Question

Since the equals function in array only check the instance, it doesn’t work well with Set.
Hence, I wonder how to make a set of arrays in Java?

One possible way could be put each array in an object, and implement equals function for that class, but will that decrease the performance too much?

Answer 1

Since the ArrayList class already wraps an array, you can extend it and override the equals and hashCode methods. Here is a sample:

public MyArrayList extends ArrayList<MyClass> {

    @Override
    public boolean equals(Object o) {
        if (o instanceof MyArrayList) {
            //place your comparison logic here
            return true;
        }
        return false;
    }

    @Override
    public int hashCode() {
        //just a sample, you can place your own code
        return super.hashCode();
    }
}

UPDATE:

You can even override it for a generic use, just changing the code to:

public MyArrayList<T> extends ArrayList<T> {
    //overrides the methods you need
    @Override
    public boolean equals(Object o) {
        if (o instanceof MyArrayList) {
            //place your comparison logic here
            return true;
        }
        return false;
    }
}