UF 06: JAVA

Classes in Pseudo Java™

Alta Formazione Professionale - ITT Marconi

February 2021

NOTE: Code in slides sometimes is too long, see all on Github

My mommy always said there were no monsters. No real ones.

But there are.

Ellen Ripley, Alien Resurrection

WARNING: what follows is not proper Java style

• complex classes are ugly in the short term

• simple classes are ugly in the long term

Your package

package it.yourcompany.adventure;

At the beginning of source files you have to declare the package.

• Choose your company domain (i.e. yourcompany.it)

• reverse the parts (it.youtcompany)

• if you want add further subdomains for your specific project (adventure):

Your files then MUST reside in folders like these:

- it
   - yourcompany
      - adventure
         Monster.java
         Game.java

Most IDE offer some automation to automatically place the files in the folders

Join The Game

Create a file called Game.java with a public class Game inside:

• to be able to run a class, you need main method

package it.yourcompany.adventure;

public class Game {
    
    public static void main(String[] args){
        
        System.out.println("Welcome to my dungeon, stranger....");        
        
       }
}

Running The Game

Run Game class in Netbeans:

Monsters: the first encounter

Create a file called Monster.java with a public class Monster inside

• filename must be the same as public class name

package it.yourcompany.adventure;

public class Monster {
    public String name = "";    
    public int lifepoints = 0;                   
}

Java dogma says...

• NOTE 1: fields should be private, but we're using public ones

• NOTE 2: we should define a getter/setter for each field, we don't do that

• NOTE 3: we should also define a constructor, but we're using the implicit default empty constructor which is public

Invoke the Monster

public class Game {
    
    public static void main(String[] args){
        
        System.out.println("Welcome to my dungeon, stranger....");        
        
        Monster monster1 = new Monster();
        
        monster1.name = "Cerberus";
        monster1.lifepoints = 7;
                        
        System.out.println();
        System.out.println("monster1.name is       : " + monster1.name);
        System.out.println("monster1.lifepoints are: " + monster1.lifepoints);
                
       }
}

Run the Game.java, you should see some output.

Join the Java Cult

toString Ritual

This output printing is verbose:

System.out.println("monster1 name is       : " + monster1.name);
System.out.println("monster1 lifepoints are: " + monster1.lifepoints);

monster1 name is       : Cerberus
monster1 lifepoints are: 7

This would be shorter, but....:

System.out.println("monster1.toString() is : " + monster1);

monster1 toString() is : it.yourcompany.adventure.Monster@b44d430f

It shows the physical address, not nice.

After you defined your fields:

• generate a toString method implementation
• (if you wonder why, it's because the cult told you to do so)

Select all fields:

package it.yourcompany.game;

public class Monster {
    
    public String name = "";
    public int lifepoints = 0;    


    // Java rituals  ......................
    
    @Override
    public String toString() {
        return "Monster{" + "name=" + name + ", lifepoints=" + lifepoints + '}';
    }    
    
}

You should now have a printable monster:

public class Game {
    
    public static void main(String[] args){
        
        System.out.println("Welcome to my dungeon, stranger....");        
        
        Monster monster1 = new Monster();
        
        monster1.name = "Cerberus";
        monster1.lifepoints = 7;
        
        System.out.println();
        System.out.println("monster1.toString() is: " + monster1);
                
    }
}

Welcome to my dungeon, stranger....

monster1.toString() is: Monster{name=Cerberus, lifepoints=7}

All equal monsters should be equal.

Are they?

import java.util.List;

public class Game {    
    public static void main(String[] args){
        
        System.out.println("Welcome to my dungeon, stranger....");        
        
        Monster monster1 = new Monster();
        
        monster1.name = "Cerberus";
        monster1.lifepoints = 7;
        
        System.out.println();
        System.out.println("monster1.toString() is: " + monster1);

        Monster monster2 = new Monster();
        
        monster2.name = "Cerberus";
        monster2.lifepoints = 7;
                
        System.out.println();
        System.out.println("monster2.toString() is: " + monster2);

        System.out.println();
        System.out.println("monster1.equals(monster2) : " + monster1.equals(monster2));
        
        System.out.println();
        System.out.println("List.of(monster1).contains(monster2): " + List.of(monster1).contains(monster2));                
    }
}

Welcome to my dungeon, stranger....

monster1.toString() is : Monster{name=Cerberus, lifepoints=7}

monster2.toString() is : Monster{name=Cerberus, lifepoints=7}

monster1.equals(monster2) : false

List.of(monster1).contains(monster2): false

That's not ok.

Both .equals() and in general collections search methods do not work by default with new classes.

Equals and Hashcode Ritual

Let's fix the problem.

You should now have a ton of more Java bla bla, see all on Github

package it.yourcompany.adventure;

import java.util.Objects;

public class Monster {
    
    // stuff that matters    
    public String name = "";
    public int lifepoints = 0;    
    
    
    // Java rituals  ......................
    
    @Override
    public String toString() {
        return "Monster{" + "name=" + name + ", lifepoints=" + lifepoints + '}';
    }    
    @Override
    public int hashCode() {
        int hash = 3;
        hash = 89 * hash + Objects.hashCode(this.name);
        hash = 89 * hash + this.lifepoints;
        return hash;
    }
    @Override
    public boolean equals(Object obj) {
        if (this == obj) {
            return true;
        }
        if (obj == null) {
            return false;
        }
        if (getClass() != obj.getClass()) {
            return false;
        }
        final Monster other = (Monster) obj;
        if (this.lifepoints != other.lifepoints) {
            return false;
        }
        if (!Objects.equals(this.name, other.name)) {
            return false;
        }
        return true;
    }        
}

Show me the equality - same monster

Rerun the Game:

Welcome to my dungeon, stranger....

monster1.toString() is : Hero{name=Cerberus, lifepoints=7}

monster2.toString() is : Hero{name=Cerberus, lifepoints=7}

monster1.equals(monster2) : true

List.of(monster1).contains(monster2): true

Show me the equality - many monsters

Monster monster3 = new Monster();

monster3.name = "Zombo";
monster3.lifepoints = 4;


System.out.println();
System.out.println("monster1.equals(monster3) : " + monster1.equals(monster3));

System.out.println();
System.out.println("List.of(monster1).contains(monster3): " + List.of(monster1).contains(monster3));

System.out.println();
System.out.println("List.of(monster1, monster3).contains(monster3): "
                    + List.of(monster1, monster3).contains(monster3));

monster1.equals(monster3) : false

List.of(monster1).contains(monster3): false

List.of(monster1, monster3).contains(monster3): true