APCSA Unit 3: College Board Learning Objectives and Standards

Learning Objectives

The objective of today’s lesson is to…

Evaluate Boolean expressions that use relational operators in program code

Essential Knowledge

College Board wants you to know that…

Primitive values and reference values can be compared using relational operators (i.e., == and !=)
Arithmetic expression values can be compared using relational operators (such as <, >, <=, >=)
An expression involving relational operators evaluates to a Boolean value

Warm Up

Answer the following questions as a group or individually. Write down your answers in your hacks notebook.

What is a boolean? A boolean is a conditional that represents a true or false value
What values can a boolean represent? How many? A boolean can represent 2 values, true and false
What is an example of when we’d use a boolean? If we want to check if a number is greater than 50 we can use a boolean and set to true if this is the case

Recap on Booleans

A boolean is a type of variable that can evaluate to true or false. In Java, there are various comparison operators that can be used in order to compare two values. What are some?

The format is val1 <oper> val2, and will return a boolean given the state of the system. Like all booleans, this can only return two values true or false

public class Example {

    private static boolean isAdult = false;
    private static int myAge = 16;

    public static void main(String [] args){

        if(myAge >= 18){
            System.out.println("You are an adult!");
            isAdult = true;
        } else {
            System.out.println("You are not an adult!");
        }  

        System.out.println(isAdult);
    }
}

Example.main(null);

You are not an adult!
false

Comparison Operators

Whenever you want to make a boolean statement (such as setting a boolean to be true or false), you want to use the "=" or "!=" sign and not the "==" sign.

On the other hand, if you are ever trying to compare String objects, use the String methods such as .equals or .compareTo, NOT the Boolean expressions mentioned above.

Setup Class

Note the utilization of the compareTo in a custom class in order to compare two classes

public class ComparisonExample implements Comparable<ComparisonExample> {
    private int comp;      // First comparison variable
    private int comp2;     // Second comparison variable

    public ComparisonExample(int _comp, int _comp2) {
        this.comp = _comp;
        this.comp2 = _comp2;
    }

    // Override the compareTo method from the Comparable interface
    @Override
    public int compareTo(ComparisonExample s) {
        // Compare objects based on the 'comp' variable
        return Integer.compare(this.comp, s.comp);
    }

    // Custom comparison method for ComparisonExample objects
    public static int compare(ComparisonExample a, ComparisonExample b) {
        // If the 'comp' values are equal, compare based on 'comp2'
        if (Integer.compare(a.comp, b.comp) == 0) {
            return Integer.compare(a.comp2, b.comp2);
        } else {
            // If 'comp' values are not equal, continue comparing based on 'comp'
            return Integer.compare(a.comp, b.comp);
        }
    }
}

Comparing using compareTo

Describe this code and the code above using comments. Talk about how we define the compareTo in the class definition and how that compares each instance of a class using a certain comparison variables.

In your notes Compare and contrast Comparator<> and compareTo

compareTo is used for natural ordering within a class, while Comparator<> is used for custom comparisons and is more versatile when working with external classes or defining multiple comparison criteria.

ComparisonExample c = new ComparisonExample(2, 2);
ComparisonExample v = new ComparisonExample(2, 3);
System.out.println(c.compareTo(v)); // Allows comparison of these two instances of a class
System.out.println(ComparisonExample.compare(c, v)); // Also allows for the instances

if(ComparisonExample.compare(c, v) < 0){ // Using the comparison 
    System.out.println("This is less than!");
}
else { System.out.println("it is not"); }

0
-1
This is less than!

Comparing using .equals

You can use the .equals method to compare the contents of two strings. To test if the two strings are the same or not, you can use Sysout (System.out.println) so that the console will provide a value that is either true or false.

System.out.println("hello".equals("hello")); // will return true, since the strings are the same
System.out.println("Hello".equals("hello")); // will return false, as the one thing that separates these two strings is that the h is capitalized in the first string

true
false

Comparing two arrays

The code below compares the contents of two arrays to see if they are equal. It uses the equals method. However, as we see below, this is rather shallow as opposed to deep comparison.

  int arr1[] = { 1, 2, 3 };
  int arr2[] = { 1, 2, 3 };

  if (Arrays.equals(arr1, arr2)){

      System.out.println("Same");
  }
  else {


      System.out.println("Not same");
  }

Same

Deep Equality

All of the methods you just saw are examples of regular equality. Whereas regular equality methods such as the .equals method compares the content of the objects themselves to check if they are equal (i.e. point to the same object), deep equality methods such as the .deepequals method compares the content of nested objects or arrays to determine equality.

import java.util.Arrays;

public class DeepEquality {

    public static void main(String[] args) {
        
        String[][] fruit1 = {
            {"apple", "banana"},
            {"orange", "grape"}
        };

        String[][] fruit2 = {
            {"apple", "banana"},
            {"orange", "grape"}
        };

        // Regular comparison (compares array references, not contents)
        System.out.println("Regular/shallow equality result: " + (fruit1.equals(fruit2))); // will output false since the arrays do not reference the same object

        // Deep comparison (compares array contents, not just references)
        System.out.println("Deep equality result: " + (Arrays.deepEquals(fruit1, fruit2))); // will output true since the arrays have the same content
    }
}

DeepEquality.main(null);

Regular/shallow equality result: false
Deep equality result: true

Challenge!

Identify the issue(s) in the code below (hint: try running it yourself). Then, make the necessary corrections to ensure that the program runs as intended.

public class Challenge {

    private static boolean isName = false;
    private static String name = new String("John");


    public static void main(String [] args){

        Scanner sc = new Scanner(System.in);

        System.out.println("Guess my name!");

        String guess = sc.nextLine();
        System.out.println("Your guess: "+guess);

    
        if(guess.equals(name)){ 
            isName = true;
        } else {
            System.out.println("Wrong! L Cope");
        }

        System.out.println(isName);

    }
}

Challenge.main(null);

Guess my name!


Your guess: John
true

U3 Boolean and If statements • 16 min read

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