泛型Java类中的反思

Question

I have followings classes (it's just a simplified example):

public abstract class Material {
    public abstract String name();
    /* ... */
}

public class Wood extends Material {
    @Override
    public String name() {
        return "<WOOD>";
    }
    /* ... */
}

public class Metal extends Material {
    @Override
    public String name() {
        return "{Metal}";
    }
    /* ... */
}

public class Car<T extends Material> {
    public void printName() {
        System.out.println(T.name()); // Here is the problem!
    }
    /* ... */
}

public class Main {
    public static void main(String[] args) {
        Car<Wood> myCar1 = new Car<Wood>();
        Car<Metal> myCar2 = new Car<Metal>();
        myCar1.printName();
        myCar1.printName();
    }
}

The problem is indicated in the code.

Answer 1

This is probably how I would have implemented it. If it satisfactory or not for your situation, I can't tell.

enum Material {

    WOOD("<WOOD>"),
    METAL("{Metal}");

    String name;
    private Material(String name) {
        this.name = name;
    }
    public String toString() {
        return name;
    }
}

class Car {
    Material material;
    public Car(Material material) {
        this.material = material;
    }
    public void printName() {
        System.out.println(material);
    }
}

public class Main {
    public static void main(String[] args) {
        Car myCar1 = new Car(Material.WOOD);
        Car myCar2 = new Car(Material.METAL);
        myCar1.printName();
        myCar2.printName();
    }
}

Answer 2

Here is one problem:

name is instance method ( not static ), but you are trying to call it as static.

This should fix it

public class Car<T extends Material> {
    public void printName( T material ) {
        System.out.println( material.name()); 
    }
    /* ... */
}

Answer 3

问题是您试图在没有实例对象实例的情况下调用实例方法。

Answer 4

There are a lot of misunderstanding in your question:

1. name() is not a static function, therfore you need an instance of a material object to call it.
2. even if name() were static, you cannot call a static method from a generic name.

A solution? Create a enumeration of material:

public enum Material {
    WOOD("wood"), 
    METAL("metal");

    private final String name;

    Material(String name) {
        this.name = name;
    }
}

And in the car class became:

public class Car {
    private final Material m;

    public Car(Material m) {
        this.m = m
    }

    public void printName() {
        System.out.println(m.name);
    }
    /* ... */
}

Answer 5

T defines the type, and the name() should be static to be called in that context. Also, consider:

public class Car<T extends Material> {
    protected T material;
    // initialize material in constructor or wherever you want.
    ....
    public void printName() {
        System.out.println(material.name()); // there is no problem
    }
    /* ... */
}

Answer 6

This will never work, since your 'T' is only a "compiler hint", and you can't access the class defined by T at runtime, unless you refer to it explicitly in your functions.

That's not how java generics work: when you compile your code, any reference to Wood and Metal is lost, and your 2 "Car" objects are identical.

Try this:

myCar1 = new Car<Wood>();
System.out.println(myCar1 instanceof Car<Metal>);