Java II¶

Classes¶

Classes are the primary organizational unit in Java
They contain both variables and functions
Each class is generally given its own file
No prototypes are needed

Class Syntax¶

The syntax for creating a class is

< accessLevelModifier > class < className >{
 < code goes here >
}

In general, each class is defined in a file that has the same name as the class
- The class UMBCer would be in file named UMBCer.java

Class Syntax Example¶

public class UMBCer{

}

Class Members¶

The members of the class are the variables used to hold the state of the class
Each member must be declared with an access level modifier and a type.

public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;
}

Methods¶

Similar to functions in procedural languages
Should be the primary way to interact with the state of an object
Just like members, methods are declared with an access level modifier
Methods have access to all the members of a class

Method Examples¶

public class UBMCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;

    ...

    public void printFullName(){
        System.out.println(firstName + " " + lastName);
    }

    ...
}

Method Examples¶

public class UBMCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;

    ...

    public String getFullName(){
        return firstName + " " + lastName;
    }

    ...
}

Method Examples¶

public class UBMCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;

    ...

    public boolean setEmail(String email){
        if(email.endsWith("@umbc.edu"))
        {
          this.email = email;
          return true;
        }

        return false;
    }

    ...
}

Constructors¶

Constructors are special methods used to initialize objects of a class
Constructors have no return type and have the same name as the class.
Each class can have multiple constructors
- In this case the compilier differentiates them by their parameter types

Constructor Example¶

import java.util.Random;
public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;

    public UMBCer(String firstName, String lastName)
    {

        this.firstName = firstName;
        this.lastName = lastName;
        this.email = firstName + "." + lastName + "@umbc.edu";

        String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
        Random rnd = new Random();

        this.id = alphabet.charAt(rnd.nextInt(alphabet.length())) + "" +  
                  alphabet.charAt(rnd.nextInt(alphabet.length())) +
                  String.valueOf(rnd.nextInt(9) + 1) + String.valueOf(rnd.nextInt(9) + 1)  +
                  String.valueOf(rnd.nextInt(9) + 1) + String.valueOf(rnd.nextInt(9) + 1);
    }
    ...
}

Special Methods¶

Two common tasks don't behave the way we expect with objects by default
- Remember that using = to compare objects will compare their reference
  - We need to implement .equals(Object obj) in order to test equality
- If we print an object, by default it will print the type of the object and a hash value
  - .toString() allows us to control what happens when we print an object.

Equals¶

The equals method should always take one parameter of type Object
- This is to make sure we are overriding the default method all of the time.
A value of null being passed to the method as well as an object of another class should be handled gracefully
When comparing member objects, be sure to use the equal methods of those classes.

Equals Example¶

public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;

    public boolean equals(Object other)
    {
        if(!(other instanceof UMBCer) || other == null)
        {
            return false;
        }
        UMBCer otherUMBCer = (UMBCer)other;

        return this.firstName.equals(otherUMBCer.firstName) && 
               this.lastName.equals(otherUMBCer.lastName) && 
               this.email.equals(otherUMBCer.email) && 
               this.id.equals(otherUMBCer.id);
    }

}

toString Example¶

The toString method is called automatically when the object is passed to a function that produces a string, like System.out.println()
Remember this is the default method and should return something legible, but in many cases shouldn't be relied upon in code to produce formatted strings

public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;

    public String toString(){
        return "UMBCer " + id + ": " + firstName + " " + lastName;
    }

}

Method Overloading¶

Due to Java's typing we can have many methods with the same name, even in the same object.
The compiler and the runtime system use the method signature along with the parameters being passed at runtime to deterimine which method to use
The return type is not part of the method signature
The following code does not work

public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;

    public void setName(String firstName){
        this.firstName = firstName;
    }

    public boolean setName(String lastName){
        this.lastName = lastName;
    }

Method Overloading Example¶

import java.util.GregorianCalendar

public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;
    private GregorianCalendar birthday;


    public void setBirthday(GegorianCalendar date){
        this.birthday = (GregorianCalendar)date.clone();
    }

    public boolean setBirthday(int month, int day, int year){
        this.birthday = new GegorianCalendar(year,month,day);
    }

Returning Objects¶

A method can return an object just like any other data type
We need to take special precautions when returing object
- Always return a new object

import java.util.GregorianCalendar

public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;
    private GregorianCalendar birthday;

    public GegorianCalendar getBirthday(){
        return  (GregorianCalendar)birthday.clone();
    }

Copy Constructors¶

In the previous example, we had to call clone to ensure we were returning a new object.
It is often easier to use new and pass the existing object as a parameter

public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;
    private GregorianCalendar birthday;

    public UMBCer(UMBCer other){
        this.firstName = new String(other.firstName);
        this.lastName = other.lastName;
        this.email = other.email;
        this.id = other.id;
        this.birthday = (GregorianCalendar)other.birthday.clone();
    }

}

Private Methods¶

So far all methods have been public
When we declare a method as private, only other methods of that class can call it.
This is useful for helper methods that we don't want to make available to all users of the class.

public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;
    private GregorianCalendar birthday;

    private boolean validEmail(String email){
        return email.endsWith("@umbc.edu");
    }

Static¶

Sometimes, we want a variable or a method to exist for the whole class rather than an instance of it.
Use the keyword static to accomplish this.
This goes against OOP principles
- But it is really useful and can make implementation more efficent

import java.util.Random;
public class UMBCer{
    private String firstName;
    private String lastName;
    private String email;
    private String id;
    private static String alphabet = "ABCDEFGHIJKLMNOPQRSTUVWXYZ";
    private static Random rnd = new Random();

    public UMBCer(String firstName, String lastName)
    {

        this.firstName = firstName;
        this.lastName = lastName;
        this.email = firstName + "." + lastName + "@umbc.edu";

        this.id = alphabet.charAt(rnd.nextInt(alphabet.length())) + "" +  
                  alphabet.charAt(rnd.nextInt(alphabet.length())) +
                  String.valueOf(rnd.nextInt(9) + 1) + String.valueOf(rnd.nextInt(9) + 1)  +
                  String.valueOf(rnd.nextInt(9) + 1) + String.valueOf(rnd.nextInt(9) + 1);
    }
    ...
}

File IO¶

Working with files in Java requres two objects generally
- Their instantiaions can be combined into one line
  - A stream represents the flow of text into or out of a file
  - The PrintWriter class implements functions used in printing
  - The Scanner class implements functions used in reading.

Reading from a File¶

To read from a file we create a FileInputStream that we then pass to a Scanner instance
Some of the methods are nextLine() , nextInt(), and nextFloat()

import java.util.Scanner;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
public class ScannerExample{

    public static void main(String[] args){

        Scanner input = new Scanner( new FileInputStream("words.txt"));

        while(input.hasNextLine())
        {
            line = input.nextLine();
        }
    }
}

Writing to a File¶

Writing is very similar, except we use a PrintWriter instance
The main methods to use in this class are print and println

import java.io.PrintWriter
import java.io.FileInputStream;
import java.io.FileNotFoundException;

public class WriterExample{
    public static void main(String[] args){

        PrintWriter output = new PrintWriter( new FileOutputStream("out.txt"))

        output.println("Print this to the file");

        output.close();
    }
}

Exceptions Preview¶

Exceptions are them main way to indicate and handle errors in Java
When attempting to read a file, what happens if the file doesn't exist?

import java.util.Scanner;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
public class ScannerExample{

    public static void main(String[] args){

        try{
            input = new Scanner( new FileInputStream("words.txt"));

            while(input.hasNextLine())
            {
                line = input.nextLine();
            }
        }
        catch(FileNotFoundException)
        {
            System.err.println("Could not open file words.txt");
        }
    }
}