Using Contains for objects in List collection

Question

I'm trying to find an object in the List collection by his name using Contains method, but, somehow, it doesn't work. How should I use it?

This is how I try to use this

CandyDao.getAllCandys().contains("Caramel")

But it can't find an object which I need.

CandyDao.java

public class CandyDao {
  private List<Candy> candys = Arrays.asList(new Candy("Caramel", 3, false),
          new Candy("Marmelade", 2, true));

  public List<Candy> getAllCandys(){
    return candys;
  }
}

Candy.java

  public class Candy {
  private String name;
  private float price;
  private boolean InStock;

  public Candy() {
  }

  public Candy(String name, float price, boolean InStock) {
    setName(name);
    setPrice(price);
    setInStock(InStock);
  }

  public String getName() {
    return name;
  }

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

  public float getPrice() {
    return price;
  }

  public void setPrice(float price) {
    this.price = price;
  }

  public boolean getInStock() {
    return InStock;
  }

  public void setInStock(boolean InStock) {
    this.InStock = InStock;
  }
}

Andreas · Accepted Answer · 2018-10-14 15:19:44Z

3

Since the list contains Candy objects, the contains() method needs a Candy object for the comparison, so you can't use contains("Caramel").

To check if the list contains a Candy object with a name of "Caramel", you can use Java 8+ Streams to do the search:

CandyDao.getAllCandys().stream().Map(Candy::getName).anyMatch("Caramel"::equals);

The equivalent non-stream version would be:

boolean hasCaramel = false;
for (Candy candy : CandyDao.getAllCandys()) {
    if ("Caramel".equals(candy.getName())) {
        hasCaramel = true;
        break;
    }
}

answered Oct 14, 2018 at 15:19

Andreas

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Comments

Pankaj Singhal · Accepted Answer · 2018-10-14 15:03:19Z

2

Override the equals & hashcode method like below :

@Override
public boolean equals(Object o) {
    if (this == o) return true;
    if (o == null || getClass() != o.getClass()) return false;
    Candy candy = (Candy) o;
    return Objects.equals(name, candy.name);
}

@Override
public int hashCode() {
    return Objects.hash(name);
}

Now, Since the equals function only checks name for equality of Candy object, the following should work:

CandyDao.getAllCandys().contains(new Candy("Caramel", 0, true)) .   //2nd & 3rd arg of Candy constructor are inessential/dummy

answered Oct 14, 2018 at 15:03

Pankaj Singhal

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Comments

Jacob G. · Accepted Answer · 2018-10-14 15:01:20Z

0

You should override the Object#equals method in Candy as shown below:

@Override
public boolean equals(Object o) {
    if (!(o instanceof Candy)) {
        return false;
    }

    Candy that = (Candy) o;

    return Objects.equals(that.getName(), this.getName());
}

After overriding, List#contains should return true if the name matches.

edited Oct 14, 2018 at 15:01

Jacob G.

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answered Oct 14, 2018 at 14:56

Vinoth A

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3 Comments

Andreas Over a year ago

That still won't make candy.equals("Caramel") return true.

Vinoth A Over a year ago

Agreed, he should be using candy object with name "Caramel", instead of using String.

Andreas Over a year ago

Like answer by Pankaj Singhal shows.

Hulya · Accepted Answer · 2018-10-14 15:03:22Z

0

getAllCandys() method returns a list of Candyobjects. You should look each element's nameproperty on the list. So cis an element of candyslist. Use c.getName().contains("Caramel") to look for "Caramel"

for(Candy c : candys) {
            System.out.println(c.getName() + " contains? " + c.getName().contains("Caramel"));
        }

answered Oct 14, 2018 at 15:03

Hulya

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2 Comments

Andreas Over a year ago

list.contains("Caramel") is a search of the list for a object that is equal to argument, i.e. c.getName().equals("Caramel"), not c.getName().contains("Caramel").

Hulya Over a year ago

Yes, you're right and i also know the difference. But I misunderstood the question.

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