CodeGym/Help with Java Tasks/didnt understand the problem error

Level 23

Chandigarh

18.09.2023
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10comments

didnt understand the problem error

Question about the task

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Second requirement is not being met.. Requirements: • The IPhone class must override the public boolean equals(Object) method. • The equals method must return true for two equal IPhone objects and false for different ones. • The equals method must return false if null is passed to it. it also says "Two different IPhones must not be equal." The code:

package en.codegym.task.pro.task10.task1010;

import java.util.Objects;

/*
Two iPhones
*/

public class Iphone {
    private String model;
    private String color;
    private int price;

    public Iphone(String model, String color, int price) {
        this.model = model;
        this.color = color;
        this.price = price;
    }
    public boolean equals(Object o){
        if(this == o) return true;
        if(o==null) return false;

        if(!( o instanceof Iphone)) return false;
        Iphone iphone = (Iphone) o;



       if((iphone.model!=null) &&(this.model).equals(iphone.model))
        {
            if((iphone.color!=null)&&(this.color).equals(iphone.color)){
                if((this.price)==(iphone.price))
                return true;
            }
        }
        return false;
    }
    //write your code here

    public static void main(String[] args) {
        Iphone iphone1 = new Iphone("X", "Black", 999);
        Iphone iphone2 = new Iphone("X", "Black", 999);

        System.out.println(iphone1.equals(iphone2));
    }

}

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Harshit Garg

Level 23 , Chandigarh, India

Expert

10 October 2023, 11:56

solution

package en.codegym.task.pro.task10.task1010;

import java.util.Objects;

/*
Two iPhones
*/

public class Iphone {
    private String model;
    private String color;
    private int price;

    public Iphone(String model, String color, int price) {
        this.model = model;
        this.color = color;
        this.price = price;
    }

    @Override
    public boolean equals(Object o) {
        if (this == o) {
            return true;
        }    else return false;}

    public static void main(String[] args) {
        Iphone iphone1 = new Iphone("X", "Black", 999);
        Iphone iphone2 = new Iphone("X", "Black", 999);

        System.out.println(iphone1.equals(iphone2));
    }

}

why isn't this much enough

Thomas

Level 41 , Bayreuth, Germany

10 October 2023, 15:11

solution

Here you just compare references. See a reference as a pointer to an object. If two references point to the same object, then they are equal. A reference you can save in a variable, an object not. new Iphone(...) creates a new Iphone object and returns a reference to that neIpwly created object. That reference you an save in a variable Iphone variable = new Iphone(...); // variable contains the reference to an Iphone object. Iphone anotherVariable; // creating another reference variable (that can hold references to Iphones) anotherVariable = variable; // assigning a value to the second reference, the value of the first reference variable. Now both, variable and anotherVariable are pointing to the same Iphone object. ==> variable == anotherVariable Iphone differentVariable = new Iphone(...); // creating a new Iphone object, assigning a value to the reference variable differentVariabale. variable points to the first Iphone object differentVariable points to the second created object. Means the point to different memory addresses. A comparison will evaluate to false. varibale == differentVariable // false and that even if you initialize that two objects with the same data, eg. the "X", "Black", 999 That's what you do. You compare the references ignoring the objects properties.

Harshit Garg

Level 23 , Chandigarh, India

Expert

16 October 2023, 04:53

THANK YOU SO MUCH!

Harshit Garg

Level 23 , Chandigarh, India

Expert

19 September 2023, 05:37

i have changed the but still doesnt work for second test case:

package en.codegym.task.pro.task10.task1010;

import java.util.Objects;

/*
Two iPhones
*/

public class Iphone {
    private String model;
    private String color;
    private int price;

    public Iphone(String model, String color, int price) {
        this.model = model;
        this.color = color;
        this.price = price;
    }
    public boolean equals(Object o){
        if(this == o) return true;
        if(o==null) return false;

        if(!( o instanceof Iphone)) return false;
        Iphone iphone = (Iphone) o;



       if((iphone.model!=null) &&(this.model).equals(iphone.model))
        {
            if((iphone.color!=null)&&(this.color).equals(iphone.color)){
                if((this.price)==(iphone.price))
                return true;
            }
        }
        return false;
    }
    //write your code here

    public static void main(String[] args) {
        Iphone iphone1 = new Iphone("X", "Black", 999);
        Iphone iphone2 = new Iphone("X", "Black", 999);

        System.out.println(iphone1.equals(iphone2));
    }

}

Thomas

Level 41 , Bayreuth, Germany

19 September 2023, 05:56

you forgot to check the fields for null. If eg. model is null then line 28 still will throw a NullPointerException (you can not invoke a method on null like null.equals(...)). But you should not only avoid that exception but also consider this and that model to be equal if both are null. If you'd use to test your code the main method Lupe posted, then you'd see the problem.

Harshit Garg

Level 23 , Chandigarh, India

Expert

19 September 2023, 08:17

(iphone.color!=null) checks null values. whats lupe?

Thomas

Level 41 , Bayreuth, Germany

19 September 2023, 10:23

it checks if iphone.color is not null but it does not check if this.color is not null and if this.color is null and you invoke equals on this.color you get a NullPointerException. And as said, you do not check if this.color == null && iphone.color == null. If both are null, both fields are considered equal. Lupe == Guadalupe Gagnon, the fellow student who helped you with everything you need yesterday that's how this can be done (the commented out code is using the Objects helper method equals and that's how it is usually done today). I hope I got it right ;)

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || this.getClass() != o.getClass()) return false;

    Iphone that = (Iphone) o;

    if (this.price != that.price) return false;
    if (this.color == null ? that.color != null : !this.color.equals(that.color)) return false; // if (!Objects.equals(this.color, that.color)) return false;
    return this.model == null ? that.model == null : this.model.equals(that.model); // return Objects.equals(this.model, that.model)
}

If you use IntelliJ you can let that IDE create the equals method for you. But you should understand what has to be done and why.

Harshit Garg

Level 23 , Chandigarh, India

Expert

20 September 2023, 16:56

it quite difficult to understand, i think this question isn't that complex I changed it this way:

package en.codegym.task.pro.task10.task1010;

import java.util.Objects;

/*
Two iPhones
*/

public class Iphone {
    private String model;
    private String color;
    private int price;

    public Iphone(String model, String color, int price) {
        this.model = model;
        this.color = color;
        this.price = price;
    }
    public boolean equals(Object o){

        if(o == null) return false;
        if(this == o) return true;
        if(this==null) return false;

        if((this.getClass() != o.getClass())) return false;

        Iphone iphone = (Iphone) o;

        if(((this.model==null)&&(iphone.model==null))||((this.color==null)&&(iphone.color==null))) return true;
        else
        if((iphone.model!=null) &&(this.model).equals(iphone.model))
        {
            if((iphone.color!=null)&&(this.color).equals(iphone.color)){
                if((this.price)==(iphone.price))
                return true;
            }
        }

        return false;
    }
    //write your code here

    public static void main(String[] args) {
        Iphone iphone1 = new Iphone("X", "Black", 999);
        Iphone iphone2 = new Iphone("X", "Black", 999);

        System.out.println(iphone1.equals(iphone2));
    }

}

I hv included every case where #1 1 val is null #2 both values are null #3 no value is null please just edit my code using comments

Guadalupe Gagnon

Level 37 , Tampa, United States

20 September 2023, 17:11

#1 Line 23 is unnecessary. Calling any method on a null value results in a null pointer exception so if the object was null then calling the equals() method on it would result in the Exception and the code wouldn't even be reached. #2 line 29 returns true if either of the current object and compared object's Strings are both null. This is not logically sound because if the other values are not a match then the objects are still not equal. Say, for example, you had 2 objects where:

Iphone iphone1 = new Iphone(null, "Red", 176);
Iphone iphone2 = new Iphone(null, "Blue", 257);

These two objects are not equal but if the equals method is called comparing them then line 29 would be true since the String model for each is null.

Guadalupe Gagnon

Level 37 , Tampa, United States

18 September 2023, 17:33

#1 line 20 would throw a class cast exception if the passed in Object,o, wasn't an instance of Iphone. #2 Strings can be null. This means lines 24 and 25 would throw a null pointer exception if the model or color of the current object is null. Here is some test code:

public static void main(String[] args) {
   Iphone iphone1 = new Iphone (null, "Black", 9);
   Iphone iphone2 = new Iphone ("null", "Black", 9);
   Iphone iphone3 = new Iphone ("null", "Black", 9);
   Iphone iphone4 = new Iphone (null, "Black", 9);

   System.out.println(iphone3.equals(iphone1)); // false
   System.out.println(iphone3.equals(iphone2)); // true

   // these three lines throw Exceptions with your code
   // but should be what is indicated as noted
   System.out.println(iphone1.equals(iphone2)); // false
   System.out.println(iphone1.equals(new Object())); // false
   System.out.println(iphone1.equals(iphone4)); // true
}