Return object from a list as CustomObject not just raw Object

Question

I have a CustomObject declared as raw type of <T>. And when I populate a List<CustomObject> with new instances of it, I can't get them back as a CustomObject, only as an Object.

public class CustomObject<T> {
    private String name;
    private T value;

    // getters and setters
}

But obviously when I use subclass, all is working as expecting;

public class CustomObject {

    private class SubCustomObject<T> {
        private String name;
        private T value;
    }

    public CustomObject() {
        this.customObject = new SubCustomObject();

    private SubCustomObject customObject;

    // getters and setters
}

Is there a way to make the first example to behave like the second one, and avoid using extra object and so I could do this:

public class CustomObject<T> {
    private String name;
    private T value;
    private boolean isGroup;

    // getters and setters

    private void setValue(T value) {

        if (value instanceof String) {
            this.value = value;
            this.isGroup = false;
        }

        if (value instanceof CustomObject) {
            if (isGroup()) {
                ((List<CustomObject>) this.value).add((CustomObject) value);
            } else {
                this.value = (T) new ArrayList<CustomObject>();
                this.isGroup = true;
                setValue(value);
            }
        }
    }
}

public void getItemByName(String name) {
    // say the list is already populated
    for (CustomObject object : listOfCustomObject) {
        String nameField = object.getName();
        if (name.equals(nameField) {
            System.out.println(nameField);
        }
    }
}

Instead of this one:

public void getItemByName(String name) {
    // say the list is already populated
    for (Object object : listOfCustomObject) {
        String nameField = ((CustomObject)object).getName();
        if (name.equals(nameField) {
            System.out.println(nameField);
        }
    }
}

// Apply that behavior to this and avoid to use inner class.

public class MetadataEntry {

    public MetadataEntry() {
        this.entity = new Entry();
    }

    private class Entry<T> {
        private String name;
        private T value;
        private boolean isGroup;

        private void setValue(T value) {

            if (value instanceof String) {
                this.value = value;
                this.isGroup = false;
            }

            if (value instanceof MetadataEntry) {
                if (isGroup()) {
                    ((List<MetadataEntry>) this.value).add((MetadataEntry) value);
                } else {
                    this.value = (T) new ArrayList<MetadataEntry>();
                    this.isGroup = true;
                    setValue(value);
                }
            }
        }

    }

    private Entry entity;


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

    public String getName() {
        return this.entity.name;
    }

    public void setValue(String value) {
        entity.setValue(value);
    }

    public void setValue(MetadataEntry value) {
        entity.setValue(value);
    }

    public boolean isGroup() {
        return this.entity.isGroup;
    }

    public List<MetadataEntity> getChildNodes() {
        if (isGroup()) {
            return (List<MetadataEntry>) this.entity.value;
        }

        return null;
    }

    public String getValue() {
        if (!isGroup()) {
            return (String) this.entity.value;
        }

        return null;
    }
}

Answer 1

You can not make a list of different types X,Y,Z and put it in a single container of type W. You need to define a bounding parameter on your raw type so that your items and list are of same type. probably your T should be bounded by some interface type or it should extends some class.

Answer 2

Here’s my suggestion. I have abandoned the generics. Instead of just one inner class there is now an abstract inner class with two subclasses, one for groups and one for entries that are not groups. The good news: no cast is necessary anywhere.

public class MetadataEntry {

    private String name;

    static abstract class Entry {
        abstract Entry setValue(String value);
        abstract Entry setValue(MetadataEntry value);
        abstract boolean isGroup();
        abstract List<MetadataEntry> getChildNodes();
        abstract String getSimpleValue();

    }

    static class SimpleEntry extends Entry {
        private String value;

        public SimpleEntry(String value) {
            this.value = value;
        }

        @Override
        Entry setValue(String value) {
            this.value = value;
            return this;
        }

        @Override
        Entry setValue(MetadataEntry value) {
            return new GroupEntry(value);
        }

        @Override
        public boolean isGroup() {
            return false;
        }

        @Override
        public List<MetadataEntry> getChildNodes() {
            return null;
        }

        @Override
        public String getSimpleValue() {
            return value;
        }
    }

    static class GroupEntry extends Entry {
        List<MetadataEntry> value;

        public GroupEntry(MetadataEntry value) {
            this.value = new ArrayList<>();
            this.value.add(value);
        }

        @Override
        Entry setValue(String value) {
            return new SimpleEntry(value);
        }

        @Override
        Entry setValue(MetadataEntry value) {
            this.value.add(value);
            return this;
        }

        @Override
        public boolean isGroup() {
            return true;
        }

        @Override
        public List<MetadataEntry> getChildNodes() {
            return value;
        }

        @Override
        public String getSimpleValue() {
            return null;
        }
    }

    private Entry entity;

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

    public String getName() {
        return this.name;
    }

    public void setValue(String value) {
        entity = entity.setValue(value);
    }

    public void setValue(MetadataEntry value) {
        entity = entity.setValue(value);
    }

    public boolean isGroup() {
        return this.entity.isGroup();
    }

    public List<MetadataEntry> getChildNodes() {
        return entity.getChildNodes();
    }

    public String getValue() {
        return entity.getSimpleValue();
    }
}

I have used an idea similar to what m 1987 said about a class that returns an instance of itself. I applied it to the inner classes only to free the users of the outer class from caring about this trickery. If you prefer, I am sure it could be applied to the outer class instead. Then you would have an abstrat class on the outer level with two subclasses, and would no longer need the inner classes. This is one of the things you asked for, so you may prefer it, but it comes at a cost: anyone calling setValue() on the outer class would need to remember that they got a new instance back.

Answer 3

I have a CustomObject declared as raw type of <T>.

That doesn't makes sense. You either have a raw type or a generic, not a raw type of a generic.

And when I populate a List with new instances of it, I can't get them back as a CustomObject, only as an Object.

Because your list is not generic (always bad). When you declare a List<Something> it will return Something on a get call. That Something can be generic or a raw type. A List<CustomObject<String>> will not accept a CustomObject<Integer> and using the raw type List<CustomObject> will end in disaster, hence the danger in raw types.

Now let's look at your code. The class

public class CustomObject<T> {
    private String name;
    private T value;
}

defines an object that behaves the same for any type. In essence what you have here is just a glorified Object with a String serving as its name attached to it.

However, now you do

private void setValue(T value) {

    if (value instanceof String)
        // ...

    if (value instanceof CustomObject)
        // ...
}

which separates the behavior for different types. and what happens if the generic type is not a String or a CustomObject?

Let's try to solve your problem. Since generics are meant to unify the behavior, let's look at what the unified behavior is that you're trying to get:

public void getItemByName(String name) {

    for (CustomObject object : listOfCustomObject) {
        String nameField = object.getName();
        // ...
        }
    }
}

Basically, you require that all the items in listOfCustomObject implement a String getName() method. That's it as far as I can see from your question. That means that your CustomObject<T> should either implement an interface or extend a class (call it Superthing) with that method. Then you will just declare your list as List<? extends Superthing>.

As for the CustomObject itself, it doesn't need to be generic as you hint that there are only 2 types of generics you want to deal with (you have 2 ifs, but no else to deal with a general case). It looks like what you want are 2 different classes with different behaviors that both implement or extend a common supertype.

Try something like this:

abstract class AbstractEntry {

    private String name;
    protected boolean isGroup;

    public void setName(String name) {

        this.name = name;
    }

    public String getName() {

        return name;
    }

    public boolean isGroup() {

        return isGroup;
    }
}

class MetaEntry extends AbstractEntry {

    AbstractEntry value;

    MetaEntry(AbstractEntry value) {

        this.value = value;
        // handle isGroup
    }   

    public void setValue(AbstractEntry value) {

        this.value = value;
    }

    public AbstractEntry getValue() {

        if (!isGroup)
            return value;
        return null;
    }
}

class StringEntry extends AbstractEntry {

    String value;

    StringEntry(String value) {

        this.value = value;
        isGroup = false;
    }

    public void setValue(String value) {

        this.value = value;
    }

    public String getValue() {

        return value;
    }
}

Answer 4

I think there is no need of list as it always hold only one element. As @Ole V.V mentioned, the requirement naturally calls for the use of composition and in fact, generic does not fit into your requirements. Here is how I would tackle your requirements:

public interface Named {

public String getName();
public String getValue();
}

public class CustomObject implements Named {
private String name;
private String value;
private boolean isGroup;

// getters and setters

private boolean isGroup() {

    return isGroup;
}

public String getName() {
    return name;
}

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

public String getValue() {
    return value;
}

public void setValue(String value) {
    this.value = value;
}
}

public class CustomObject2 implements Named {

private String name;
private CustomObject value;


public String getName() {
    return name;
}
public void setName(String name) {
    this.name = name;
}
public String getValue() {
    return value.getValue();
}
public void setValue(CustomObject value) {
    this.value = value;
}

}

public class DriverCustomObject {

public static void main(String arg[]) {
    CustomObject t = new CustomObject();
    t.setName("key1");
    t.setValue("value1");
    CustomObject2 t2 = new CustomObject2();
    t2.setName("complex");
    t2.setValue(t);

    List<Named> list = new ArrayList<Named>();
    list.add(t);
    list.add(t2);
    for (Named l : list) {
        System.out.println(l.getName());
        System.out.println(l.getValue());

    }
}

}