Questions and Answers
What is a class?
It is a template that defines the form of an object. It defines the data (variables) and code that acts on the data.
What is an object?
An instance (copy) of a class
What are the two basic parts of a class?
Instance variables and methods
What is another name for the code contained within a class?
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What are members of a class?
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What are instance variables?
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What is a key point for a well-designed class?
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What method is the starting point for any program?
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What is the general form of the dot operator?
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Given the piece of code shown below, in which line(s) is (are) an object(s) created? 1 class Vehicle { 2 int passengers; 3 int fuelcap; 4 int mpg; 5 } 6 class VehicleDemo { 7 public static void main(String args[]) { 8 Vehicle minivan = new Vehicle(); 9 Vehicle truck = new Vehicle(); 10 int range;
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What are the names of the instance variables in this class?
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Which lines represent the main program?
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Which lines represent the class?
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Should a class have many themes?
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What's the name of the class in the code above that represents the main program?
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How many classes are created when this program is compiled and what are the names of them?
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How do the contents of the variables of minivan compare to the contents of the variables of truck?
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Write a piece of code that creates an instance of Vehicle called sportscar
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Does the class in the code above have a method associated with it?
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Between which lines could you insert a method?
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Write a method called range that computes and prints out the range of the vehicle within the method itself.
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How many parameters does your method have?
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What is the return type of the method you just wrote?
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What two items are used to cause your method to end?
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Write a piece of code that invokes the range method you wrote on minivan.
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Where does execution resume after a call to a method?
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What would be the return type of the square root function you used before?
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What is the form of a piece of code to return a calculation from a method?
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Is the return type of a method important?
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What is an argument? For the code below, give an example. What does the program do? class Fact { boolean isFact(int a, int b) { if( (b % a) == 0) return true; else return false; } } class IsFact { public static void main(String args[]) { Fact x = new Fact(); if(x.isFact(2, 20)) System.out.println("Yes it is"); if(x.isFact(3, 20)) System.out.println("this won't be displayed"); }}
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What is a parameter?
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How many parameters does isFact have?
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Do parameters within a method have to be of the same type?
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What is a constructor and what is it used for?
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Study Notes
Classes and Objects
- A class serves as a blueprint for creating objects, defining data and methods.
- An object is an instance or copy of a class.
- Two fundamental components of a class are instance variables and methods.
- Instance variables represent the data attributes of a class.
Class Members
- Members of a class include its instance variables and methods, defining the state and behavior.
- The code within a class that performs actions is known as a method.
- A well-designed class encapsulates a single concept or entity.
Program Structure
- The main method is the entry point for execution in a Java program.
- Dot operator's general syntax is
Object.member
, used to access members of a class.
Code Examples
- In a defined class, objects are created with the syntax:
ClassName objectName = new ClassName();
- Example variables in a class
Vehicle
includepassengers
,fuelcap
, andmpg
. - The snippet of code provided creates instances of the
Vehicle
class at lines 8 and 9.
Class Relationships
- A class should focus solely on one theme to avoid complexity.
- In the provided code, the class representing the main program is
VehicleDemo
. - Compiling the program results in the creation of two class files:
Vehicle.class
andVehicleDemo.class
.
Independence of Objects
- Each object, like
minivan
andtruck
, has its own state; their variable contents are independent.
Methods and Parameters
- A method can be added between specified lines in the class to define behavior.
- An example method
range()
calculates a vehicle's range based on its variables. - The
range()
method has no parameters and returnsvoid
.
Execution Flow
- Execution resumes after a method call with the line following the method invocation.
- A method can return a value using the syntax
return value;
, with return types likeint
,double
, orboolean
.
Arguments and Parameters
- An argument is a value passed to a method, while a parameter is a variable defined in the method's signature that receives an argument.
- The provided example uses
2
and20
as arguments, determining if2
is a factor of20
.
Constructors
- Constructors are special methods with the same name as a class, responsible for initializing new objects and setting up initial conditions.
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