q. Write a method called selectionsort that takes an array of integers and that uses indexofmaxinrange and swapelement to sort the array from largest to smallest. public static int[] swapelement(int[] arr, int x, int y) { int z = arr[y-1]; arr[y-1] = arr[x-1]; arr[x-1] = z; return arr; public static int[] selectionsort(int[] a) { for (int i = 1; i < a.length; i++) { if (a[i] > a[i-1]) { swapelement(a,i,i+1); return a; Exercise 12.12: Q. A word is said to be a doubloon if every letter that appears in the word appears exactly twice. Write a method called isdoubloon that returns true if the given word is a doubloon and false otherwise. public static boolean isdubloon(string s) { int[] histo = new int[26]; for(int i = 0; i < s.length(); i++) { char curchar = s.tolowercase().charat(i); if ('a' <= curchar && curchar <= 'z') { histo[curchar-'a']++; for (int i = 0; i < histo.length; i++) { if (histo[i]!= 0 && histo[i]!= 2) { return false; return true;

18 CONCLUSION i) Constructors: Constructors are used to initialize the instances of our classes. A constructor can be visualized as a template of the program. All objects created using the new keyword will be given the parameters as defined in the constructor. ii) Encapsulation: Encapsulation is the technique of making the fields in a class private and providing access to the fields via public methods. Encapsulation is also known as data hiding. The instance variables are almost every time declared as private. Instance variables are made private to force the users of that class to use methods to access them. iii) Using the this keyword: Within an instance method or a constructor, this is a reference to the current object the object whose method or constructor is being called. You can refer to any member of the current object from within an instance method or a constructor by using this. iv) Syntaxes related to Arrays: The index of any array always starts from 0. Built-in methods in Java can be used effectively for various applications. Array is a powerful concept which can be used for manipulation (sorting, deleting an element, etc.) very effortlessly for further processing. v) Array methods: An array may be passed to a method which may either be a modifier method (void) or return an array to the calling method (pure). The most preferable method to go through an array is by using the for loop.