Data Types and Casting
What is the result of 7 / 2 in Java?
3. This is integer division, and the decimal is truncated.
What is the difference between a class and an object?
A class defines the blueprint (structure/behavior); an object is a specific instance of that class.
What is the result of "AP" + 5 + 3?
"AP53". Java evaluates left to right, so it concatenates "AP" with 5, then with 3.
What is the purpose of a constructor in a class?
To initialize new objects of that class—usually by assigning values to instance variables.
What does Math.sqrt(25) return?
5.0 (a double).
What is the result of (double) 11 / 2 and why?
5.5. Casting 11 to a double causes the division to be floating-point.
What keyword is used to define a method that belongs to a class rather than an instance?
static
What does "Computer".substring(1, 4) return?
"omp". Characters at index 1 to 3 (4 is exclusive).
What is constructor overloading? Give an example.
Having multiple constructors with the same name but different parameter lists.
How do you generate a random integer from 1 to 10 (inclusive)?
(int)(Math.random() * 10) + 1
Why must you cast a double to an int before assigning it to an int variable?
Because Java does not automatically convert from double to int due to potential loss of data. You must cast it explicitly using (int).
Given a class Student with a constructor Student(String name, int grade), write the code to create a new object named s1.
Student s1 = new Student("Alice", 11);
What is the output of the following code?System.out.println("The\tEnd");
The End
The \t inserts a tab space.
Write a constructor for a class Book that takes in a String title and sets a default number of pages to 100.
public Book(String myTitle) {
title = myTitle;
pages = 100;
}
What is the output of Math.pow(2, 3) and what type is it?
8.0 — the result is a double.
Is this declaration valid: int x = 3.14;? Explain why or why not.
Invalid. 3.14 is a double, and assigning it directly to an int without casting causes a type mismatch error.
What's the difference between an instance method and a static method in terms of how you call them?
Instance methods are called using an object: obj.method().
Static methods are called using the class name: ClassName.method().
Given a String s = "Jeopardy", what does s.indexOf("p") return?
3. Index starts at 0, and "p" is the 4th character.
Why will these two methods NOT overload?
void doSomething(int x)
void doSomething(int y)
They have the same parameter type and order. The parameter names do not matter, so they are considered duplicates.
Write an expression to generate a random integer from 5 to 15.
(int)(Math.random() * 11) + 5
What is printed by the following code?
int a = 5;
double b = 2.0;
System.out.println(a / b);
2.5. Since b is a double, a is promoted, and floating-point division occurs.
Write a class method named getAv that takes in two int values: score and total, and returns the percentage score as a double.
public static double getAv(int score, int total) {
return (double) score / total * 100;
}
Write a class method named comb that takes in two Strings, returns a new String consisting of the first 2 letters of the first String and the last 2 of the second. Use substring.
public static String comb(String a, String b) {
return a.substring(0,2)+b.substring(b.length()-2);
}
Assume a class contains an instance variable "level". Write an instance method called changeLevel that takes an int and sets the instance variable level to that value.
public void changeLevel(int newLevel) {
level = newLevel;
}
You are writing a method that simulates rolling two dice. Show how to generate the two values using Math.random().
int die1 = (int)(Math.random() * 6) + 1;
int die2 = (int)(Math.random() * 6) + 1;