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Java generic class & wildcards

I have a generic Box class with the following function:

  • one of methods 2 putshould allow the client to insert a field and get its contents in the current field.

I want this method to get two types of boxes: Box<Number>and Box<Integer>, therefore, I changed the method put(Box<T>)to put(Box<? extends Number> box). But I get a compilation warning. What am I doing wrong here?

This is my current code: warning I get: Security Type: Unchecked throw from capture # 1-of? extends the number to T

public class Box<T> {
    public T get() {
        return element;
    }

    public void put(T element) {
        this.element = element;
    }

    public void put(Box<? extends Number> box) {
        put((T) box.get()); // this is where i get the warning
    }

    private T element;
}

public class BoxClient {

    public static void main(String[] args) {
        Box<Number> nBox = new Box<Number>();
        Box<Integer> iBox = new Box<Integer>();
        nBox.put(iBox);

    }

}
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5 answers

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:

public void put(Box<? extends Number> box) {
    put(box.get());
}

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public class BoxClient {

    public static void main(String[] args) {
        Box<String> sBox = new Box<String>();
        Box<Integer> iBox = new Box<Integer>();

        iBox.put(1);
        sBox.put(iBox);

        System.out.println(sBox.get()); 
    } 

}

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java.lang.ClassCastException: java.lang.Integer java.lang.String

, Integer 1 String.

, , Box, T extends Number, :

public class Box<T extends Number> {
    public T get() {
        return element;
    } 

    public void put(T element) {
        this.element = element;
    }

    public void put(Box<? extends T> box) {
        put((T) box.get());
    }

    private T element;
}

UPDATE: , . , Box<Number> Box<Integer>, .

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UPDATE2: newacct, T .

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+1

public void put(Box<? extends T> box) {
    put(box.get());
}
+6

, :

public void put(Box<? extends Number> box) {
    put( (T) box.get());
}

:

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+3

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+1

, Java 5. - .

:

public abstract class Draft<T> {
    public abstract void put(T element);
    public abstract void putDraft(Draft draft);
}

.

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