ReadMe.md

Week 3 Note

1. Conditional Statements
   1. if statement
   2. if else statement
   3. if else if ..else statement
   4. nested if ( if statement inside another if statement)
   5. switch statement
   6. Ternary operator (conditional assignment)
2. Creating Interactive Program
   1. Scanner Class
   2. Importing Scanner
   3. Creating Scanner Object
   4. Using existing methods from Scanner Class
   5. Common Scanner methods
   6. Common Erros:
      1. Data type mismatch :
      2. Errors while mixing nextLine() and nextX() methods
   7. How do we get consistent result if we have to mix it like this?
3. Using Scanner with Conditional Statements
   1. Task 1 :
   2. Task 2 :

Conditional Statements

if statement

 int num = 75;
 
 if(num>60){
     System.out.println("Passed");
 }

if else statement

 int num = 75;
 
 if(num>60){
     System.out.println("Passed");
 }else{
     System.out.println("Failed");
 }

if else if ..else statement

int score = 56 ; 
	
if( score>100 || score < 0 ) {	
	System.out.println("INVALID NUMBER!!!!");
}else if( score>=90 ) { 
	System.out.println("A");		
}else if ( score>=80 ) {
	System.out.println("B");
}else if ( score>=70  ) {
	System.out.println("C");		
}else {
	System.out.println("Failed");
}

nested if ( if statement inside another if statement)

int price = 34;
boolean primeMember = false;

if(! primeMember ) {

	if(price>30){
		System.out.println("Free Shipping");	
	}else{
		System.out.println("5$ Shipping!!");	
	}
}else{		
	System.out.println("Free Shipping");
}

switch statement

When you have a variable and you want to check several different possible values and take action accordingly , switch statement can be used.

You can do everything switch can do using if else if else, other way is not true since switch only check for equality.

For example :

	int choice = 1;

	switch(choice) {

	case 1 :
		System.out.println("You have selected Option 1 ");
		break ;
	case 2 :
		System.out.println("You have selected Option 2 ");
		break ;
	case 3 :
		System.out.println("You have selected Option 3 ");
		break ;
	case 4 :
		System.out.println("You have selected Option 4 ");
		break ;
	default:
		System.out.println("You did not select any of 1,2,3,4 ");
		System.out.println("this is like else section of if else");
		break; //optional here
	}

ONLY VARIABLE TYPES SUPPORTED BY switch(variable) are :

byte, short, char, and int primitive data types and String (Later Enum and Wrapper classes)

Variable with any other data type WILL NOT COMPILE

Another Example :

String season = "summer";
switch(choice) {

	case "spring" :
		System.out.println("Spring is here!!!");
		break ;
	case "summer" :
		System.out.println("Summer!! go swimming!!!");
		break ;
	case "fall" :
		System.out.println("Spring go hiking!!");
		break ;
	case "winter" :
		System.out.println("Go Skiing!!");
		break ;
	default:
		System.out.println("NOT A VALID SEASON!!");
		System.out.println("this is like else section of if else");
		break; //optional here
}

Ternary operator (conditional assignment)

Ternary operator is used to simplify conditional assignment for if else statement.

Here is the simple if else example

	String result = ""; 
	int score = 70
	if(score>60){
		result = "Pass" ;
	}else{
		result = "Fail" ;
	}

This code can be one liner using ternary operator as below.

String result = (score>60) ? "Pass" : "Fail" ; 

NEAT!!!

Syntax :

DataType variableName = (condition here ) ? IfTrueAssignThis : IfFalseAssignThis ;  

---

Creating Interactive Program

We have been creating a variable and assiging value directly in our source code.

For example :

String name = "Adam" ;
System.out.println("You have entered "+ name); 
// output You have entered Adam

What if We want to make the program interactive to let user enter the name when program is running and capture that user input to assign the name value ?

That's exactly where Scanner class comes in.

Scanner Class

Scanner is a class coming from JDK and It's under a package called java.util and we can use the existing functionality already provided to capture users input just like we were able to directly use System.out.println to print something on the console.

Importing Scanner

In order to use it , we need to tell compiler exactly where is the class using import statement.

import java.util.Scanner ;

More precisely, It should be right under package declaration and before class declaration as below.

package your_package_name;

import java.util.Scanner ;

public class Practice{
    // codes here
}

Creating Scanner Object

We will talk a lot more about what's class and object in later classes. right now we will just take it and use it since the line is always the same.

Once imported, We need to create Scanner object and assign it to variable.

• When you create a variable with data type int , you can store a number
• When you create a variable with data type Scanner, then you can store Scanner object.

This is how we create Scanner object and assign it to a variable with type Scanner . (You may use any variable name , in this case we called it scan)

Scanner scan = new Scanner(System.in) ; 

Using existing methods from Scanner Class

Now you may use the variable scan to call various methods (behaviour of objects) already exists to capture users input.

For example, in order to capture single word and save into a variable :

String name = scan.next(); // the program will pause here until user enter a word and hit enter. the value will be saved into variable name. we can use it anywhere just like we would do with any variables.
System.out.println("You have entered " + name);

We can also capture the user inputs like numbers and boolean and save into corresponding data types.

Common Scanner methods

Here is the list of common methods we can call :

// assuming we already created scanner object and named it scan.
byte   num1 = scan.nextByte();
short  num2 = scan.nextShort();
int    num3 = scan.nextInt();
long   num4 = scan.nextLong();
float  num5 = scan.nextFloat();
double num6 = scan.nextDouble();
boolean b   = scan.nextBoolean();

When user enter the input , it can be separated by space or enter keystroke to distinguish one input from another.

Here is a sample program to ask user 3 numbers and print sum

package capturing_user_input;
import java.util.Scanner ; 

public class SumCalculator {

	public static void main(String[] args) {
		
		Scanner scan = new Scanner(System.in) ;  
		
		int num1, num2, num3 , sum ; 
		
		System.out.println("Enter 3 numbers");
		num1 = scan.nextInt();
		num2 = scan.nextInt();
		num3 = scan.nextInt();
		
		sum = num1+num2+num3 ; 
		
		System.out.println("Sum of 3 numbers is : " + sum );	
	}
}

Here is the sample input and output for running the program 2 times.

Fact : There is no nextChar() method to capture single character.

If we need to capture more than one word , like sentences we can use nextLine() method

String sentence = scan.nextLine();

The way we indicate we are done with the line is by hitting Enter, so whenever a enter is pressed in keyboard it will see it as you have finished typing.

It's always better to print a message before the methods to ask question so it's clear what's expected when entering value in console.

Here is the sample program to combine few methods :

package capturing_user_input;

import java.util.Scanner ; 

public class ScannerPractice {

	public static void main(String[] args) {

		Scanner scan = new Scanner(System.in) ;  
		
		System.out.println("Please enter your full name : ");
		String fullName =  scan.nextLine() ; 
		System.out.println("You have entered full name :  " + fullName);
		
		
		System.out.println("Please enter your age : ");	
		int age = scan.nextInt(); 
		System.out.println("You have entered  age : " + age); 
		
		System.out.println("You study full time , true or false ?");	
		boolean isFullTime = scan.nextBoolean(); 
		System.out.println("You have entered : " + isFullTime );
		
		System.out.println("Thanks for the information");
		
		
	}

}

Here is the sample input and output :

Common Erros:

Data type mismatch :

If user input does not match the data type user input, it will throw and error while program run.

For example :

System.out.println("Enter a number :");
int num = scan.nextInt() ; 

If user enter abc instead of a valid number the output is as below:

Because int num = "abc"; is not a valid statement.

Same apply for other issues like entering fractional number for nextInt() method or entering anything not true or false for nextBoolean() method.

You may however enter anything for next() or nextLine() methods because anything can be stored as String.

For example :

String s1 = scan.next() ;
// all below values will be valid as user input.
123 
true
12.99
A
// It will be stored as String : "value here"

Errors while mixing nextLine() and nextX() methods

There is no nextX method , I will be using it just to refer all Scanner methods : nextInt(), nextDouble() , nextBoolean() and anything other than nextLine().

If you remember for nextLine() method, the only way to know if user done typing is by Enter key(⏎). And all nextX() methods only capture the actual input and will not capture Enter key(⏎). check below example.

Scanner scan = new Scanner(System.in); 

System.out.println("Enter your age : ");
int age = scan.nextInt();
System.out.println("You have entered " + age);

System.out.println("Enter full name : ");
String fullName = scan.nextLine();
System.out.println("You have entered full name : "+ fullName);

First program run : User input the age and hot Enter key(⏎) to try to type full name.

Unexpectedly the program finished running before user even have a chance to enter something in console.

The reason is when you enter number it's captured by nextInt() and when you hit enter nextLine() method assume you are done typing and capture that and save into fullName variable as illustrated in the picture.

However that was not what we intended.

Second Program run : User input the age and hit space key to try to type full name.

The Program executed as expected because nextInt() captured 21 and space indicate that you are done and it will continue.

You typed Adam Jones after the space and hit enter.

nextLine() method start capturing right after space until you hit enter ,

In this case : Adam Jones And it worked! as illustrated below.

Same program , 2 different result according to the way we run due to the nature of nextLine() method use Enter key as end of input.

How do we get consistent result if we have to mix it like this?

Well first , if you do not have to use nextLine() just stick to next() method to avoid issues.

If you really have to use it though, just add extra scan.nextLine() every time you use nextLine() right after nextX() method to capture that Enter key(⏎).

For example :

Scanner scan = new Scanner(System.in); 

System.out.println("Enter your age : ");
int age = scan.nextInt();  // capture age
System.out.println("You have entered " + age);

// adding extra scan.nextLine() just to capture Enter key
scan.nextLine() ;

System.out.println("Enter full name : ");
String fullName = scan.nextLine(); // capture full name
System.out.println("You have entered full name : "+ fullName);

Now with this code , we will not have to worry about the Enter Key accidentally captured by scan.nextLine() that supposed to capture full name.

Bottom line is : Whenever you have to mix nextInt or other similar method with nextLine , always follow below pattern :

int num = scan.nextInt() ; // capture number 
scan.nextLine();  //capture Enter key , do not need to save it.
String sentence = scan.nextLine() ; // capture one or more words

Here is the full program

---

Using Scanner with Conditional Statements

Task 1 :

Write a program to ask user to enter age and print you are eligible to vote if age is more than or qual to 18 and print you are not eligible for vote if less than 18.

Task 2 :

Write a program to ask user to enter name of our school and print Correct if user entered Cybertek else `print incorrect.

Checking primitive types equality , == always return consistent result.

String is not primitive and equality check is little bit more complicated than primitives, in order to get consistent result, always use equals method, we will explain why when we learn String class deeper.

For example :

String str1 = "Apple";
String str2 = "Apple";
System.out.println( str1 == str2)  );      // true
System.out.println( str1.equals(str2)  );  // true

With Scanner 🚨

Scanner scan = new Scanner(System.in); 
String str1 = scan.next(); 
String str2 = scan.next(); 
// assuming user enter "Adam" for both of them
System.out.println( str1 == str2)  );      
// false??!! OMG WHY ?  for now just think of 2 person with same name are not same person. 
System.out.println( str1.equals(str2)  );  // true

Bottom line : ALWAYS USE .equals for String equality check NOT ==