Chapter 4b - If Else

Conditional Structure

👀 Reading Notes

If Else

public class Main {
  public static void main(String[] args) {
    if(false){
      System.out.println("Is True");
    }else {
      System.out.println("Is False");
    }
  }
}

🧪 Try the code out~!

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Practice

• What do you think the program will be printing if we change false to true in line 3?

it prints 'Is True'

it prints 'Is False'

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Using Comparisons to resolve If Else conditionals

class Main{
 public static void main (String args[]){
    int my_age    = 21;
    int age_marie  = 25;
   
     if(my_age < age_marie){
       System.out.println("I am Younger than Marie");
     }else if(my_age > age_marie){
       System.out.println("I am Older than Marie");
     }
   }
}

🧪 Try the code out~!

Comparison Operators

Comparison Operator	Definition	Example
==	Equals	2==2 -> True, 2==4 -> False
!=	Not Equal	2!=3 -> True, 2!=2 -> False
>	Larger	3>2 -> True
<	Smaller	4 < 5 -> True
>=	Larger or Equals	4 >= 2 -> True, 2>=2 -> Tru

Else IF

note

• I created this using draw.io

public class Main {
  public static void main(String[] args) {
    
    int age = 17;
    if(age == 17){
      System.out.println("Age is 17");
    }else if(age>17) {
      System.out.println("You are an adult now");
    }else{
      System.out.println("You are still a kid.");
    }
  }
}

Explaination:

If (Boolean condition1) Then

    (perform computation or action)

Else if (Boolean condition2) Then

    (perform another computation or action)

Else
    (perform a default computation or action)

🧪 Try the code out~!

Try it: if / else if / else

Quiz

if / else if / else

1 / 2

How many of the branches below can run for a single execution?

if (x > 10) {
  A();
} else if (x > 5) {
  B();
} else {
  C();
}

Pick an answer to see if you are right.

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Exercise

👀 Exercise in the curriculum

Complete the following program so that it prints if number is positive or not.

• If the input is positive it should print: num is positive.
• else If the input is negative it should print: num is negative
• else (the case where input is neither positive or negative) it should print: num is ZERO (0)

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🙋‍♀️ Sample expected program:

• Try entering 5
• Try entering -5
• Try entering 0

Nested Conditionals

👀 Curriculum - Nested Conditionals

public class Main{
 public static void main(String args[]){ 
 int num = 25;
 
 
 if (num >5){
   System.out.println("num is greater than 5");
   if(num>10){
      System.out.println("num is larger than 10");
      if(num>20){
        System.out.println("num is larger than 20");       
      }
   }
  }else if(num<0){  
    System.out.print ("num is negative");
    
  }else{
    System.out.print ("num is ZERO (0)");
  }
 }
}

Answer the following:

1. What do you think it will print if num is 15

num is greater than 5

num is greater than 5, num is larger than 10

num is greater than 5, num is larger than 10, num is larger than 20

2. What do you think it will print if num is 20

num is greater than 5

num is greater than 5, num is larger than 10

num is greater than 5, num is larger than 10, num is larger than 20

3. What do you think it will print if num is 25

num is greater than 5

num is greater than 5, num is larger than 10

num is greater than 5, num is larger than 10, num is larger than 20

🧪 Try the code out~!

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Chapter summary

Key takeaways

1. if / else if / else runs exactly one branch — the first condition that is true, or the final else if none match.
2. Order matters: conditions are checked top to bottom, and the chain stops at the first match, so put the most specific checks first.
3. Nested ifs (an if inside another if's block) let you check a second condition only when the first already passed.
4. Leaving out the final else is fine if you genuinely have nothing to do for the "none of the above" case — but it is easy to forget a case by accident, so double check.

Exercises

Activity: Sign of a number

Read an integer. Print positive, negative, or zero.