Java泛型分配全局变量

Question

In this example, I get the warning that "List is a raw type." How can I use generics to properly satisfy this warning? I'm having trouble because I can't figure out how to define "T" for the global variable like I was able to for the constructor signature.

public class MyClass{

    private List input;

    public <T extends Comparable<T>> MyClass(List<T> input){
        this.input = input;
    }

}

Answer 1

You must define your class as parameterized

public class MyClass<T extends Comparable<T>>{

    private List<T> input;

    public MyClass(List<T> input){
        this.input = input;
    }

}

Answer 2

'Raw Types' refer to generic types that have not had their generic parameter types specific. In this case, this is because you've omitted the generic type in your declaration of input . Changing the declaration to:

private List<T> input;

Should resolve the error. In addition, your constructor declaration doesn't need generic parameters, and can be shortened to:

public MyClass(List<T> input){
    this.input = input;
}

Answer 3

You probably need to do something like:

public class MyClass<T> {
    private List<T> input;
}

Answer 4

As you've seen, the issue is caused by your member variable input using List without a generic parameter. Hence, the type is being used "raw". To resolve this, you have three options. You can:

1. Make your whole class generic

    public class MyClass<T extends Comparable<T>> { private List<T> input; public MyClass(List<T> input) { this.input = input; } } 

or

2. Use a wildcard

    private List<? extends Comparable<T>> input; 

or

3. Rethink your strategy

     public class MyClass { private List<Comparable<T>> input; public <T extends Comparable<T>> MyClass(List<T> input) { this.input = new LinkedList<>(); // Obv. there are better ways of copying a list... for (Comparable<T> obj : input) { this.input.add(obj); } } } 

I would recommend option 1 or 3.

Answer 5

I believe your problem is where you say private List input; , you game no parameter for the generic type that it's looking for. You can fix it by saying private List<T> input;

Answer 6

You could make the class generic.

public class MyClass<T extends Comparable<T>> {

    private final List<T> input;

    public MyClass(Collection<? extends T> input){
        this.input = new ArrayList<T>(input);
    }
}

I have also changed the line this.input = input to instead make a copy of input . This is really quite important because otherwise anyone with a reference to input could modify your class by modifying input . I've also made your constructor a bit more flexible by allowing any Collection of any subtype of T .