Unit 1 Notes

Unit 1: Primitive Types – AP Computer Science A

🔑 Key Concepts

• Primitive Data Types: int, double, boolean, char
• Variable Declaration and Initialization
• Arithmetic Expressions
• Casting (Type Conversion)
• Constants (final keyword)

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📌 Primitive Data Types

Type	Example	Description
int	int x = 5;	Whole numbers
double	double y = 3.14;	Decimal numbers (floating point)
boolean	boolean done = true;	True or false values
char	char grade = 'A';	A single Unicode character

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🧠 Arithmetic Expressions

Basic math:

int x = 5;
int y = 2;
System.out.println(x + y); // 7
System.out.println(x / y); // 2 (integer division)

With doubles:

double a = 5;
double b = 2;
System.out.println(a / b); // 2.5

Operator Precedence (same as math):

1. () Parentheses
2. *, /, %
3. +, -

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🔄 Type Casting

Widening (safe): int → double

int i = 3;
double d = i; // 3.0

Narrowing (manual):

double pi = 3.14;
int approx = (int) pi; // 3

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✅ Constants

Declare a constant using final:

final double PI = 3.14159;
PI = 3.2; // ❌ Error: cannot assign a value to final variable

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💡 Tips

• Integer division truncates the decimal.
• % is the modulus operator (remainder).

  10 % 3 → 1
  8 % 2 → 0
  

• Use char for single characters only, " " for strings:

  char c = 'A';
  String s = "A";
  

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🧪 Practice Prompt

What will the following code print?

int x = 10;
int y = 3;
System.out.println(x / y);
System.out.println((double) x / y);

Answer:

3
3.3333333333333335

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🧪 Unit 1 Practice Multiple Choice (5 Questions)

1. What is the result of the expression 7 / 2 in Java?
(A) 3.5
(B) 3
(C) 4
(D) 3.0
✅ Answer: (B) — Integer division truncates the decimal.

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2. Which of the following is a valid declaration of a char?
(A) char c = "A";
(B) char c = 'A';
(C) char c = A;
(D) char c = 'ABC';
✅ Answer: (B) — A char holds one character in single quotes.

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3. What will be printed by the following code?

double x = 5;
int y = 2;
System.out.println(x / y);

(A) 2
(B) 2.0
(C) 2.5
(D) 2.00
✅ Answer: (C) — A double divided by an int results in a double.

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4. What keyword is used to define a constant in Java?
(A) const
(B) static
(C) final
(D) constant
✅ Answer: (C) — The final keyword is used for constants.

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5. What is the value of result in the following code?

int result = (int)(3.7 + 2.5);

(A) 6
(B) 6.2
(C) 7
(D) 5
✅ Answer: (A) — 3.7 + 2.5 = 6.2 → cast to int gives 6.