Unit 2: Using Objects – AP Computer Science A
🔑 Key Concepts
- Creating objects with
new
- Calling methods on objects
- String objects and their methods
- Wrapper classes (
Integer
,Double
, etc.) - Autoboxing and unboxing
📦 Object Instantiation
Objects are created using the new
keyword.
Scanner input = new Scanner(System.in);
Random rand = new Random();
📚 String Methods
Strings are objects with built-in methods.
String s = "APCSA";
System.out.println(s.length()); // 5
System.out.println(s.indexOf("C")); // 2
System.out.println(s.substring(1, 3)); // "PC"
Strings are immutable: methods return a new String, not modify the original.
📐 Wrapper Classes
Used to store primitives as objects.
Integer num = new Integer(5);
int x = num.intValue(); // unboxing
⚙️ Autoboxing & Unboxing
Java automatically converts between primitives and wrapper objects.
Integer a = 10; // autoboxing
int b = a; // unboxing
✅ Comparing Objects
Use .equals()
for objects (not ==
):
String a = "hello";
String b = "hello";
System.out.println(a == b); // false (sometimes true due to string pool)
System.out.println(a.equals(b)); // true
🧪 Practice Multiple Choice (5 Questions)
1. What is printed by the following?
String s = "Java";
System.out.println(s.substring(1, 3));
(A) “Ja”
(B) “av”
(C) “va”
(D) “Jav”
✅ Answer: (B)
2. Which of the following correctly creates a Scanner
object to read input from the keyboard?
(A) Scanner scan = Scanner();
(B) Scanner scan = new Scanner();
(C) Scanner scan = new Scanner(System.in);
(D) Scanner scan = System.in;
✅ Answer: (C)
3. What does the Integer
wrapper class allow you to do?
(A) Store multiple integers
(B) Convert an integer to a string
(C) Treat an int
as an object
(D) Use decimals with integers
✅ Answer: (C)
4. What will the following code print?
String x = "abc";
String y = "abc";
System.out.println(x == y);
(A) true
(B) false
(C) error
(D) depends on the compiler
✅ Answer: (A) (true because of string interning)
5. What will be the output?
Integer i1 = 100;
Integer i2 = 100;
System.out.println(i1 == i2);
(A) true
(B) false
(C) 100
(D) error
✅ Answer: (A) (cached for values between -128 and 127)