First part Introduction to Java

Objectives
    • Learn the basics of java programming, syntax, types and basic operations
    • Understand how to fix errors
    • Create simple programs
1. Introduction to Java
Many of you have taken ITI1120 in the fall, where you have used Python. Here is an overview of the main differences:
    • Java is an Object-Oriented language. All the code is within classes (as can be seen in the HelloWord program from the lab 0).
    • Every program must contain a main method, public static void main(String[] args)..
    • Java is a strongly typed language. You must declare the type of each variable.
    • Every instruction ends with a semi-colon “;”.
    • Instructions that belong to the same block must be between curly brackets “{…}”.
Here are some resources specifically designed to migrate from Python to Java:
    • From Python to Java by Ken Lambert
    • Java for Python Programmers by Brad Miller
    • Java Tutorials — Lesson: Language Basics
1.1 Conventions
In order to make your code clear and easy to understand, you must follow some conventions when it comes to naming your class, methods, variables and constants. Whenever possible, avoid names that are too vague, not representative of what it is or names that are too long.
Name
Conventions
Example
Class name
Starts with an uppercase letter, if it is composed of many words, the following words starts with a capital letter, the words are not separated by a space. Class name are usually common names.
Bouton, StudentSchedule, String
Method name
Starts with a lowercase letter, if it is composed of many words, the following words starts with a capital letter, the words are not separated by a space. Method name are usually verbs.
main(), println(), moveLeft()
Variable name
Starts with a lowercase letter, if it is composed of many words, the following words starts with a capital letter, the words are not separated by a space.
flag, firstName, result
Constant name
The entire name should be in uppercase letter, if it is composed of many words, the words separated by an underline (_).
MAX_NUMBER, RED
Reminder : it is important to follow conventions. Points can be deducted from your mark if they are not respected. You can find many resources online about theses conventions. Don’t forget to consult them!
    • java.sun.com/docs/codeconv/html/CodeConvTOC.doc.html.
    • www.site.uottawa.ca/~turcotte/teaching/iti-1521/assignments/directives.html
It is also important to add comments to your code as it will help other programmers to understand it. Comments in java start with two dash (//), if the comments are too long for one line, the first line starts with “/*” and ends with “*/”.

/*this is the beginning of
the class HelloWorld
that prints "Hello World!" to the console*/
public class HelloWorld {

    //this is the main method
    public static void main(String[] args) {
        System.out.println("Hello World!");
    }
}
  
Take a close look at the code above and how the different conventions are implemented. You should also note the code indentation which makes it easier to read and understand
1.2 Introduction to the basic types
As mentioned above, Java is strongly typed, it is important to understand the primitive types. The most commonly used types are:
    • int : An integer with a possible value between -231 and 231-1

        int myInt=3;
    • double : The most common type to represent a decimal value

        double myDouble=1.213;
    • boolean : Can have the value true or false

        boolean myBool= true;
    • char : A character such as a letter, a number or a symbol

        char myChar='x';
    • String :An array of char, the content of a String is identified by the symbol " at the beginning and end of the String. Note that a String is not a primitive type but the reason is beyond the scope of this lab.

        String myString= "Hello World!";
In java it is important to declare the type of each variable as well as the return type of each method (see Section 3.1).
In the following example we forget to declare the variable year before using it.
1 public class Test {
2       public static void main ( String[] args ) {
3               year = 2017 ;
4       }
}
We get an error while trying to compile this class. The error is error: cannot find symbol
Test.java:3: error: cannot find symbol
                year = 2017 ;
                ^
  symbol:   variable year
  location: class Test
1 error
This error is easily fixed by adding the declaration of the variable of type int to line 3.
1 public class Test {
2       public static void main ( String[] args ) {
3               int year;
4               year = 2017 ;
5       }
6}
Exercises: declaration of types
Identify the error in the following codes
Question 1.21 :
1 public class Test {
2       public static void main ( String[] args ) {
3               char hello = "hello";
4       }
5}
Answer
Question 1.22 :
1 public class CourseCode {
2       public static void main ( String[] args ) {
3               String subjectArea = "ITI";
4               int catalogNumber = 1121;
5               int numCredits = 3.0;
6               System.out.print(subjectArea + catalogNumber + " : " + numCredits + " credits");
7       }
8}
Answer
Question 1.23 :
1 public class Test {
2       public static void main ( String[] args ) {
3               double a,b;
4               a = 24;
5               b = 8/9;
6               sum = a + b;
7       }
8}
Answer
Question 1.24 :
1 public class Test {
2       public static void main ( String[] args ) {
3               boolean winner, tie, loser;
4               winner = loser = tie = true;
5               System.out.println(winner + "  " +  tie + "  " + loser);
6       }
7}
Answer
Question 1.25 :
1 public class Test {
2       public static void main ( String[] args ) {
3               int numOfApples = 5;
4               numOfApples = numOfApples - 2;
5               double numOfApples = numOfApples - 1/2 ;
6       }
7}
Answer
1.3 Basic operators
The operators are used to manipulate variables. Here is a list of the operators that you need to know. You can find a more complete list of operator and their priority HERE.
Arithmetic operators :
    • + (Addition)
    • - (Subtraction)
    • * (Multiplication)
    • / (Division)
    • % (Modulus) : Divides left-hand operand by right-hand operand and returns remainder.
    • ++ (Increment) : Increases the value of operand by 1.
    • -- (Decrement) : Decreases the value of operand by 1.
Relational Operators:
These operators return a Boolean (true or false)
    • > (greater than)
    • < (less than)
    • >= (greater than or equal to)
    • <= (less than or equal to)
    • == (equal to)
    • !=(not equal to)
Logical Operators (From the highest to the lowest priority) :
These operators return a Boolean (true or false)
    • ! (logical NOT) : Use to reverses the logical state of its operand. If a condition is true then Logical NOT operator will make false and vice-versa
    • && (logical AND) : If both the operands are true, then the condition becomes true.
    • || (logical OR) : If any of the two operands are true, then the condition becomes true.
Exercises. Watch out for the priority of operation...
What will be printed if we replace the missing line by...?
1 public class PrintLine {
2       public static void main(String[] args) {
3               //missing line
4       }
5 }
  
Question 1.31 :
System.out.println(4 + 2 > 6 == !(8 <= 8)); 
Answer
Question 1.32 :
System.out.println(27 % 4 == 0 && 500 / 100 != 5 || true);
Answer
Question 1.33 :
System.out.println(27 % 4 == 0 && (500 / 100 != 5 || true)); 
Answer
Question 1.34 :
System.out.println('o' >= 'j' && 'B' > 'A' && '&' == '&');
Answer
Question 1.35 :
char c = 'a';
System.out.println(c == 'c');
Answer
Question 1.36 :
double x = 17.24;
System.out.println((x + 32) < true);
Answer
2.1 For and while loop
Loops are efficient tools used to act on a range of values
The for loop is a loop that repeats itself until a condition is satisfied. It follows this form:
for (initialization; termination; increment) {
statement(s)
}
Where :
    1. The Initialization expression initializes the loop; it's executed once, as the loop begins.
    2. Termination : if it evaluates to false, the loop terminates.
    3. Increment: it is invoked after each iteration through the loop; it is perfectly acceptable for this expression to increment or decrement a value
Here is a simple class that uses a for loop:
1 class ForDemo {
2     public static void main(String[] args){
3          for (int i=1; i<11; i++){
4               System.out.println("Count is: " + i);
5          }
6     }
7 }
If you compile and run this class, the output will be:
Count is: 1
Count is: 2
Count is: 3
Count is: 4
Count is: 5
Count is: 6
Count is: 7
Count is: 8
Count is: 9
Count is: 10
Note that we added the variable "i" to the String "Count is: " by using the operator +
The while loop is similar to the for loop. It takes a logical expression as a parameter and execute itself as long as that expression is true. A variation of the while loop is do while, this form of the loop will always execute at least once.
Here is the same program as before using the while and do while loop
1class WhileDemo {
2    public static void main(String[] args){
3        int i = 1;
4        while (i < 11) {
5            System.out.println("Count is: " + i);
6            i++;
7        }
8    }
9}
1class DoWhileDemo {
2    public static void main(String[] args){
3        int i = 1;
4        do {
5            System.out.println("Count is: " + i);
6            i++;
7        } while (i<11);
8    }
9}
Note that the variable i was declared before the loop and that we had to increment it inside the loop
if, else and else if
The if and else if statements take a logical expression as a parameter. If this expression is true, the following operation will be executed, otherwise it will be skipped. Else statements are used for code that will be executed only if no if or else if statement was executed. An if statement can be alone, an else if has to follow an if or another else if. The else statement has to follow an if or else if statement.
The use of these statement is as follows:
if(/*expression*/){
        //do something
}
else if(/*boolean expression*/){
        //do something
}
else if(/*boolean expression*/){
        //do something
}
else {
        //do something
}
Only the first statement (if, else if or else) whose expression evaluates to true will be executed. It is therefore important to use the good statements in the right order and to use the brackets properly. Using the right indentation helps to prevent mistakes.
Here is a program that prints each number from 1 to 10 and indicates if they are even or odd:
1class IfDemo{
2       public static void main(String[] args){
3               int i = 1;
4               while (i<11){
5                       if(i % 2 == 0){
6                               System.out.println(i + " is even");
7                       }
8                       else{
9                               System.out.println(i + " is odd");
10                      }
11                      i++;
12              }
13      }
14}
This program has a similar loop to the program from 2.1. At line 5 we check if the modulus 2 of i is 0. If that is true we know that the number is even and the program executes the line 6. Otherwise we know that the number is odd and the program execute the line 9.
Note that the else at line 8 could have been replaced by else if (i % 2 == 1) to get the same result
The output is:
1 is odd
2 is even
3 is odd
4 is even
5 is odd
6 is even
7 is odd
8 is even
9 is odd
10 is even
Exercice 2 : FizzBuzz
Make a class Q2 that goes throught the numbers from 1 to 30, if the number is divisible by 3 print "Fizz", if it is divisible by 5 print "Buzz". If the number is divisible by 15 print "FizzBuzz".
public class Q2{

        public static void main(String[] args){
                // Your code here
        }

}
  
The output should look like this:
3 Fizz
5 Buzz
6 Fizz
9 Fizz
10 Buzz
12 Fizz
15 FizzBuzz
18 Fizz
20 Buzz
21 Fizz
24 Fizz
25 Buzz
27 Fizz
30 FizzBuzz
3. Methods, array and solving errors
3.1 Methods
Up to this point we have only used the main method. In order to write efficient programs, you will need to divide you code in different methods according to the functionality of the code. A method had to indicate the type of value it will return or void if it returns nothing. To return a value, use the keyword return followed by the value to return. A method takes zero to many parameters, parameters are variables that are passed to the methods when it is called. A method finishes its execution when it encounters the keyword return or if it reaches its closing bracket }. The syntax of a method is as follows:
public static int addition(int a, int b){
        int result = a + b;
        return result;
}
Where...
    • public static : Method modifiers
    • int : Return type
    • addition : Name of the method
    • int a, int b : List of parameters ("int" is the type, "a" and "b" are the name of the variables)
    • return : Keyword preceding the value to return
If we want to use that method, we need to call it from the main method like this:
class MethodDemo{
        public static void main(String[] args){
                System.out.println("1 + 2 = " + addition(1,2));
         }


        public static int addition(int a, int b){
                int result = a + b;
                return result;
        }
}
Running this program outputs:
1 + 2 = 3
Array and errors
Arrays are tables of fixed size that contain one type of data. The size is determined during initialization. Every “thing” in an array is called an element; each element is accessible via its index. The index of an element is its position the array. The first element of an array is at the index 0..
An array is declared by indicating the type follow by two square brackets [] and the name of the variable. To initialise an array you need to use the keyword new followed by the size of the array within square brackets.
Example :
int[] anArray;
anArray = new int[10];
Now let’s try to create a method that creates an array of a size passed in parameter that will fill the array with an integer equal to the index of the element * 100.
Let’s start with this code. Try to compile and then run this code:
1class ArrayDemo{
2       public static void main(String[] args){
3               populateArray(10);
4       }
5
6       //methode that populates the array with the index of the element*100
7       public static int[] populateArray(int size){
8               int[] anArray;
9               anArray = new int[size];
10              for(int i = 1; i <= anArray.length; i++){
11                      anArray[i] = i * 100;
12              }
13              return anArray;
14      }
15}
When we try to run the code we get the following error message:
Exception in thread "main" java.lang.ArrayIndexOutOfBoundsException: 10
        at ArrayDemo.populateArray(ArrayDemo.java:11)
        at ArrayDemo.main(ArrayDemo.java:3)
This program returns an error when we try to run it. The first line of the message indicates the type of error. The following line indicates where the error occurred. In this case the error is of type “ArrayIndexOutOfBoundsException” with the additional information “10”, this indicates that we tried to access the index 10 of the array but that the array does not contain an index 10. While the size of the array is in fact 10, since the index start at 0 the array contains the index from 0 to 9. In the previous example we tried to access the index 1 to 10, which is why we got an error.
It is possible to fix the error by modifying the code like this (Identify the 2 changes to the line 10) :
1class ArrayDemo{
2       public static void main(String[] args){
3               populateArray(10);
4       }
5
6       //methode that populates the array with the index of the element*100
7       public static int[] populateArray(int size){
8               int[] anArray;
9               anArray = new int[size];
10              for(int i = 0; i < anArray.length;i++){
11                      anArray[i] = i * 100;
12              }
13              return anArray;
14      }
15}
Now that the method populateArray does not cause any error, we can print the array to make sure that the method does the right operation to the array
Verifying the logic of every method as soon as it is completed is a good practice that helps prevent future errors.
1class ArrayDemo{
2       public static void main(String[] args){
3               int[] arrayToPrint = populateArray(10);
4               for(int i = 0; i < arrayToPrint.length; i++){
5                       System.out.println("value for index " + i + " is: " + arrayToPrint[i]);
6               }
7       }
8
9       //methode that populates the array with the index of the element*100
10      public static int[] populateArray(int size){
11              int[] anArray;
12              anArray = new int[size];
13              for(int i = 0; i < anArray.length; i++){
14                      anArray[i] = i * 100;
15              }
16              return anArray;
17      }
18}
The output is:
value for index 0 is: 0
value for index 1 is: 100
value for index 2 is: 200
value for index 3 is: 300
value for index 4 is: 400
value for index 5 is: 500
value for index 6 is: 600
value for index 7 is: 700
value for index 8 is: 800
value for index 9 is: 900
Selection Sort
Let’s use a selection-sort algorithm to review what we learned. This sorting algorithm first identified the location of smallest element in an array, and exchange the value at this location with the value at the location 0. Then, it identifies the second smallest value in the array, and exchange the value at this location with the value at location 1. The algorithm keeps on going iteratively until the values are sorted in increasing order.
    • The method sort declared at line 3 implements our algorithm. This method has one parameter, called xs. This parameter is of type reference, a reference to an array of integers int[].
    • Line 5, we declare four variables of type integer (int).
    • Line 7, we have the outermost for loop. A for loop test contains three parts: initialization, halting criteria and incrementation. Here, in the initialization part we initialize the variable i to the value. The loop will stop (halting criteria) once the value of i is more than or equal to the length of the array referenced by the (reference) variable xs. Finally, at each iteration, the value of i is incremented by 1 (i++, which is a short cut for i=i+1).
    • Line 8, we assign the value of i to the variable argMin. The value of variable argMin is going to be used to record the position of the smallest entry in the array between i and xs.length-1.
    • Line 9, we have a nested for loop. This loop is used to traverse positions from i+1 to xs.length-1 of the array: the value of j is initialized with the value of i+1 and the loop finishes once j is larger than or equal to the length of the array. At each iteration, the value of j is incremented by 1.
    • Line 10, we have an if statement. Note that the test is between parentheses. Here, we compare the values of the positions j and argMin in the array. If the value at position j is less than the value at position argMin, we store the value at position j in the variable argMin (line 11). It’s the smaller value so far.
    • Lines 15, 16 and 17. Once the nested loop is finished, argMin contains the location of the smallest value for the part of the array between locations i and xs.length-1. We exchange the values at location i and argMin.
    • At line 18 we have the closing bracket for the block starting at line 7.
    • At line 20 we have the closing bracket for the block starting at line 3.
    • The main method is declared at line 22. This method has one parameter, called args, a reference to an array of Strings.
    • At line 24, we define a reference variable, which points at an array of integers. This variable is called marks.
    • At line 25. The right hand side of the statement created an array of integers. The reference of the array is stored in a variable called marks.
    • At line 27. Call to the method sort. The value of the actual parameter marks is copied into the formal parameter xs.
    • In lines 29 35. Printing of the array referenced by the variable marks. Note the declaration and initialization of a new variable and its type (int i=0) right inside the initialization of the for loop..
1 public class Sort {
2
3       public static void sort(int[] xs){
4
5               int i, j, argMin, tmp;
6
7               for (i = 0; i < xs.length - 1; i++) {
8                       argMin = i;
9                       for (j = i + 1; j < xs.length; j++) {
10                              if (xs[j] < xs[argMin]) {
11                                      argMin = j;
12                              }
13                      }
14
15                      tmp = xs[argMin];
16                      xs[argMin] = xs[i];
17                      xs[i] = tmp;
18              }
19
20      }
21
22      public static void main(String[] args) {
23
24              int[] marks;
25              marks = new int[]{100,34,72,56,82,67,94};
26
27              sort(marks);
28
29              for(int i=0; i< marks.length; i++) {
30                      if (i>0) {
31                              System.out.print(", ");
32                      }
33                      System.out.print(marks[i]);
34              }
35
36              System.out.println();
37      }
38
39 }
Exercices!
Question 3.1
Create a class containing a method name createArray and a main method. The method createArray creates an array where each element contains the square of its index. The size of the array is determined by the method’s parameter. createArray then returns this array.
The main methods calls createArray with the right parameter to get the squares of the numbers from 0 to 12, it then prints the values of the array.
public class Q3_SquareArray{

        public static int[] createArray(int size) {
                // Your code here
        }

        public static void main(String[] args){
                // Your code here
        }
}

The output should be similar to this:
The square of 0 is: 0
The square of 1 is: 1
The square of 2 is: 4
The square of 3 is: 9
The square of 4 is: 16
The square of 5 is: 25
The square of 6 is: 36
The square of 7 is: 49
The square of 8 is: 64
The square of 9 is: 81
The square of 10 is: 100
The square of 11 is: 121
The square of 12 is: 144
Question 3.2
Complete this code so that it calculates the average of the 7 numbers passed via the array valuesArray:
public class Q3_AverageDemo{
        public static void main(String[] args){
                double[] valuesArray;
                valuesArray = new double[]{100.0,34.0,72.0,56.0,82.0,67.0,94.0};
                System.out.println("The average is: " + calculateAverage(valuesArray));
        }

        //method that calculates the average of the numbers in an array
        public static double calculateAverage(double[] values){
                double result;
                //your code here
                return result;
        }
}
Question 3.3
Create a method that inserts an element to a specific index of an array. This method has three parameters: an array, an index and a number to insert. Use the main method to print the array before and after the insertion. (Hint: arrays have fixed size; you must create a second array that is bigger than the first)
public class Q3_ArrayInsertionDemo{

        public static int[] insertIntoArray(int[] beforeArray, int indexToInsert, int valueToInsert){
                // Your code here
        }

        public static void main(String[] args){
                // Your code here
        }
}

Example of the output:
Array before insertion:
1
5
4
7
9
6
Array after insertion of 15 at position 3:
1
5
4
15
7
9
6
Question 3.4
Complete this code so that it prints the letters from the char array unorderedLetter in decreasing order:
public class Q3_ReverseSortDemo {
        public static void main(String[] args){
                char[] unorderedLetters;
                unorderedLetters = new char[]{'b', 'm', 'z', 'a', 'u'};
                reverseSort(unorderedLetters);
                for (int i = 0 ; i < unorderedLetters.length; i++ )
                        System.out.print(unorderedLetters[i]);
        }

        //method that sorts a char array into its reverse alphabetical order
        public static void reverseSort(char[] values){

                //your code here
        }

}
4. Java Scanner
In Java there is a class named Scanner. It is the quickest way of obtaining data from the user. To use this class, you must first import it using import java.util.Scanner;. To create a Scanner, we use this line:
Scanner sc = new Scanner(System.in); 
The creation of an object of type Scanner take the argument in which is a variable from the class System. (similar to System.out use to get the method println). You will further explore the creation of objects in another lab
To read the values entered by a user, we need to know what kind of value we expect to get…
For an int, we use :
int myInt = scanNum.nextInt();
For a double, we use:
double myDouble = scanName.nextDouble();
For a String, we use:
String myString = scanName.nextLine();
Compile and run the following code:
import java.util.Scanner;

class ScannerDemo{
        public static void main(String[] args){

                System.out.print("What is your name? ");
                Scanner scanName = new Scanner(System.in);
                String myString = scanName.nextLine();

                System.out.print("How many countries have you visited? ");
                Scanner scanNum = new Scanner(System.in);
                int myInt = scanNum.nextInt();


                System.out.println(myString + " has visited " + myInt + " countries.");

        }
}
Question 4 : My first Scanner:
Create a small program named VotingRight that asks the user to enter his age. If the user is not old enough to vote, the program needs to print the number of years until the person is allowed to vote.
Example of the output:
How old are you? 11
You will be allowed to vote in 7 years.
How old are you? 24
You have the right to vote!
Exercices : Test and error correction!
Question 5
For this section, many methods are given. They however contain errors. Some errors are syntax error while others are logical error. The main method contains test that will execute the two other methods and verify if they work properly.
The method isPrime checks if the number received as an argument is a prime number. A prime number is a number bigger then 1 that can only be divided by 1 or itself.
The Fibonacci method takes a number as argument and returns the value at this position in the Fibonacci series. In the Fibonacci series, each number is the sum of the two preceding position in the series. The position 0 has the value 0 and the position 1 has the value 1. This table describes the series:
position
0
1
2
3
4
5
6
7
8
9
Value
0
1
1
2
3
5
8
13
21
34
public class Q5{

        public static void main(String[] args){
                boolean[] test = new boolean[8];

                //test for isPrime
                test[0]  = (isPrime(0) == false);
                test[1] = (isPrime(2) == true);
                test[2] = (isPrime(7) == true);
                test[3] = (isPrime(15) == false);

                test[4]  = (getFibonacci(1) == 1);
                test[5]  = (getFibonacci(3) == 2);
                test[6]  = (getFibonacci(5) == 5);
                test[7]  = (getFibonacci(8) == 21);


                boolean testFlag=true;
                for (int i = 0; i < test.length; i++){
                        if(test[i]){
                                System.out.println("test " + i + " passed");
                        } else {
                                System.out.println("test " + i + " failed");
                                testFlag = false;
                        }
                }

                if(testFlag ){
                        System.out.println("All tests are successful");
                } else {
                        System.out.println("Not all tests are successful");
                }

        }

        //method that determines if the number x is prime
        public static boolean isPrime(int x){
                boolean prime = true;
                if(x < ){
                        prime = ;
                }
                if(x== ){
                        prime = ;
                }
                int i = 2;
                while(prime && x < i){
                        if(x % i == 0){
                                prime = false;
                        }
                        i++;
                }
                return prime;
        }

        //returns the fibonacci number at the position in parameter
        public static int getFibonacci(int position) {
        
                int num = 0;
                int num2 = 1;
                int fibonacci = 0;

                if (position == ) {
                        fibonacci = 0;
                } else if (position == 1) {
                        fibonacci = ;
                } else {
                        for (int i = 2; ; ) {
                                fibonacci = ;
                                num = num2;
                                num2 = fibonacci;
                        }
                }
                return num;
        }

}
You need to correct the code above until all tests pass successfully and that you get the following output message:
test 0 passed
test 1 passed
test 2 passed
test 3 passed
test 4 passed
test 5 passed
test 6 passed
test 7 passed
All tests are successful

Question 6
For this exercice, you will need to write a method that asks for 10 numbers from the user in the main method. These numbers represent 10 students' grade. Once the 10 numbers are obtained, you will need to call the four other methods passing the 10 numbers as an argument and print the results from the main. The method calculateAverage needs to calculate and then return the average of the grades. The method calculateMedian calculates and returns the median of the grades. The method calculateNumberFailed calculates and returns the number of students that failed and calculateNumberPassed calculates and returns the number of students that passes. Grades are numbers between 0.0 and 100.0. The passing grade is 50%.
public class Q6{
        public static void main(String[] args){

                //your code here

        }

        public static double calculateAverage(double[] notes){
                //your code here
        }

        public static double calculateMedian(double[] notes){
                //your code here
        }

        public static int calculateNumberFailed(double[] notes){
                //your code here
        }

        public static int calculateNumberPassed(double[] notes){
                //your code here
        }

}