How do I compare two ObservableLists?

[Scott Vallery]

I wasn't sure if this should be in the Beginning Java - though I'm a greenhorn

The Question:
How do I compare two ObservableList?

What I'm trying to do:
Since there can be many sources (e.g. book, video, magazine, etc) )with the same exact title in the database but possibly different authors. I'm attempting to compare a list of Authors for each existing source found in the database to the list of authors entered by a user. So this is my test code for planning and constructing this method in "DoSourceAuthorsExst()" which will most likely become a boolean method. The 'sourceAuthors' will be the result of retrieving all the authors of an existing source as Author objects and compare that list to the user's list of author objects, which the authors table is based.

My Problem:
The two ObservableList<Authors> objects are identical. I literally copied one and pasted it -- renaming the second as 'sourceAuthors'. It seems no matter what I do, the result is always false. I feel I'm overlooking something simple, I'm just too thick to see it. I could use a second or more set of eyes to see it.

    void DoSourceAuthorsExist {
        
        
        ObservableList<Authors> tblAuthors = FXCollections.observableArrayList();
        ObservableList<Authors> sourceAuthors = FXCollections.observableArrayList();
        
        tblAuthors.add(new Authors("Arthur", "S.", "Doe", null));
        tblAuthors.add(new Authors("Jane", "Lynne", "Doe", null));
        tblAuthors.add(new Authors("Edward", null, "Fisher", ""));
        tblAuthors.add(new Authors("Elizabeth", "Elaine", "Strange", null));

        sourceAuthors.add(new Authors("Arthur", "S.", "Doe", null));
        sourceAuthors.add(new Authors("Jane", "Lynne", "Doe", null));
        sourceAuthors.add(new Authors("Edward", null, "Fisher", ""));
        sourceAuthors.add(new Authors("Elizabeth", "Elaine", "Strange", null));

        tblAuthors.forEach((Authors a) -> {
            if(sourceAuthors.contains(a)){
                System.out.println("True");
            }else{
                System.out.println("False");
            }
        });
    }

the Authors Class:

public class Authors {
    private final SimpleStringProperty FirstName;
    private final SimpleStringProperty Middle;
    private final SimpleStringProperty LastName;
    private final SimpleStringProperty Suffix;

    Authors(String firstName, String middle, String lastName, String suffix){
        this.FirstName = new SimpleStringProperty(firstName);
        this.Middle = new SimpleStringProperty(middle);
        this.LastName = new SimpleStringProperty(lastName);
        this.Suffix = new SimpleStringProperty(suffix);
    }

    public String getFirstName() {
        return FirstName.get();
    }
    public String getMiddle() {
        return Middle.get();
    }
    public String getLastName() {
        return LastName.get();
    }
    public String getSuffix() {
        return Suffix.get();
    }

    void setFirstName(String firstName) {
        this.FirstName.set(firstName);
    }
    void setMiddle(String middle) {
        this.Middle.set(middle);
    }
    void setLastName(String lastName) {
        this.LastName.set(lastName);
    }
    void setSuffix(String suffix) {
        this.Suffix.set(suffix);
    }

    public SimpleStringProperty firstNameProperty() {
        return FirstName;
    }
    public SimpleStringProperty middleProperty() {
        return Middle;
    }
    public SimpleStringProperty lastNameProperty() {
        return LastName;
    }
    public SimpleStringProperty suffixProperty() {
        return Suffix;
    }
}

[Carey Brown]

You probably need to override equals() and hashCode() in your Authors class. That may be why "contains()" doesn't return true.

[Daniel Demesmaecker]

It's not the same list twice...
It might contain the same values, but you create a new object each time.
If you would implement your own toStringmethod and check that for equality on each object in the list it would return true.
Naming conventions would dictate to call your objectclass Author, not Authors and you can also initialize your list on creation instead of adding each object seperately

The toString():

@Override
    public String toString() {
        return "Authors{" +
                "FirstName=" + FirstName +
                ", Middle=" + Middle +
                ", LastName=" + LastName +
                ", Suffix=" + Suffix +
                '}';
    }

Checking ObservableList:

 ObservableList<Author> tblAuthors = FXCollections.observableArrayList(
                new Author("Arthur", "S.", "Doe", null),
                new Author("Jane", "Lynne", "Doe", null),
                new Author("Edward", null, "Fisher", ""),
                new Author("Elizabeth", "Elaine", "Strange", null)
        );
        ObservableList<Author> sourceAuthors = FXCollections.observableArrayList(
                new Author("Arthur", "S.", "Doe", null),
                new Author("Jane", "Lynne", "Doe", null),
                new Author("Edward", null, "Fisher", ""),
                new Author("Elizabeth", "Elaine", "Strange", null)
        );

        for (int i = 0; i < sourceAuthors.size(); i++){
            if(sourceAuthors.get(i).toString().equals(tblAuthors.get(i).toString())){
                System.out.println("true");
            }else{
                System.out.println("false");
            }
        }

And this would actually be a javaFx question

[Campbell Ritchie]

Why have you got null as a middle name anywhere? Avoid nulls as much as possible. Work out how to create an array of middle names, so you can give somebody an array of different sizes starting from 0 depending on how many middle names they have.

[Scott Vallery]

Hi Daniel and Campbell!

Good points and I totally agree with you both - So I appreciate this site and your patience and efforts.
This is the first application in Java for me and it is further compounded by needing JavaFX and Databases - and I've had 1 weeks worth of high-level training and hit-miss tutorials. -- Sorry for the wrong forum.

After continuing to research, I understand now why the object comparison will always return false. So I was attempting to build out an override of 'equals' as suggested but I was failing miserably. So I think the 'toString' override is a good suggestion too and I should try. I will refactor the naming the 'Authors' to Author.

I will reply with my success or failure! Thanks! It is greatly appreciated.

[Daniel Demesmaecker]

A way that wouldn't require any overide of methods would be to create your objects first and thenb assing them to both lists.

Author author1 = new Author("Arthur", "S.", "Doe", null);
...
ObservableList<Author> sourceAuthors = FXCollections.observableArrayList(author1,...);
ObservableList<Author> tblAuthors = FXCollections.observableArrayList(author1,...);
...

[Campbell Ritchie]

What does the documentation for different kinds of observable List say about the equals() method? I thought there was an equals() method in AbstractList, so all different kinds of List return whether they have the same elements, so a linked list can be equal to an array list if they contain the same elements.

[Carey Brown]

Note that this will throw a null pointer exception if any field is null. I would suggest modifying your constructor to either: a) throw an exception if any argument is null, or b) convert nulls to empty Strings ("").

@Override
    public boolean equals( Object o ) {
        if( o == this )
             return true;
        if( ! (o instanceof Author ) )
            return false;
        Author a = (Author)o;
        return firstName.equals(a.firstName) &&
            middle.equals( a.middle  ) &&
            lastName.equals( a.lastName ) &&
            suffix.equals( a.suffix ) );
    }

Also, your member variables (aka fields) should begin with a lower case letter.

[Campbell Ritchie]

Alternatively you can use the long form of equality test:-

... || this.xyz == null ? that.xyz == null : this.xyz.equals(that.xyz) ...

or the medium version:-

... || Objects.equals(this.xyz, that.xyz) ...

I bet you will go for the medium version.

[Scott Vallery]

Daniel, Carey, Campbell!

These are some good thoughts. Let me try these. As a Greenhorn, I need to run them through my thick head a couple times to understand what is happening with each. I did construct the override of "toString" but it has to be too precise. The difficult piece for me every time is dealing with null vs "". I did create an Object compare method to help with this in another section of the code, which maybe I can incorporate.

Thanks! I should have something soon!

[Scott Vallery]

Oh... and the table entries may be in any order, which I can handle with some looping. I forgot to mention that caveat.

[Campbell Ritchie]

Scott Vallery wrote:. . . Thanks! . . .

That's a pleasure

I hope my mistake yesterday didn't confuse you; I have just noticed that I wrote || instead of &&. Please show us your compare() method, but there is a risk that I shall tell you to change its name as soon as I see it.
Remember that List equality includes both identity of elements and their order.

List<String> words = new ArrayList<>(Arrays.asList(
      "Mary had a little lamb, she had it with mint sauce.".split("\\s+")));
List<String> rhyme = new ArrayList<>(words);
System.out.println(rhyme.equals(words)); // true
rhyme.add(0, "Mary");
System.out.println(rhyme.equals(words)); // false

And rhyme isn't a good name for a List at all.

[Daniel Demesmaecker]

Scott Vallery wrote:Oh... and the table entries may be in any order, which I can handle with some looping. I forgot to mention that caveat.

What tableentries are you talking about? In to a tableview? Cause there's a simpeler way to do it, setting the celValueFactory to the propertyValueFactory.

[Scott Vallery]

Hi Everyone!

Daniel -- sorry, I wasn't as clear as I should have been, but if a user doesn't know a source exists, they may add the authors in a different order that listed on the database retrieval. However, I see what you are saying... and I could probably just us the 'sorted()' method on each ObservableArrayList... duh!. Like I said, I can be thick.

Right now the goal is to get something working then begin to refactor to be more efficient. I feel (obviously ) this application can be far more efficient and understandable. I'm just not experienced enough and naive at the present time.

Anyhow, this is working at present. I continued with the overriding of the "toString()" and applied part of Carey's suggestion to convert any 'null' value as a "". My Convert method may be in efficient, as I mentioned before, the null and "" and " " are what I'm trying to handle.

    public static void main(String[] args){

        ObservableList<Author> tblAuthors = FXCollections.observableArrayList();
        ObservableList<Author> sourceAuthors = FXCollections.observableArrayList();

        tblAuthors.add(new Author("Arthur", "S.", "Doe", null));
        tblAuthors.add(new Author("Jane", "Lynne", "Doe", " "));
        tblAuthors.add(new Author("Edward", "", "Fisher", "Jr."));
        tblAuthors.add(new Author("Elizabeth", "Elaine", "Strange", ""));

        sourceAuthors.add(new Author("Arthur", "S.", "Doe", ""));
        sourceAuthors.add(new Author("Jane", "Lynne", "Doe", ""));
        sourceAuthors.add(new Author("Edward", null, "Fisher", "Jr."));
        sourceAuthors.add(new Author("Elizabeth", "Elaine", "Strange", ""));

        boolean tf = false;

        for (Author tblAuthor : tblAuthors) {
            tf = false;
            for (Author sourceAuthor : sourceAuthors) {
                if (sourceAuthor.toString().equals(tblAuthor.toString())) {
                    tf = true;
                    break;
                }
            }
            if (!tf)
                break;
        }
        if(tf)
            System.out.println("True");
        else
            System.out.println("False");
    }
}

Author Class:

public class Author {
    private final SimpleStringProperty FirstName;
    private final SimpleStringProperty Middle;
    private final SimpleStringProperty LastName;
    private final SimpleStringProperty Suffix;

    Author(String firstName, String middle, String lastName, String suffix) {
        this.FirstName = new SimpleStringProperty(Convert(firstName));
        this.Middle = new SimpleStringProperty(Convert(middle));
        this.LastName = new SimpleStringProperty(Convert(lastName));
        this.Suffix = new SimpleStringProperty(Convert(suffix));
    }

    @Override
    public String toString(){
        return "Author{FirstName=" + Convert(FirstName) + "Middle=" + Convert(Middle) +
                "LastName=" + Convert(LastName) + "Suffix=" + Convert(Suffix);
    }

    private String Convert(Object obj1) {
        if (obj1 == null || obj1.toString().trim().isEmpty()) {
            obj1 = ""; }
        return obj1.toString();
    }

    public String getFirstName() {
        return FirstName.get();
    }

    public String getMiddle() {
        return Middle.get();
    }

    public String getLastName() {
        return LastName.get();
    }

    public String getSuffix() {
        return Suffix.get();
    }

    void setFirstName(String firstName) {
        this.FirstName.set(firstName);
    }

    void setMiddle(String middle) {
        this.Middle.set(middle);
    }

    void setLastName(String lastName) {
        this.LastName.set(lastName);
    }

    void setSuffix(String suffix) {
        this.Suffix.set(suffix);
    }

    public SimpleStringProperty firstNameProperty() {
        return FirstName;
    }

    public SimpleStringProperty middleProperty() {
        return Middle;
    }

    public SimpleStringProperty lastNameProperty() {
        return LastName;
    }

    public SimpleStringProperty suffixProperty() {
        return Suffix;
    }
}

[Daniel Demesmaecker]

I wouldnt check of the object is null, but if any of the fields is null

[Campbell Ritchie]

Apart from the fact that it shouldn't start with a CapitalLetter, your convert method does some unnecessary things. Just about the only object to return "" from its toString() method is the empty String. I think you can reduce it to this:-

public static String convertToEmptyString(Object o)
{
   return o == null || o.equals("") ? "" : o;
}

You can probably reduce it to this, too:-

public static String convertToEmptyString(Object o)
{
   return o == null ? "" : o;
}

I tried my list equality test again with a LinkedList:

List<String> words = new ArrayList<>(Arrays.asList(
      "Mary had a little lamb, she had it with mint sauce.".split("\\s+")));
List<String> rhyme = new LinkedList<>(words);
System.out.println(rhyme.equals(words)); // Only this part changed: prints true

[Campbell Ritchie]

Somebody has pointed out that convertToEmptyString() is a bad name for that method.