Big Idea 1 Lesson 1.4

Popcorn Hacks

• showed popcorn hack 1.1
• popcorn hack 1.1, 2.1, 2.2 are verbal (no code cells needed)
• showed popcorn hack 3

int x = 10;
x = 20;
int y = x;

System.out.println(y);

20

import java.util.Scanner;

public class InputLesson {
    public static void main(String[] args) {
        System.out.println("=== User Information Program ===\n");
        Scanner sc = new Scanner(System.in);

        // String input
        System.out.print("Enter your full name: ");
        String name = sc.nextLine();
        
        // Integer input
        System.out.print("Enter your age: ");
        int age = sc.nextInt();
        
        // Double input
        System.out.print("Enter your GPA: ");
        double gpa = sc.nextDouble();
        
        // Display results
        System.out.println("\n--- Summary ---");
        System.out.println("Name: " + name);
        System.out.println("Age: " + age + " (next year: " + (age + 1) + ")");
        System.out.println("GPA: " + gpa);
        
        sc.close();
    }
}

Homework Hacks

• picked the second homework hack (as the lesson said to choose one)

Assignment Operator (=) vs Comparison Operator (==) in Java

In Java, the symbols = and == look similar but serve very different purposes.

1. Assignment Operator (=)

• Purpose: Assigns a value to a variable.
• Effect: Changes the value stored in that variable.

int x = 5;         
String y = "Hello"; 

System.out.println(x);  
System.out.println(y);  

5
Hello

2. Comparison Operator (==)

• Purpose: Compares two values for equality.
• Effect: Returns a boolean (true or false).

int a = 10;
int b = 20;

System.out.println(a == b);  
System.out.println(a == 10); 

String s1 = new String("Hello");
String s2 = new String("Hello");

System.out.println(s1 == s2);        
System.out.println(s1.equals(s2));   

false
true
false
true

3. Common Bug: Mixing up = and ==

• A common mistake is to use = when you mean ==.
• In Java, this won’t raise a syntax error inside if statements (unlike Python).
• Instead, it changes the variable and the condition may always evaluate as true or false.

int z = 3;

// incorrect usage (common mistake):
if (z = 5) {
    System.out.println("z is 5");
}

// correct usage:
z = 5;
if (z == 5) { 
    System.out.println("z is 5");
}

|   if (z = 5) {   // ❌ Error: incompatible types (int cannot be converted to boolean)

incompatible types: int cannot be converted to boolean