Unit 2 - Using Objects¶

2.2 Creating and Storing Objects¶

Explain the relationship between a class and an object
Identify different constructor signatures and create objects using them
Use variables to represent object references
- I will be able to write more complex programs using classes and objects

The next two concepts will be used in most of your programs moving forward. Read through constructor and toString() below.

2.3 Calling a Void Method¶

Call non-static void methods without parameters
- I will be able to write and identify a non-static void method program

Class methods represent an object’s behavior, defining what an object can do within a program.

Recall the Child program created above. We will use the one constructor that has no defined parameters to create behaviors of our object.

class Child{

  String name;
  Integer age; //Integer is a wrapper class for the int data type

  Child(){
    System.out.println("I am a child."); // constructor without parameters
    }
  Child(String n, Integer a){ //constructor with parameters
    this.name=n;
    this.age=a; //'this' is a Java keyword and refers to a specific instance of the variable
  }
   public void running(){
     System.out.println("I am running!");
  }
     public void sleeping(){
     System.out.println("Zzzzzzz!");
  }
     public void singing(){
     System.out.println("Lah-la-la-LAH-la-la!");
  }
  
  
//Over-riding the toString function as a class function
  public String toString(){
    return "The name of the child is " + this.name + ". The age of the child is " + this.age + ".";
  }
}

class Main {
    public static void main( String args[] ) {
      Child a = new Child(); //constructor - no defined parameter
      Child p = new Child("Francis",10); // overloaded constructor - adding one or more parameter to the constructor 
      
      //Calling the class version of toString()
        System.out.println(p.toString());
        
      //Calling the original toString()
      System.out.println(Integer.toString(12));
      a.running(); // running method
      a.sleeping(); // sleeping method
      a.singing(); //singing method
    }
}

      

Sample output

I am a child
The name of the child is Francis. The age of the child is 10.
12
I am running!
Zzzzzzz!
Lah-la-la-LAH-la-la!

The void keyword will not return anything from the method declaration. While the void method of the class does not return any value, the constructors return an object reference. Thus, they can be called as a part of an expression. Explained another way, void means that nothing is returned from a method, nor will anything ever be returned. It is an absence of any value.

NOTE: ‘Null’ is a place holder for a reference that does not have an object. If defines a memory address that may eventually hold a value.

2.4 Calling a Void Method with Parameters¶

Call non-static void methods with parameters
Use overloaded methods
- I will be able to write a program using overloaded methods

Using parameters allows us the freedom to write a program that is more flexible and specific. It is similar to providing specific instructions for the program. When you pass multiple parameters, it is important to make sure that the order is correct. Incorrect order will return incorrect results. As we did with constructors, we can overload a method. Recall that the method name has also an ordered list of parameters. You can also overload a method by giving the method the same name as another method, and provide a different set of parameters.

For example:

public void school(int grade, double gpa) {}
public void school(int grade, double gpa, String name) {}

Activity 2.4.1¶

Instructions DigiPet You will create a class DigiPet

Create a DigiPet class with the following attributes:
- animal
- name
- food
- energy
- happy
Create a constructor with variables for animal and name. food, energy, and happy should be set to 20.
In the main class, create two pets with their name and print them out.
Create the following void methods according to these rules:

speak() - If your pet is a dog, print out “woof! woof!”. If you choose another animal follow the format of the dog.
play() - print out “Do you wanna play?”, increase happy by 10, decrease food and energy by 10.
feed() - If food is greater than 15, print out the pet is not hungry and decrease happy by 10. Otherwise, print “Yay! Food!”, increase food and energy by 10.
sleep() - if energy is 20 or more, print out the pet is not sleepy and decrease happy by 10. Otherwise, print “zzzzzzzzzzzzzzz!”, increase happy by 15 and set energy to 50.

Call each method according to the sample output below.

Sample Output:

Hangry the cat
Hunger: 20
Energy: 20
Happy: 20

Fido the dog
Hunger: 20
Energy: 20
Happy: 20

Purr! Purr!
Bark! Bark!
Yay! Let's play!
Hangry's not sleepy!
zzzzzzzzzzzzzzz!
Hangry's not hungry!
Yay! Food!

Hangry the cat
Hunger: 20
Energy: 20
Happy: 0

Fido the dog
Hunger: 20
Energy: 60
Happy: 45

2.5 Non-Void Methods¶

If you’ve programmed in Python, you may remember that we used functions to execute a small chunk of code in other areas of the program. Non-Void methods are very similar to functions in that they return a value of some type. You have used non-void methods already, you just may not have recognized it. For example, the Scanner Class has non-void methods like nextInt(), nextDouble(), nextLine(), and next(). There are a number of methods that can be found in the Java standard library methods that are non-void methods. Take a look at the example below.

public int sum(){
   return num1 + num2;
}

The keyword return is required in all non-void methods. Since the method sum has a return type of int the return line is required to return a value of this type.

2.5 Calling a non-void method¶

Call a non-void method.
Use a return value from a non-void method in a variable or as part of an expression.
- I will be able to write a program that utilizes a program using a non-void method

The way that a non-void method is called differs from the way that a void method is called in that the call is made from within other Java statements. Since a non-void method always returns a value, this value has to be stored in a variable, printed, or returned to a Java control structure or another method. Here are some examples.

The method call can be in an assignment statement.

int num = sum();
double avg = calculateAverage();
String name = getName();

The method call can be in a print or println statement.

System.out.println("The sum is " + sum());
System.out.println("Avg = " + calculateAverage();
System.out.println(getName() + " is my name");

The method call can be inside a Java structure such as a conditional statement or a looping structure.

if(sum() > 500)
if(calculateAverage() == 100)
if(getName().equals("John Wang"))
while(!getName().equals("stop")

The method call can be on the return line of another method.

return sum() / numItems;
return calculateAverage() > 70;

Activity 2.5.1¶

Instructions Shape using Non-Void Methods

In the Shape class, the attributes and constructor is complete. Look them over.
In the Main class, instantiate the three objects described in the comments
In the Shape class, write the following methods:
- getPerimeter() - calculates and returns perimeter depending on the shape. The perimeter of a circle is 2*3.14*length
- getArea() - calculates and returns area depending on the shape. The area of an equilateral triangle is 0.25*1.732*length*length. The area of a circle is 3.14*length*length
In the Main class, follow the comments and call the appropriate methods.

Assignment1_Shape.zip

Sample Output

Perimeter of Triangle: 18.0
Area of Triangle: 15.588

Perimeter of Square: 20.0
Area of Square: 25.0

Perimeter of Circle: 50.24
Area of Circle: 200.96

Submit your link to the finished program

Activity 2.5.2¶

Instructions RoadTrip using Non-Void Methods

Starter file: 125-RoadTrip.zip

In the Car class, the attributes and constructor are complete. Look them over.
In the Main class, create car object that has 20 mpg and a 15 gallon tank
In the Car class, create the following methods:

odometer() - prints out how many miles driven like this: Miles Driven: 300.0
milesAvailable() - prints out how many miles the car can drive on that much gas. For example: Miles Available: 240.0
addGas() - This should add amount to the tank. If the amount is greater than the capacity, set the gas to the tank capacity. The method should print Adding gas ...
drive() - checks if car can drive that number of miles, and then adds miles to totalMilesDriven and subtracts the amount of gas needed to go those miles. The method should print Driving [distance] or, if the distance is too far, it should print Can’t drive [distance]. That’s too far!

In the Main class, follow the comments and call the appropriate methods.

Sample Output:

Miles Driven: 0.0
Miles Available: 0.0
Adding gas ...
Miles Available: 300.0
Driving 100.0
Miles Available: 200.0
Adding gas ...
Miles Available: 240.0
Can't drive 365.0. That's too far!
Driving 200.0
Miles Driven: 300.0

Submit your link to the finished program

2.6 String Objects: Concatenation, Literals, and Other Things¶

Create String objects
Reference String objects
Understand that in java, String objects are immutable and can only be called by reference
- I will be able to write a program using String objects

String objects can be instantiated in two different ways, either by assigning a String literal directly to a String variable or by calling the String class constructor. In the example below, the code on the left uses a String literal assignment, and the code on the right uses the String class constructor.

String literal assignment	String class constructor
String state = “Maine”;	String state = new String(“Maine”);

String objects have the benefit of many methods, one such method is concat. Concatenation means appending one string to the end of another and is the function performed by the concat method. String objects are immutable. This means that you cannot change the String object. In this case, the program created a String object with a specific value. When the concat() method is called, it creates a new String object in memory with the new value. This does not replace the original reference.

concat() vs. the + Operator for Concatenation

concat()	+ Operator
Suppose str1 is `null` and str2 is “Java”. Result of `str1.concat(str2) will throw a NullPointerException.	Assume str1 is `null` and str2 ia “Java”. Result of `str1 + str2` returns “nullJava”.
You can only pass a String to the `concat()` method.	If one of the operands is a string and the other is a non-string value, then the non-string value is converted to a string before concatentation. For example, `"Java" + 5` will return `"Java5"`.

Example

public class Main 
{
  public static void main(String[] args) 
  {
    String str1 = "My string";
    // The string str1 does not change even though it is being invoked by the method.
    // concat(), as it is immutable must have an explicit assignment to produce the desired outcome.
    str1.concat("is immutable");
    System.out.println(str1);
    str1 = str1.concat(" is immutable, so I have to assign it explicitly here.");
    System.out.println(str1);
  }
}

Output

My string
My string is immutable, so I have to assign it explicitly here.

Example 2 Take a look at the following program:

public class Main {
    public static void main(String[] args) {
	String str1 = "Hello ";
	String str2 = "neighbor, ";
	String str3 = "How are you?";
	// concatentating 1 string
	String str4 = str1.concat(str2);
	System.out.println(str4);
	//concatenating more strings
	String str5 = str1.concat(str2).concat(str3);
	System.out.println(str5);
    }
}

Output

Hello neighbor,
Hello neighbor, How are you?

public class Main {
    public static void main(String[] args) {
	String str1 = "Coyotes";
	String str2 = "Our mascot is the ".concat(str1);

	System.out.println(str2);
    }
}

Our mascot is the Coyotes

Using Notepad: Copy and complete the code below using each of the concatenation operators where noted in the comments. When the program runs, it should display Whoopie Pie and Wild Blueberry.

public class Main
{
  public static void main(String[] args) 
  {
    String stateTreat = "Whoopie";
    String stateBerry = "Wild ";
    
    /* Use the + operator on stateTreat to append " Pie" */
    
    
    
    /* Use the += operator on stateBerry to append "Blueberry" */
    
    
    
    System.out.println(stateTreat);
    System.out.println(stateBerry);
  }
}

NOTE Java allows for implicit type conversion during concatenation. This means that even if a primitive is stored as a primitive type (be it int or double), it can still be concatenated to a String. The int is converted to a String before the + operator is applied.

NOTE Certain characters need special treatment to be included in strings in Java. Either they are “non-printable,” such as the character that indicates a new line of output (called the newline character), or they already have other meanings in the Java language.

Escape sequences begin with the \ character (back slash as opposed to a forward slash) and allow a programmer to type these characters in String literals. Take a look at the examples below.

Escape Sequence

newline	backslash	single quote	double quote	tab	backspace
`\n`	`\\`	`\'`	`\"`	`\t`	`\b`

public class Main
{
  public static void main(String[] args) 
  {
    // Notice a new line in your code does not mean
    // a newline in your output, line breaks after
    // operators are allowed
    String stateSong = 
      "\"Oh, Pine Tree State,\n Your woods\\ fields\\ and hills\\\"";
    System.out.println(stateSong);
  }
}

Output

“Oh, Pine Tree State,

Your woods\ fields\ and hills”

Create a new java file.
- Research a College Fight Song to use for this assignment.
Implement the following features (in any order) and comment where they occur.

A String created using a literal
A String created using the String class constructor
An immutable String
An implicit conversion via concatenation
The += operator for concatenation
The + operator for concatenation
Three escape sequences

Submit your program link. Do not send me a screen shot of your program.
Comment your code showing that you completed each of the bullet points above.

2.7 String Methods¶

Compare String Objects
Use String Methods
Using Java API Specification
Using ASCII table within Java
- I will be able to write a program that utitlizes String methods

Navigate to and review the information that can be found at Java API Specification.

Navigation
Select java.base > java.lang
Scroll through Class Summary until you find `String`
Review String class description
Click on `String` class link to explore the class

Click on ‘Constructor Summary’ to explore how you can call the String Constructor
Click on ‘Method’ to view the Method Summary

Note

Application programming interfaces (APIs) simplify complex programming tasks.
In APIs, classes are grouped into packages, also called modules.
The String class is part of the java.lang package. Classes in the java.lang package are available by default.
Documentation for APIs and libraries is essential to understanding the attributes and behaviors of a class and its methods.

String Class Constructors and Methods	Explanation
`String(String str)`	Constructs a new String object that represents the same sequence of characters as str.
`int length()`	Returns the number of characters in a String object.
`String substring(int from, int to)`	Returns the substring beginning at index from and ending at index to – 1.
`String substring(int from)`	Returns substring(from, length()).
`int indexOf(String str)`	Returns the index of the first occurrence of str; returns –1 if not found.
`boolean equals(String other)`	Returns true if this is equal to other; returns false otherwise.
`int compareTo(String other)`	Returns a value < 0 if this is less than other; returns zero if this is equal to other; returns a value > 0 if this is greater than other.
`String startsWith(String str)`	The method checks whether a string starts with the specified character(s).
`String equalsIgnoreCase(String str)`	The method compares two strings, ignoring lower case and upper case differences.

Take a look at the code below:

     String horse = "percheron";
     System.out.print("Percheron is ");
     //
     System.out.print(horse.length());
     //
     System.out.println(" letters long.");

Output

Percheron is 9 letters long.

Use the program as a reference, create a program that:

Creates a String of length three that references the String literal “cat”
Calls the length method of that object, storing the result in a variable of type int
Displays the value store in that variable
Concatenates a String of your choice to the end of “cat”
Displays the new length of the String

String substring(int from, int to) - Returns the substring beginning at index from and ending at index to -1.

5973746a769a40f78540dfbd24529a0c

Example 1

public class Main
{
  public static void main (String[] args)
  {
    String pangram = "The quick brown fox jumps over the lazy dog";
    System.out.println(pangram.substring(4, 19));
  }
}

Sample Output quick brown fox

public class Main
{
  public static void main (String[] args)
  {
    String pangram = "The quick brown fox jumps over the lazy dog";
    System.out.println(pangram.substring(41, 44));
  }
}

Note A String object has index values from 0 to length – 1. Attempting to access indices outside this range will result in an IndexOutOfBoundsException.

Sample Output

Exception in thread "main" java.lang.StringIndexOutOfBoundsException: String index out of range: 44
	at java.lang.String.substring(String.java:1963)
	at IndexTester.main(IndexTester.java:9)

How would you fix this error above?

class Main {
  public static void main(String[] args) {
	String pangram = "The quick brown fox jumps over the lazy dog";
    System.out.println(pangram.indexOf("b")); 

  }
}

Output 10

class Main {
  public static void main(String[] args) {
	String pangram = "The quick brown fox jumps over the lazy dog";
    System.out.println(pangram.indexOf(44)); 

  }
}

Output -1

public class Main
{
  public static void main(String[] args)
  {
    String placeName = "New Brunswick";
    /* 	What is the length of placeName?
	What is the indexOf "N"?        */
    System.out.println();
  }
}

What would go within the println statement?

public class Main
{
  public static void main(String[] args){
    String word1 = "motorcycle";
    String word2 = "recycle";
    
    String word1part2 = word1.substring(5);
    String word2part2 = word2.substring(5);
    System.out.println(word1part2.equals(word2part2));
  }
}

How would you modify the calls to substring so that the program below will print true.

compareTo
int compareTo(String other)	Returns a value < 0 if this is less than other; returns zero if this is equal to other; returns a value > 0 if this is greater than other.

public class Main
{
  public static void main(String[] args)
  {
    String string1 = "park";
    String string2 = "parkway";
    
    System.out.println(string1.compareTo(string2));
  }
}

What is the output of this program?

Answer

-3

What is the output if String1 was parkway and string2 was park?

Answer

3

What is the output of String1 and String2 if they are both ‘parkway’?

Answer

0

boolean equals
boolean equals(String other)	Compares the current string to the specified other string and returns true if they are equal, false otherwise. Given the following code segment, predict whether the output will be true or false.

String word1 = "tricycle";
String word2 = "recycle";
System.out.println(word1.equals(word2));

String Substring
String substring(int beginIndex)	Returns a string that is a substring of the current string. The substring begins with the character at the specified beginIndex and extends to the end of the current string.

Given the following code segment, predict whether the output will be true or false.

String word1 = "tricycle";
String word2 = "bicycle";

String word1part2 = word1.substring(5);
String word2part2 = word2.substring(5);
System.out.println(word1part2.equals(word2part2));

Take a look at this ASCII table: ascii-table-characters.pdf

Experiment with the program above by changing the values of the two Strings until you can answer the questions that follow definitively. Use the ASCII table resource to help guide your exploration.

Which is greater, a capital letter or a lowercase letter?
Which is greater given two Strings that vary only in their last character?
Which is greater, a String beginning with a number, or one that begins with a letter?
When two Strings begin with the same substring and one is longer, which is greater?

String Objects Practice: Activity 126 String Objects_Vers1.docx

String Object Practice Part II: Activity 126 String Objects_Vers2.docx

Activity 2.7.1 - Wordplay¶

Starter Files: WordPlay.zip

You will write a program that asks the user to enter their first name and their last name, separated by a space. Then it asks the user to enter a phrase. You will write and call the following:

Methods
getFirstName()	returns the first name from the full name of user
getLastName()	returns the last name from the full name of user
getInitials()	returns the initials from the full name of user
makePassword()	returns a password created by the following: length of first name + first half of first name + last name + last half of last name + length of last name
ceeBee()	returns the phrase with all the c’s replaced by b’s

Hint: You may want to use API documentation to understand: split(), charAt(), substring(), replace()

Sample Output:

Enter your first name and last name, separated by a space: 
Bernard Heally
Enter a phrase: 
Give papa a proper cup of coffee in a copper coffee cup.

Your first name is Bernard
Your last name is Heally
Your initials are BH

Your new password: 8BernHeallylly5

Give papa a proper bup of boffee in a bopper boffee bup.

Submit your repl.it file on Canvas.

Activity 2.7.2 - Speaking¶

Starter files: Speaking.zip

Create a program that will ask the user for their name and their partners name. Create the following:

Methods
yelling()	returns a name in all caps
whisper()	returns a name in all lower case
pigLatin()	takes a word and take the first letter and put it at the end of the word and add the letters “ay” to the end. For example, “pig” becomes “igpay”
nameJoin()	takes two parameters, name1 and name2, and combines them by taking the first half of name1 and second half of name2 and combine them

Sample Output

Enter your first name:
 Samantha
Enter your partner's name:
 Russell

Yelling your name: SAMANTHA
Whispering your partner's name: russell
Your name in Pig Latin: amanthaSay
Your partner's name in Pig Latin: ussellray
Your team name is Samaell

2.8 Wrapper Classes - Integer and Double¶

Convert between primitive data types and objects.
- I will be able to identify a wrapper class
- I will be able to convert primitives to an object of a wrapper class
- I will be able to identify autoboxing/ unboxing of primitive and reference types
I will be able to write programs using wrapper classes

NOTE: Wrapper classes have been deprecated as of Java 9. If you use this in replit, it will notify you that it has been depracated. For more information click here.

Recall that primitive data types are int, double, boolean & char. A wrapper class is a class that allows you to extend a primitive type into an object. A wrapper class contains a single attribute that is its primitive type. For example, Integer is a wrapper class for primitive type int. For every Wrapper Class there is a corresponding primitive type . Integer belongs to the Java.lang package that can be found in the API documentation that we reviewed in a prior lesson.

What are the advantages of using Wrapper Classes

They convert primitive data types into objects. Objects are needed if we want to modify arguments passed to a method.
The classes in java.util package only handles objects thus wrapper classes are appropriate to use.
Data structures such as an ArrayList, store only objects and not primitive types.
We can call multiple methods like compareTo(), equals(), toString()

Wrapper Classes

Primitive Data Type	Wrapper Class
byte	Byte
short	Short
int	Integer
long	Long
float	Float
double	Double
boolean	Boolean
char	Character

Integer Class Constructors and Methods	Explanation
Integer(int value)	Constructs a new Integer object that represents the specified int value.
Integer.MIN_VALUE	The minimum value represented by int or Integer.
Integer.MAX_VALUE	The maximum value represented by int or Integer.
int intValue()	Returns the value of this Integer as an int.

The program below retrieves the value wrapped by an Integer and stores it in a primitive int before using that value. Modify the program so that it stores the value of int1 in int2.

public class Main
{
  public static void main(String[] args)
  {
    Integer int1;
    Integer int2;
    int primitiveInt;
    
    int1 = new Integer(20);
    int2 = new Integer(int1.intValue()); // solution 1
    int2 = int1.intValue(); // solution 2
    
    System.out.println("int2 value is: " + int2.intValue());
  }
  
}

Sample Output int2 value is: 20

Double Class Constructors and Methods	Explanation
Double(double value)	Constructs a new Double object that represents the specified double value.
double doubleValue()	Returns the value of this Double as a double.

public class Main
{
  public static void main(String[] args)
  {
    Double d = new Double(1.5); // instantiates a Double
    // Uses the doubleValue method in a print statement
    System.out.println("d value is: " + d.doubleValue()); 
    
  } 
}

Sample Output d value is: 1.5

Autoboxing/ Unboxing In the program below the JVM did some extra work at lines 5 and 6. Java automatically converted from the int and double primitive types to their corresponding object reference types. This is called autoboxing, an automatic conversion that the Java compiler makes between primitive types and their corresponding object wrapper classes.

public class Main
{
 public static void main(String[] args)
 {
   Integer integerInstance = 5;
   Double doubleInstance = 2.0;
   // Newlines are allowed around operators
   System.out.println("integerInstance: " + integerInstance.intValue() +
                      " doubleInstance: " + doubleInstance.doubleValue());
}
}

Note: Autoboxing is the automatic conversion that the Java compiler makes between primitive types and their corresponding object wrapper classes, including int to Integer, double to Double, and boolean to Boolean.

The program below demonstrates the opposite of autoboxing called unboxing. Java automatically converted the Integer reference types to its corresponding int primitive types.

public class Main
{
  public static void main(String[] args)
  {
    unboxInt(new Integer(42));
    unboxDouble(new Double(3.14159));
    unboxBoolean(new Boolean(true));
  }
  
  public static void unboxInt(int i)
  {
    System.out.println(i);
  }
  
  public static void unboxDouble(double d)
  {
    System.out.println(d);
  }
  
  public static void unboxBoolean(boolean b)
  {
    System.out.println(b);
  }
}

Sample Output

42
3.14159
true

Note: Unboxing is the automatic conversion that the Java compiler makes from the wrapper class to the primitive type. This includes converting an Integer to an int and a Double to a double.

The Java compiler applies unboxing when a wrapper class object is: i. Passed as a parameter to a method that expects a value of the corresponding primitive type. ii. Assigned to a variable of the corresponding primitive type.

In the program below, which line of code contains an example of autoboxing? In the program below, which line of code contains an example of unboxing?

public class Main
{
 public static void main(String[] args){
  Integer cardboardWidth = new Integer(4);
  Integer cardboardHeight = new Integer(5);
  
  Box cardboardBox = new Box(cardboardWidth, cardboardHeight);
  cardboardBox.changeDimensions(2,3);
  System.out.println("Ending box dimensions: " + cardboardBox.getWidth() 
                     + " x " + cardboardBox.getHeight());
}
12
private static class Box
{
  private int width, height;
  
  public Box(int w, int h){
    width = w;
    height = h;
  }
  
  public void changeDimensions(Integer w, Integer h){
    width = w.intValue();
    height = h.intValue();
  }
  
  public int getWidth(){
    return width;
  }
  public int getHeight(){
    return height;
  }
}
}

Answer

Unboxing occurs on line 7 when Integer parameters are automatically converted to ints in the constructor.

Autoboxing occurs on line 8 when int parameters are automatically converted to Integers in the method.

Sample Output

Ending box dimensions: 2 x 3

Activity 2.8.1 Wrapper Classes¶

Create a program that contains an example of each of the following. Add in-line comments to describe each example.

2.9 Using the Math Class¶

Use methods from the Math package
- I will be able to write programs using math methods
- I will write a segment of code that will convert an annuity into java
Incorporate randomness into a program
- I will be able to write a program using random method

The Math class can be found in the java.lang package. The Math class contains only static methods. Some of the more commonly use Math methods are listed below.

Oracle Java Methods & Classes

Navigate to and review the information that can be found at Java API Specification.

Navigation
Select java.base > java.lang
Scroll through `Class Summary` until you find `Math`
Review Math class description
Click on `Math` class link to explore the class

Click on ‘Method Summary’ for All Methods to explore how you can call the implement a method
Click on a ‘Method’ to view the Method Summary

Math Class Methods	Explanations
int abs(int x)	Returns the absolute value of an int value
double abs(double x)	Returns the absolute value of a double value
double pow(double base, double exponent)	Returns the value of the first parameter raised to the power of the second parameter
double sqrt(double x)	Returns the positive square root of a double value
double random()	Returns a double value greater than or equal to 0.0 and less than 1.0
round()	Returns the round of the decimal number to the nearest value
ceil()	Returns a double greater than or equal to the original number
floor()	Returns a double less than or equal to the orignal number

Math Method: Primitive Type Returned

Math.round:

Math.round is designed to round a floating-point number to the nearest integer. Its purpose is to convert a floating-point value to an integer value, effectively removing the decimal part. Rounding to the nearest integer makes sense because it’s often used in scenarios where you need an integer result. For example, if you have a floating-point number that represents a measurement and you want to round it to the nearest whole number, Math.round is a convenient function to use.
The return type of Math.round is long, but if you want an integer result, you can cast it to int or store it in an int variable. This is because rounding can result in a value that may not fit into the range of a 32-bit integer, so it’s safer to return a long.

Math.sqrt:

Math.sqrt is designed to calculate the square root of a number, which may not always result in an integer value. In many cases, the square root of a number is a decimal value (a double in Java). This is because not all numbers have integer square roots. For example, the square root of 25 is exactly 5, but the square root of 26 is approximately 5.09901951359.
To accommodate the fact that square roots can have decimal parts, Math.sqrt returns a double by default to provide a precise and flexible result. If you know that the input value will always have an integer square root and you want the result as an integer, you can cast the result to an int.

Math.ceil:

Math.ceil is a method in the Math class that is used to round a floating-point number up to the nearest integer greater than or equal to the original number (the “ceiling”). This means that it always returns a double value. The result will have a decimal part of 0.0 if the input is already an integer.

Math.floor:

Math.floor is another method in the Math class, and it is used to round a floating-point number down to the nearest integer less than or equal to the original number (the “floor”). Like Math.ceil, Math.floor also returns a double value.

Math.round

Round is a another method in the Math class which returns the closest long to the argument. The result is rounded to an integer by adding 1/2, taking the floor of the result after adding 1/2, and casting the result to type long. If the argument is NaN, the result is 0.

Having seen the Math method abs method in use, refer to the Java Quick Reference to predict how you could use the pow method to raise a base of 2 to the exponent 10 (2¹⁰ ).

Answer

Math.pow(2, 10)

How would you write the sqrt method to get the square root of 9.

Answer

Math.sqrt(9)

How would you write the round method to get the round of 79.52.

Answer

Math.round(79.52) 

The random Method

For programs that require some amount of randomness, such as games, security, simulations, and statistics, Java’s Math class provides a random method. This method returns a double value that falls in the range from 0.0 to 1.0 non-inclusive, expressed mathematically as (0.0, 1.0] to indicate that it includes 0.0 but that its highest value is .999999999999999. With random values from 0 to .99…, you can mathematically generate any random number you may need. As you will see, you can manipulate the results in a variety of ways to fit your needs.

public class Main
{
  public static void main(String[] args)
  {
    int rnum = (int) (Math.random() * 24 );
    // This program produces a random integer between 0 and 23 inclusive.
    System.out.println(rnum);
  }
}

You can also create a range using the random method in the formula below.

Math.random() * n¹ + m²

¹ = Number of integers included in range

² = Lowest integer in range

Use the Math.random method to generate a double value greater than or equal to 10.0 (inclusive) and less than 15.0 (exclusive).

public class Main
{
  public static void main(String[] args)
  {
    System.out.println(Math.random() * 5 + 10);
  }
}

How would you modify a program to produce int values between 70 inclusive and 100 exclusive?

System.out.println(Math.random() * 30 + 70);

As stated above, Math methods are static. This means that you do not use an object reference like you did with non-static methods (cat.length()). Instead, you reference the class name itself followed by the static method. Some examples are listed below.

Math Method Examples
Math.abs(-5)
Math.abs(2.3)
Math.sqrt(4)
Math.random()

If you were to write your program using a constructor and reference the object your program will return an error.

Math m = new Math();
m.random();

Take a look at the program below.

public class Main
{
  public static void main(String[] args)
  {
    int path1 = -1; // The car drives West 1 block
    int path2 = -1; // The car drives West 1 block
    int path3 = -1; // The car drives West 1 block
    int path4 = 3; // The car drives East 3 blocks
    // Added calls to Math.abs in the print statement below.
    System.out.println(Math.abs(path1) + Math.abs(path2) + Math.abs(path3) + Math.abs(path4)); 
  }
}

Review the Math Class Methods in the table above and determine the correct syntax to execute each print statement.

public class MathPractice
{
public static void main(String[] args)
{
  // Find 8x8x8x8
  System.out.println(     );
  // Find the square root of 64
  System.out.println(     );
  // Find the square root of 65
  System.out.println(     );
  // Find the square root of the absolute value of the difference
  // between 12 squared and 5 squared.
  System.out.println(     );
}
}

8x8x8x8 answer

System.out.println(Math.pow(8, 4));

square root of 64 answer

System.out.println(Math.sqrt(64));

square root of 65 answer

System.out.println(Math.sqrt(65));

difference between 12 squared and 5 squared answer

System.out.println(Math.sqrt(Math.abs(Math.pow(12, 2) - Math.pow(5, 2))));

Activity 2.9.1 - Math Class Worksheet 1 & 2¶

In pairs, work on worksheet 1 in class. We will review your work in class.

Download: mathclassworksheet1.docx

ascii Table: ascii-table-characters.pdf

Complete worksheet 2 for homework. We will review your work next class.

Download: mathClassWorksheet2.docx

An example of single vs double quotes

Click Here

	
class Main {
  public static void main(String[] args) {
    System.out.println( "a" + (char)1 ); // output is a
	System.out.println( "a" ); // output is a
	System.out.println( 'a' + (char)1 ); // output is 98
	System.out.println( "a" + 1 ); //output a1
	System.out.println( 'a' + 1 ); //output 98
  }
}

Prefix & Postfix Operators

Click Here

++ and - - used as a prefix or as a postfix

Assume for what is being discussed that what works for increment operators(++) will also work with decrement operators(--). I will only use increment operators for the sake of brevity.

If you use the ++ operator as a prefix like: ++var, the value of var is incremented by 1; then it returns the value. If you use the ++ operator as a postfix like: var++, the original value of var is returned first; then var is incremented by 1.

class Main {
    public static void main(String[] args) {
        int var1 = 5, var2 = 5;

        // 5 is displayed
        // Then, var1 is increased to 6.
        System.out.println(var1++); //output is 5

        // var2 is increased to 6
        // Then, var2 is displayed
        System.out.println(++var2); //output is 6
		System.out.println(var1); // output is now 6
		System.out.println(var2); // output is still 6
    }
}

As a wrap-up, consider the ++ operator as:

x = x++; //Post-increment
x = ++x; //Pre-increment

Activity 1.2.9 - Race Car¶

Starter Files: Activity129RaceCar.zip

Complete the computeTime method and the comments in main.java

	//Length of the track in meters
        double distance = 2414; // ~ 1.5 miles
        
        // Generate a random acceleration (integer) 
        // for each car from 20 - 50 (inclusive)

        // Create two Racecar objects

        // Compute the finishing times for both cars
        // Print times of each car

You will need to complete the methods and toString within Racecar.java

Free Response Question (FRQ)

Free Response Question

Students will work in pairs during this activity.

In this part of the activity, you and a partner will answer your first Free Response Question (FRQ). FRQs define a set of requirements that an algorithm must meet. You provide answers to the FRQ in the form of handwritten Java code.

FRQs are also great ways to apply the knowledge and skills you have learned. They are also on the College Board’s AP exam, representing approximately 45 percent of the Exam. You should practice hand writing your answers to FRQs since this is the format you will use on the exam.

As to the structure of FRQs, the one you will complete now is a shortened version of an FRQ you might see on an exam. Other FRQs in this course will be longer and more like the FRQs you will see in an exam. In general, FRQs describe a scenario and specific requirements that your code must meet. Sometimes FRQs provide code to enhance. They can also omit irrelevant code, using comments similar to /* implementation not shown */.

Your first FRQ is titled Annuity. It requires you to write expressions using a Math method you learned in this activity and to use a return statement to return a value from a method.

Your teacher will distribute the Annuity FRQ. Work together with a partner to write your solution.

Once you complete the FRQ, ask your teacher for the scoring guidelines and with the guidelines in hand, join another team of two. Exchange solutions and grade each other’s FRQs according to the canonical (most well-established or ideal) solution.

Unit 2 - Using Objects¶

2.2 Creating and Storing Objects¶

5.2 - Constructors¶

Activity 2.1.1¶

Activity 2.1.2¶

2.3 Calling a Void Method¶

2.4 Calling a Void Method with Parameters¶

Activity 2.4.1¶

2.5 Non-Void Methods¶

2.5 Calling a non-void method¶

Activity 2.5.1¶

Activity 2.5.2¶

2.6 String Objects: Concatenation, Literals, and Other Things¶

2.7 String Methods¶

Activity 2.7.1 - Wordplay¶

Activity 2.7.2 - Speaking¶

2.8 Wrapper Classes - Integer and Double¶

Activity 2.8.1 Wrapper Classes¶

2.9 Using the Math Class¶

Activity 2.9.1 - Math Class Worksheet 1 & 2¶

Activity 1.2.9 - Race Car¶

Project Mad Libs¶