1

I'm learning Java and run into a problem. Why doesn't the ArrayList contains() method work with objects? It seems to work with Strings and other basic data types but not objects. Am I using the wrong method?

The program creates data in lines 8 - 11 and receives it from the command line. The data from the command line is:

avacado 10 .50 broccoli 20 .75 cauliflower 30 1.00 banana 300 .55

The last entry "banana 300 .55" on the command line matches the data on line 11 and should be bypassed as already existing but contains() says it's not there. How come? Help.

Here's my code:

package java21days;

import java.util.*;

public class FruitBasket3 {

    ArrayList<Fruit> cart;
    Fruit apple = new Fruit("apple", 100, .75F);
    Fruit orange = new Fruit("orange", 200, .90F);
    Fruit banana = new Fruit("banana", 300, .55F);
    Fruit kumkwat = new Fruit();

    public FruitBasket3(String[] userCodes) {
        cart = new ArrayList<>();
        int quantity;
        float price;

        addCode(apple);
        addCode(orange);
        addCode(banana);
        addCode(kumkwat);

        for (int j = 0; j < userCodes.length; j+=3) {
            quantity = Integer.parseInt(userCodes[j+1]);
            price = Float.parseFloat(userCodes[j+2]);
            addCode(new Fruit(userCodes[j],quantity,price));
        }

        // display all codes
        for (Fruit code : cart) {
            System.out.println("Fruit: " + code.getName() + " Price: " +
                    code.getQuantity() * code.getPrice());
        }
    }

    private void addCode(Fruit inFruit) {
        boolean match = false;
        System.out.println("Inside addCode with fruit: " + inFruit.getName() + 
                " Quantity: " + inFruit.getQuantity() + " Price: " + inFruit.getPrice());
        if (!cart.contains(inFruit)) {
            System.out.println("    Passed the contains test.");
            for (Fruit item : cart) {
                System.out.println("    Existing: " + item.getName() + 
                        " Quantity: " + item.getQuantity() + " Price: " + item.getPrice());
                System.out.println("    New:      " + inFruit.getName() + 
                        " Quantity: " + inFruit.getQuantity() + " Price: " + inFruit.getPrice());

                //if (inFruit.getName() == item.getName() &&
                //        inFruit.getQuantity() == item.getQuantity() &&
                //        inFruit.getPrice() == item.getPrice()){
                if (inFruit.getName().equals(item.getName()) &&
                        inFruit.getQuantity() == item.getQuantity() &&
                        inFruit.getPrice() == item.getPrice()){
                    match = true;
                    System.out.println("        Did not pass the individual element comparision test.");
                } else {
                    System.out.println("        Passed the individual element comparision test.");
                }
            }
            if (!match) {
                System.out.println("            >>>> adding.");
                cart.add(inFruit);
            } else {
                System.out.println("            >>>> did not add.");
            }
        } else {
            System.out.println("    Fruit: " + inFruit.getName() + " already on file ... bypassing.");
        }
    }

    public static void main(String[] arguments) {
        if (arguments.length % 3 != 0) {
            System.out.println("Error!!! Incorrect number of arguments.");
        } else {
            FruitBasket3 keeper = new FruitBasket3(arguments);
        }
    }

}

public class Fruit {
    private String name;
    private int quantity;
    private float price;

    public Fruit() {
        name = "";
        quantity = 0;
        price = 0.0F;
    }

    public Fruit(String inName, int inQuantity, float inPrice) {
        name = inName;
        quantity = inQuantity;
        price = inPrice;
    }

    public String getName(){
        return name;
    }

    public int getQuantity(){
        return quantity;
    }

    public float getPrice(){
        return price;
    }
}

The output is:

run:
Inside addCode with fruit: apple Quantity: 100 Price: 0.75
    Passed the contains test.
            >>>> adding.
Inside addCode with fruit: orange Quantity: 200 Price: 0.9
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      orange Quantity: 200 Price: 0.9
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: banana Quantity: 300 Price: 0.55
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit:  Quantity: 0 Price: 0.0
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:       Quantity: 0 Price: 0.0
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:       Quantity: 0 Price: 0.0
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:       Quantity: 0 Price: 0.0
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: avacado Quantity: 10 Price: 0.5
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      avacado Quantity: 10 Price: 0.5
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      avacado Quantity: 10 Price: 0.5
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:      avacado Quantity: 10 Price: 0.5
        Passed the individual element comparision test.
    Existing:  Quantity: 0 Price: 0.0
    New:      avacado Quantity: 10 Price: 0.5
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: broccoli Quantity: 20 Price: 0.75
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
    Existing:  Quantity: 0 Price: 0.0
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
    Existing: avacado Quantity: 10 Price: 0.5
    New:      broccoli Quantity: 20 Price: 0.75
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: cauliflower Quantity: 30 Price: 1.0
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing:  Quantity: 0 Price: 0.0
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing: avacado Quantity: 10 Price: 0.5
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
    Existing: broccoli Quantity: 20 Price: 0.75
    New:      cauliflower Quantity: 30 Price: 1.0
        Passed the individual element comparision test.
            >>>> adding.
Inside addCode with fruit: banana Quantity: 300 Price: 0.55
    Passed the contains test.
    Existing: apple Quantity: 100 Price: 0.75
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: orange Quantity: 200 Price: 0.9
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: banana Quantity: 300 Price: 0.55
    New:      banana Quantity: 300 Price: 0.55
        Did not pass the individual element comparision test.
    Existing:  Quantity: 0 Price: 0.0
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: avacado Quantity: 10 Price: 0.5
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: broccoli Quantity: 20 Price: 0.75
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
    Existing: cauliflower Quantity: 30 Price: 1.0
    New:      banana Quantity: 300 Price: 0.55
        Passed the individual element comparision test.
            >>>> did not add.***
Fruit: apple Price: 75.0
Fruit: orange Price: 180.0
Fruit: banana Price: 165.0
Fruit:  Price: 0.0
Fruit: avacado Price: 5.0
Fruit: broccoli Price: 15.0
Fruit: cauliflower Price: 30.0
BUILD SUCCESSFUL (total time: 0 seconds)
Improve this question

2 Answers 2

Reset to default
6

It doesn't work with your class because it doesn't have a valid public boolean equals(...) method override. Give it one, one that uses the key fields to test for functional equality, and contains will work.

Per the contains method section of the ArrayList API:

Returns true if this list contains the specified element. More formally, returns true if and only if this list contains at least one element e such that (o==null ? e==null : o.equals(e)).

Use the name field only for the test of equality and make it a final field, since you don't want to use fields that could change. Also give it a hashCode override that also uses the same fields tested in the equals method.

Something like:

@Override
public int hashCode() {
    final int prime = 31;
    int result = 1;
    result = prime * result + ((name == null) ? 0 : name.hashCode());
    return result;
}

@Override
public boolean equals(Object obj) {
    if (this == obj)
        return true;
    if (obj == null)
        return false;
    if (getClass() != obj.getClass())
        return false;
    Fruit other = (Fruit) obj;
    if (name == null) {
        if (other.name != null)
            return false;
    } else if (!name.equals(other.name))
        return false;
    return true;
}

could work

Improve this answer
Sign up to request clarification or add additional context in comments.

Comments

1

The method contains() in ArrayList class uses method equals() to evaluate two objects. The ArrayList equals() works if the two objects are the SAME object. You have to override equals() method in your class and define evaluating criterion to define if a Fruit is already contained in your list. Add this code to your Fruit class

@Override
public boolean equals(Object o1) {
if (this == obj) {
    return true;
}
Fruit fruit = (Fruit)o1
if (name != null && fruit.getName() != null) {
   if (name.equas(fruit.getName())) {
        return true;
   }
   return false;
}
return false;
Improve this answer

Comments

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.