"... unchecked or unsafe operations" generic container class

Question

I want to make a generic container class that can contain one object of some other class. I thought this might be a reasonable approach:

class Container <T> {
    private T thing;

    public void Store(T obj) {
        thing = obj;
    }

    public T ReturnIt() {
        return thing;
    }
}

When I try this together with let's say a Book class, I get the following error message: "Note: GenericContainer.java uses unchecked or unsafe operations. Note: Recompile with -Xlint:unchecked for details."

Could it be that the public T ReturnIt() { return thing; } is the cause of the error, and is this the wrong way to go about returning object that is contained in the Container-class? I did not get any further information when I tried to compile it with -Xlint:unchecked. What do I make of the error message?

Code that caused the error:

class GenericContainer {

    public static void main(String[] args) {
        Container BookStorage = new Container <Book>();
        Book thejavabook = new Book("The Java book");   
        BookStorage.Store(thejavabook);
    }       
}

class Book {
    private String title;
    Book(String title) {
        this.title = title;
    }
}

class Container <T> {
    private T thing;

    public void Store(T obj) {
        thing = obj;
    }

    public T ReturnIt() {
        return thing;
    }
}

Did you try -Xlint:unchecked? What did it tell you? Usually Eclipse will give more detailed warnings about these things, if you use Eclipse. — Ryan Amos, Jan 23 '13 at 20:04
Yes, i did. Sorry if i was not clear on that point - i got the exact same error message when i compiled it with the Xlint:unchecked-command. But as jdb pointed out, i forgot to use in the assignment. Thanks :) — user2005142, Jan 23 '13 at 20:11

score 3 · Answer 1 · answered Jan 23 '13 at 20:00

3

Your BookStorage variable should be defined like this:

Container<Book> BookStorage = new Container <Book>();

answered Jan 23 '13 at 20:00

jdb

4,419
21
21

1

Or in the current version of Java SE, `Container bookStorage = new Container<>();` – Tom Hawtin - tackline Jan 23 '13 at 21:42
What does it mean to not have the `<>`? will it be incorrect/What is the Java compiler going to do with it? – Matthew Woo Sep 21 '17 at 00:25

score 2 · Answer 2 · answered Jan 23 '13 at 20:02

I've rewritten your code to fix the problem and to use java naming standards:

package com.sandbox;

public class Sandbox {

    public static void main(String[] args) {
        Container<Book> bookStorage = new Container<Book>();  //fix for your warning!
        Book theJavaBook = new Book("The Java book");
        bookStorage.store(theJavaBook);
    }
}


class Book {
    private String title;   //this is unused

    Book(String title) {
        this.title = title;
    }
}


class Container<T> {
    private T thing;

    public void store(T obj) {
        thing = obj;
    }

    public T returnIt() {
        return thing;
    }
}

Emphasis on this line:

Container<Book> bookStorage = new Container<Book>();  //fix for your warning!

You forgot to put the <Book> on the left hand side of your assignment.

Thanks! I did not even realize that i had to specify on both sides in the assignment. — user2005142, Jan 23 '13 at 20:12

"... unchecked or unsafe operations" generic container class

2 Answers2