Questions and Exercises: Generics - Java Tutorial 5.0 英文版

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Questions and Exercises: Generics

Questions

1. Consider the following classes:

public class AnimalHouse<E> {
    private E animal;
    public void setAnimal(E x) {
        animal = x;
    }
    public E getAnimal() {
        return animal;
    }
}

public class Animal{
}

public class Cat extends Animal {
}

public class Dog extends Animal {
}

For the following code snippets, identify whether the code:

1. fails to compile
2. compiles with a warning
3. generates an error at runtime
4. none of the above (compiles and runs without problem)

a. AnimalHouse<Animal> house = new AnimalHouse<Cat>();

b. AnimalHouse<Dog> house = new AnimalHouse<Animal>();

c. AnimalHouse<?> house = new AnimalHouse<Cat>();
   house.setAnimal(new Cat());

d. AnimalHouse house = new AnimalHouse();
   house.setAnimal(new Dog());

Exercises

1. Write an example that sorts a set of predefined strings based on string length. Use the version of the Collections.sort method that takes a Comparator parameter.

Check your answers.

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