将 object 添加到 Java 中的数组列表中

Question

I am trying to add an object to an array list and have the following -

public static void main(String[] args) {

        authors Authors = new authors();
        ArrayList<authors> tabAuthors = new ArrayList<authors>();

        Authors.setAuthId(1);
        Authors.setAuthName("Roald Dahl");
        System.out.println(Authors.toString());
        tabAuthors.add(Authors);


        Authors.setAuthId(2);
        Authors.setAuthName("Julia Donaldson");
        System.out.println(Authors.toString());
        tabAuthors.add(Authors);

        for (int counter =0; counter < tabAuthors.size(); counter++) {
        System.out.println(tabAuthors.get(counter).getAuthId() + " " + tabAuthors.get(counter).getAuthName() );
        }
   }
}

The authors class

public class authors {

    private int authId;
    private String authName;


    public int getAuthId() {
        return authId;
    }

    public void setAuthId(int authId) {
        this.authId = authId;
    }

    public String getAuthName() {
        return authName;
    }

    public void setAuthName(String authName) {
        this.authName = authName;
    }

    @Override
    public String toString() {
        return "authors{" +
                "authId=" + authId +
                ", authName='" + authName + '\'' +
                '}';
    }
}

I was expecting the code to return - 1 Roald Dahl 2 Julia Donaldson

Instead, I am getting - 2 Julia Donaldson 2 Julia Donaldson

Why is the array list not reflecting the first object values?

Answer 1

Because you create only one object and override second time when you set properties.

You need to create new object when you insert second object.

Answer 2

You are setting the value Julia Donaldson on the same Object. reference the variable Authors to a new object and the problem is solved.

authors Authors = new authors();
ArrayList<authors> tabAuthors = new ArrayList<authors>();

Authors.setAuthId(1);
Authors.setAuthName("Roald Dahl");
System.out.println(Authors.toString());
tabAuthors.add(Authors);

Authors = new Authors(); //Reference Authors to a new object here, or else 
//you're using the same object and you'll overwrite the value Roald Dahl with 
//Julia Donaldson 
Authors.setAuthId(2);
Authors.setAuthName("Julia Donaldson");
System.out.println(Authors.toString());
tabAuthors.add(Authors);

Answer 3

In addition to the answer above by @Maurice, you can also add a constructor in authors class to set values for every object.

public authors(int authId, String authName) {
        super();
        this.authId = authId;
        this.authName = authName;
}

In your main() , create two objects as below and add them to the list:

public static void main(String[] args) {

        Authors authors1 = new Authors(1, "Roald Dahl");
        Authors authors2 = new Authors(2, "Julia Donaldson");
        
        List<Authors> tabAuthors = new ArrayList<Authors>();

        System.out.println(authors1.toString());
        tabAuthors.add(authors1);

        System.out.println(authors2.toString());
        tabAuthors.add(authors2);

        for (int counter = 0; counter < tabAuthors.size(); counter++) {
        System.out.println(tabAuthors.get(counter).getAuthId() + " " + tabAuthors.get(counter).getAuthName());
      }
} 

Output:

authors{authId=1, authName='Roald Dahl'}
authors{authId=2, authName='Julia Donaldson'}
1 Roald Dahl
2 Julia Donaldson

Answer 4

Thanks for your prompt responses.
I added the below in the code as I need to add indeterminate number of objects - tabAuthors.add(new authors(authId,authName,authCountry,numBooks));

and declared authId,authName,authCountry,numBooks as variables. The object values are being fetched from a database table.