package cn.xf.cp.ch02.item27;

import java.util.HashSet;

import java.util.Set;

public class Union

{

    /**

     * 这个方法就会有警告

     * @param s1

     * @param s2

     * @return

     */

    public static Set union1(Set s1, Set s2)

    {

        Set result = new HashSet(s1);

        result.addAll(s2);

        return result;

    }

    /**

     * 这里使用泛型就是安全没有警告的

     * @param s1

     * @param s2

     * @return

     */

    public static <E> Set<E> union(Set<E> s1, Set<E> s2)

    {

        Set<E> result = new HashSet<E>(s1);

        result.addAll(s2);

        return result;

    }

}

package cn.xf.cp.ch02.item27;

import java.util.HashMap;

import java.util.List;

import java.util.Map;

public class GenericStaticFactory

{

    public static <K, V> HashMap<K, V> newHashMap()

    {

        return new HashMap<K, V>();

    }

    public static void main(String[] args)

    {

        //这里创建对象的时候，就会根据前面map中的String和List<String>自动转换

        Map<String, List<String>> anagrams = newHashMap();

    }

}

package cn.xf.cp.ch02.item27;

public class GenericSingletonFactory

{

    //先创建object对象，暂时替代T对象

    private static UnaryFunction<Object> IDENTITY_FUNCTION = new UnaryFunction<Object>()

    {

        public Object apply(Object arg)

        {

            return arg;

        }

    };

    //根据T，吧IDENTITY_FUNCTION转换为相应的类型，由于IDENTITY_FUNCTION返回未被修改的object类型参数，所以下面类型转换时安全的

    @SuppressWarnings("unchecked")

    public static <T> UnaryFunction<T> identityFunction()

    {

        return (UnaryFunction<T>) IDENTITY_FUNCTION;

    }

    public static void main(String[] args)

    {

        String[] strings =

        { "jute", "hemp", "nylon" };

        UnaryFunction<String> sameString = identityFunction();

        for (String s : strings)

            System.out.println(sameString.apply(s));

        Number[] numbers =

        { 1, 2.0, 3L };

        UnaryFunction<Number> sameNumber = identityFunction();

        //这里用来判断是否真的实现了单例

        Object t1 = sameString;

        Object t2 = sameNumber;

        if(t1 == t2)

        {

            System.out.println("相同引用");

        }

        String s1 = "123456";

        String s2 = new String("123456");

        if(s1.equals(s2))

            System.out.println("相同内容");

        for (Number n : numbers)

            System.out.println(sameNumber.apply(n));

    }

}