If

if is a conditional statement. It tests "if the condition is true, then run the code within the if statement"

The if statement syntax is like this:

if (condition) {
    // code
}

Types of conditions:

  • a < b
  • a <= b
  • a > b
  • a >= b
  • a == b
  • a != b
  • true
  • false
public class IfElseLesson{
    public static void main(String[] args) {
        String weather = "rainy"; // assign variable weather to the current weather (rainy)

        // if the weather is rainy, remind the user to bring an umbrella
        if (weather == "rainy") {
            System.out.println("Don't forget to bring an umbrella!"); 
        }
    }
}
IfElseLesson.main(null)

Don't forget to bring an umbrella!


If-else

The syntax for if-else statements are as follows:

if (condition) {
    // run this if condition is true
} else {
    // run this if condition is false
}
public class IfElseLesson{
    public static void main(String[] args) {
        String weather = "sunny"; // assign variable weather to sunny

        // if weather is rainy, remind to bring an umbrella
        // otherwise, no need to bring an umbrella
        if (weather == "rainy") {
            System.out.println("Don't forget to bring an umbrella!");
        } else {
            System.out.println("No need for an umbrella!");
        }
    }
}
IfElseLesson.main(null)

No need for an umbrella!


Else-if

else-if statements are written like this:

if (condition) {
    // run if condition is true
} else if (condition 2) {
    // run if condition is false and condition 2 is true
}
public class IfElseLesson{
    public static void main(String[] args) {
        String weather = "rainy"; // assign variable weather to rainy

        // if weather is rainy, remind to bring an umbrella
        // if weather is sunny, remind to bring a hat
        if (weather == "rainy") {
            System.out.println("Don't forget to bring an umbrella!"); 
        } else if (weather == "sunny") {
            System.out.println("Don't forget to bring a hat!");
        }
    }
}
IfElseLesson.main(null)

Don't forget to bring an umbrella!

Setting variable weather to sunny:

public class IfElseLesson{
    public static void main(String[] args) {
        String weather = "sunny";

        if (weather == "rainy") {
            System.out.println("Don't forget to bring an umbrella!");
        } else if (weather == "sunny") {
            System.out.println("Don't forget to bring a hat!");
        }
    }
}
IfElseLesson.main(null)

Don't forget to bring a hat!


Putting It All Together

#1 

public class IfElseLesson{
    public static void main(String[] args) {
        String weather = "cloudy"; // assign variable weather to cloudy

        // because it is cloudy, print that the weather is nice
        if (weather == "rainy") {
            System.out.println("Don't forget to bring an umbrella!");
        } else if (weather == "sunny") {
            System.out.println("Don't forget to bring a hat!");
        } else {
            System.out.println("Nice weather outside!");
        }
    }
}
IfElseLesson.main(null)

Nice weather outside!

#2 

import java.util.Scanner; 

public class Jeopardy{
    public static void main(String[] args) {
        System.out.println("Welcome to Jeopardy! Choose 100, 200, 300, 400, 500");
        int questionNum = 0; // keep track of selecting the 100, 200, 300, 400, or 500 question
        int score = 0;
        String answer = "";

        Scanner sc = new Scanner(System.in);
        questionNum = sc.nextInt(); // for questionNum input
        System.out.println(questionNum);

        Scanner questionManySpace = new Scanner(System.in); // for answer input
        
        if (questionNum == 100) {
            // ask question
            System.out.println("What programming language does this code run in? "); 
            answer = questionManySpace.nextLine();
            // if answer is correct, add 100 points to score
            System.out.println(answer);
            if (answer.equals("java")) {
                score += 100;
            }
        // the format is the same for all of the answers below
        } else if (questionNum == 200) {
            System.out.println("How do you create a string called 'name' and assign it to 'Bob' in Java? ");
            answer = questionManySpace.nextLine();
            System.out.println(answer);
            if (answer.equals("String name = Bob;")) {
                score += 200; 
            }
        } else if (questionNum == 300) {
            System.out.println("What is the conditional called to do something if something happens or do something else if something else happens? ");
            answer = questionManySpace.nextLine();
            System.out.println(answer);
            if (answer.equals("if-else")) {
                score += 300; 
            }
        } else if (questionNum == 400) {
            System.out.println("How do you create a scanner object with name scanner? ");
            answer = questionManySpace.nextLine();
            System.out.println(answer);
            if (answer.equals("Scanner scanner = new Scanner(System.in)")) {
                score += 400;
            }
        } else if (questionNum == 500) {
            System.out.println("What backend framework runs on lwu1822.tk? ");
            answer = questionManySpace.nextLine();
            System.out.println(answer);
            if (answer.equals("Spring")) {
                score += 500; 
            }
        }
        System.out.println("Your total score is: " + score);
    }
}
Jeopardy.main(null);

Welcome to Jeopardy! Choose 100, 200, 300, 400, 500
500
What backend framework runs on lwu1822.tk? 
Spring
Your total score is: 500

Switch case statements 

import java.util.Scanner; 

public class Jeopardy{
    public static void main(String[] args) {
        System.out.println("Welcome to Jeopardy! Choose 100, 200, 300, 400, 500"); 
        int questionNum = 0;  // keep track of selecting the 100, 200, 300, 400, or 500 question
        int score = 0;
        String answer = "";

        Scanner sc = new Scanner(System.in);
        questionNum = sc.nextInt(); // for questionNum input
        System.out.println(questionNum);

        Scanner questionManySpace = new Scanner(System.in);  // for answer input

        switch (questionNum) {
            case 100:
                // ask question
                System.out.println("What programming language does this code run in? "); 
                answer = questionManySpace.nextLine();
                System.out.println(answer);
                // if answer is correct, add 100 points to score
                if (answer.equals("java")) {
                    score += 100;
                }
            // the format is the same for all of the answers below
            case 200:
                System.out.println("How do you create a string called 'name' and assign it to 'Bob' in Java? ");
                answer = questionManySpace.nextLine();
                System.out.println(answer);
                if (answer.equals("String name = Bob;")) {
                    score += 200; 
                }
            case 300:
                System.out.println("What is the conditional called to do something if something happens or do something else if something else happens? ");
                answer = questionManySpace.nextLine();
                System.out.println(answer);
                if (answer.equals("if-else")) {
                    score += 300; 
                }
            case 400:
                System.out.println("How do you create a scanner object with name scanner? ");
                answer = questionManySpace.nextLine();
                System.out.println(answer);
                if (answer.equals("Scanner scanner = new Scanner(System.in)")) {
                    score += 400;
                }
            case 500: 
                System.out.println("What backend framework runs on lwu1822.tk? ");
                answer = questionManySpace.nextLine();
                System.out.println(answer);
                if (answer.equals("Spring")) {
                    score += 500; 
                }
        }

        System.out.println("Your total score is: " + score);
    }
}
Jeopardy.main(null);

Welcome to Jeopardy! Choose 100, 200, 300, 400, 500
500
What backend framework runs on lwu1822.tk? 
Spring
Your total score is: 500


De Morgan's Law

First, some vocab:

  • Set: A group of objects
  • Universal set: A set that has all of the elements
  • Subsets: Contains elements from the universal set
  • Complement: For example, if we have set A, the complement of set A would have the elements in the universal set, but not in set A.

  • Union: New set has elements from both sets

    Symbol: ∪

  • Intersection: New sets has elements that are shared between both sets

    Symbol: ∩

De Morgan's Law is used in boolean algebra to help simplify expressions (this is important, remember this for later).

De Morgan's Law of Union states that: "The complement of the union of the two sets A and B will be equal to the intersection of A' (complement of A) and B' (complement of B)." 1

Formula: (A ∪ B)' = A'∩ B'

De Morgan's Law of Intersection states that: "The complement of the intersection of A and B will be equal to the union of A' and B'." 2

Formula: (A ∩ B)’ = A’ ∪ B’


The following is an example:

Universal set: {1, 3, 5, 6, 8, 9} Set A: {1, 3} Set B: {5, 6, 8}

De Morgan's Law of Union: (A ∪ B)' = {9}

A'∩ B' = {9}

(A ∪ B)' = A'∩ B'

De Morgan's Law of Intersection: (A ∩ B)' = {1, 3, 5, 6, 8, 9}

A’ ∪ B’ = {1, 3, 5, 6, 8, 9}

(A ∩ B)’ = A’ ∪ B’


Now how is this useful?

Remember this?

Translating De Morgan's law into code, De Morgan's Law of Union would mean: !(a || b) = (!a && !b).

De Morgan's Law of Intersection means: !(a && b) = (!a || !b) 3

1. Reference↩

2. Reference↩

3. Reference↩

Example in Coding

because what would De Morgan's law be if it's not applicable to coding?

public class DeMorgansLaw{
    public static void main(String[] args) {
        boolean rainy = false;
        boolean sunny = false;

        if (!!!(rainy || sunny) && ((!rainy && !sunny) || !(!(rainy || sunny))) ) {
            System.out.println("It's a cloudy day, not too hot, not too cold");
        }
    }
}
DeMorgansLaw.main(null)

It's a cloudy day, not too hot, not too cold

Well this is awfully confusing, how about we clean it up a little?

First, remove all of the !! (negative + negative = positive)

This code reads: If it's not rainy or sunny, and: it's not rainy and it's not sunny OR it's rainy or sunny

Evaluating each part of the code:

  1. If it's not rainy or sunny: True
  2. If it's not rainy and it's not sunny: True
  3. It's rainy or sunny: False

2. and 3. are OR, therefore, it is True.

1. is ANDed to 2. and 3. which is True. -> True AND True is true, therefore, the message is outputted.

public class DeMorgansLaw{
    public static void main(String[] args) {
        boolean rainy = false;
        boolean sunny = false;

        if (!(rainy || sunny) && ((!rainy && !sunny) || (rainy || sunny)) ) {
            System.out.println("It's a cloudy day, not too hot, not too cold");
        }
    }
}
DeMorgansLaw.main(null)

It's a cloudy day, not too hot, not too cold

More examples:

This code reads: If it's not sunny and it's not cloudy and it's not snowy and it's not windy, then bring an umbrella.

public class DeMorgansLaw{
    public static void main(String[] args) {
        boolean rainy = true;
        boolean sunny = false;
        boolean cloudy = false;
        boolean snowy = false;
        boolean windy = false;

        if (!sunny && !cloudy && !snowy && !windy) {
            System.out.println("Bring an umbrella!");
        }
    }
}
DeMorgansLaw.main(null)

Bring an umbrella!

This code is kind of messy, huh?

How about this code?

This code reads: If it's not sunny or cloudy or snowy or windy, then bring an umbrella.

Better?

public class DeMorgansLaw{
    public static void main(String[] args) {
        boolean rainy = true;
        boolean sunny = false;
        boolean cloudy = false;
        boolean snowy = false;
        boolean windy = false;

        if (!(sunny || cloudy || snowy || windy)) {
            System.out.println("Bring an umbrella!");
        }
    }
}
DeMorgansLaw.main(null)

Bring an umbrella!


A more complicated example...

public class DeMorgansLaw{
    public static void main(String[] args) {
        boolean pineappOnPizza = true;
        boolean nopineappOnPizza = false; 

        System.out.println("Is pineapple on pizza bad?");

        // huh???
        if (!(!nopineappOnPizza && !pineappOnPizza)) {
            System.out.println("No!");
        }
    }
}
DeMorgansLaw.main(null)

Is pineapple on pizza bad?
No!

Is this better?

public class DeMorgansLaw{
    public static void main(String[] args) {
        boolean pineappOnPizza = true;
        boolean nopineappOnPizza = false; 

        System.out.println("Is pineapple on pizza bad?");

        // if no pineapple on pizza or yes pineapple on pizza...
        // since there is a pineapple on pizza, then pineapple on pizza is good
        if ((pineappOnPizza || nopineappOnPizza)) {
            System.out.println("No!");
        }
    }
}
DeMorgansLaw.main(null)

Is pineapple on pizza bad?
No!