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? What is usually contained in the code block of a constructor?
To create and initialize new objects of that class—usually by assigning values to instance variables.
In a class, attributes are typically ___ and methods are typically ___.
private and public
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. A constructor Rectangle(5) would assign 5 to both length and height, but a constructor Rectangle(6, 7) would assign 6 to length and 7 to height.
___ refers to the attributes/methods belonging to the class itself, and ___ refers to the attributes/methods belonging to an instance of the class.
static and non-static
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 Student 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.
The class Book has two instance variables; a String title, and an int pages. Write a constructor for Book that takes in one parameter, String myTitle. It will set the number of pages to a default of 100.
public Book(String myTitle) {
title = myTitle;
pages = 100;
}
The name of a method you can include in your class in order to return a pre-defined string when an instance of the class is directly called (ex: print(object)).
toString
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.
Used to return the flow of control to the point immediately following where the method or constructor was called
return
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, the division results in a double.
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 with a parameter int "newLevel", and sets the instance variable level to that value.
public void changeLevel(int newLevel) {
level = newLevel;
}
Type of variable declared in the body of constructors and methods
local variable