Compiler error while converting array to list?

Question

I am trying to transform an array of Cause to an ArrayList of Cause and i get this compiler error:

Type mismatch: cannot convert from ArrayList<Cause> to Cause[]

This is the implementation:

import java.util.ArrayList;
import java.util.Arrays;
import java.util.List;

public class MyException extends Exception

    private List<Cause> causes;

    public MyException(Cause... causes) {
        causes = new ArrayList<Cause>(Arrays.asList(causes));
    }

}

How can I convert that causes that I am receiving as a parameter to the List member? (I don't want to change parameter's type). Do you have any idea?

In think the error is in this portion - (Arrays.asList(causes)). — Razib
– Razib, Commented Mar 26, 2015 at 22:16

fabian · Accepted Answer · 2015-03-26 22:18:12Z

7

The left hand side of the assignment in your constructor refers the parameter, not to the field. (Also there's a { missing). You can fix this by adding this.:

public class MyException extends Exception {

    private List<Cause> causes;

    public MyException(Cause... causes) {
        this.causes = new ArrayList<Cause>(Arrays.asList(causes));
    }

}

answered Mar 26, 2015 at 22:18

fabian

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Comments

DNA · Accepted Answer · 2015-03-26 22:22:29Z

3

The confusing error is due to name shadowing (i.e. using the same name for two different things in the same scope); you have causes as a field of your class, and also as a parameter to the constructor. Rename it as shown and all is well:

public class MyException extends Exception {

    private List<Cause> causes;

    public MyException(Cause... cs) {
        causes = new ArrayList<Cause>(Arrays.asList(cs));
    }
}

Update: as the comments correctly point out, you commonly get around this problem by making it explicit to the compiler when you are referring to the field, using this:

public class MyException extends Exception {

    private List<Cause> causes;

    public MyException(Cause... causes) {
        this.causes = new ArrayList<Cause>(Arrays.asList(causes));
    }
}

edited Mar 26, 2015 at 22:22

answered Mar 26, 2015 at 22:15

DNA

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3 Comments

aioobe Over a year ago

Or use the common pattern this.causes = ... to initialize the member.

JustinKSU Over a year ago

Although your answer is helpful, why are your Lists of String?

DNA Over a year ago

Good point, I changed them to String when I was testing the code; will edit the answer.

JustinKSU · Accepted Answer · 2015-03-26 22:18:09Z

3

The parameter String... causes is the same as causes[]. If you want to convert it to a list you need to declare a new variable:

  List<Cause> causesList = Arrays.asList(causes);

Or simply explicitly reference your member class variable

   this.causes = Arrays.asList(causes);

answered Mar 26, 2015 at 22:18

JustinKSU

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Comments

Razib · Accepted Answer · 2015-03-26 22:18:47Z

0

You have declared cause as List<Cause> -

private List<Cause> causes;

and trying to convert causes (which is an arrayList already) to an array at here -

causes = new ArrayList<Cause>(Arrays.asList(causes));

Try to change either of causes from declaration or from method parameter.

answered Mar 26, 2015 at 22:18

Razib

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