这是转换ArrayList的好方法 <Object> 到ArrayList <MyObject> ？

Question

Based on this Spring tutorial: http://www.roseindia.net/tutorial/spring/spring3/ioc/springlistproperty.html I had a problem. i used Spring framework to create a list of objects, but I wanted to get list of list. Casting from ArrayList to ArrayList is not possible, so I've made my own static method to do it. We have two classess:

Student:

public class Student {
    private String name;
    private String address;

   //getters and setters
}

College:

import java.util.List;

public class College {
    private List<Object> list;

    public List<Object> getList() {
        return list;
    }

    public void setList(List<Object> list) {
        this.list = list;
    }
}

And context.xml

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
       xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xsi:schemaLocation="http://www.springframework.org/schema/beans
                           http://www.springframework.org/schema/beans/spring-beans.xsd">
    <bean id="Student" class="testing.Student">
        <property name="name" value="Thomas"/>
        <property name="address" value="London"/>
    </bean>

    <bean id="College" class="testing.College">

        <property name="list">
            <list>
                <value>1</value>
                <ref bean="Student"/>
                <bean class="testing.Student">
                    <property name="name" value="John"/>
                    <property name="address" value="Manchester"/>
                </bean>
            </list>
        </property>
    </bean>
</beans>

Here's my main method:

public static void main(String[] args) {
        BeanFactory beanFactory = new ClassPathXmlApplicationContext(
                "context.xml");
        College college = (College) beanFactory.getBean("College");
}

What I wanted to do here is to make generic Student ArrayList by receiving it from college object which contains Object list. Here's my solution:

import org.springframework.beans.factory.BeanFactory;
import org.springframework.context.support.ClassPathXmlApplicationContext;

import java.util.ArrayList;

public class MainTest {

    //This is my casting static method:

    public static ArrayList<Student> castListToStudent(College college) {
        ArrayList<Student> casted = new ArrayList<Student>();
        Student s = null;
        for (int i = 0; i < college.getList().size(); i++) {

            if (college.getList().get(i) instanceof Student) {
                s = (Student) college.getList().get(i);
                casted.add(s);
            }
        }
        return casted;
    }

    public static void main(String[] args) {
        BeanFactory beanFactory = new ClassPathXmlApplicationContext(
                "context.xml");
        College college = (College) beanFactory.getBean("College");
        ArrayList<Student> list = castListToStudent(college);

        for (Student s : list) {
            System.out.println(s);
        }
    }
}

Looks like it's working, but the question is - is it the best way to safely cast one list to another ?

Answer 1

Using Guava:

List<Object> objects = Lists.newArrayList();
objects.add("A");
objects.add("B");
List<String> strings = FluentIterable.from(objects).filter(String.class).toList();

This example returns an ImmutableList If you need a mutable list ( ArrayList ):

List<String> strings = Lists.newArrayList(Iterables.filter(objects, String.class));

Any elements in objects that are not String (in my example) will be ignored. This is a completely type-safe solution that does not require any self-written methods.

Answer 2

Using an intermediate wildcard to do the casting does the job of casting.

List<Student> casted = (List<Student>)(List<?>) college.getList();

This is more compact. However your method is much safer as this causes an unchecked cast warning.

Your method is the best way to go.

Proof : List<Object> can have objects that extends Object. Thus if you want to cast safely, you should check that each object in List is an instance of Student. Therefore you have to traverse all the list. Thus in terms of performance you can't do any better than traversing the whole list.