java泛型与通配符

1、泛型类、泛型接口、泛型方法

泛型类

package com.asiainfo.group.generic;
/**
 * 泛型类
 *
 */
public class GenericClassDemo<T> {
    private T obj;
    public GenericClassDemo(T obj) {
        this.obj = obj;
    }
    public T getObj(){
        return obj;
    }
}

泛型接口与实现类

package com.asiainfo.group.generic;
/**
 * 泛型接口
 *
 */
public interface GenericInterfaceDemo<T> {
    public T getObj();
}
package com.asiainfo.group.generic;
/**
 * 泛型接口实现类
 *
 */
public class GenericInterfaceImpl<T> implements GenericInterfaceDemo<T>{
    private T obj;
    public GenericInterfaceImpl(T obj){
        this.obj = obj;
    }
    @Override
    public T getObj() {
        return this.obj;
    }
}

泛型方法

package com.asiainfo.group.generic;
import java.util.Map;
public class GenericMethodDemo {
    /**
     * <T>表明该方法是个泛型方法
     */
    public <T> T getObj(Class<T> c){
        T obj = null;
        try {
            obj = c.newInstance();
        } catch (InstantiationException e) {
            e.printStackTrace();
        } catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        return obj;
    }
    /**
     * 泛型的数量可以任意多个
     */
    public <K,V> V getValueByKey(K key,Map<K, V> map){
        V value = map.get(key);
        return value;
    }
}

测试

package com.asiainfo.group.generic.main;
import java.text.SimpleDateFormat;
import java.util.Date;
import java.util.HashMap;
import java.util.Map;
import com.asiainfo.group.generic.GenericClassDemo;
import com.asiainfo.group.generic.GenericInterfaceImpl;
import com.asiainfo.group.generic.GenericMethodDemo;
public class GenericDemo {
    public static void main( String[] args ){
        //测试泛型类
        GenericClassDemo<String> g = new GenericClassDemo<String>("hello");
        String obj = g.getObj();
        System.err.println(obj);
        //测试泛型接口
        GenericInterfaceImpl<Date> g2 = new GenericInterfaceImpl<Date>(new Date());
        Date d = g2.getObj();
        System.err.println(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(d));
        //测试泛型方法
        GenericMethodDemo g3 = new GenericMethodDemo();
        //单个
        StringBuffer sb = g3.getObj(StringBuffer.class);
        sb.append("ok");
        System.err.println(sb.toString());
        //多个
        Map<String, Integer> map = new HashMap<String,Integer>();
        map.put("zhangsan", 20);
        map.put("lisi", 25);
        map.put("wangwu", 80);
        Integer value = g3.getValueByKey("lisi", map);
        System.err.println(value);
    }
}

2、通配符

一些实体类

package com.asiainfo.group.generic.wildcard;
public interface Animal {
    String sleep();
}
package com.asiainfo.group.generic.wildcard;
public class Cat implements Animal{
    @Override
    public String sleep() {
        return "cat----sleep";
    }
}
package com.asiainfo.group.generic.wildcard;
public class Dog implements Animal{
    @Override
    public String sleep() {
        return "dog----sleep";
    }
}
package com.asiainfo.group.generic.wildcard;
public class Apple{
}

测试

package com.asiainfo.group.generic.main;
import java.util.ArrayList;
import java.util.List;
import com.asiainfo.group.generic.wildcard.Animal;
import com.asiainfo.group.generic.wildcard.Apple;
import com.asiainfo.group.generic.wildcard.Cat;
import com.asiainfo.group.generic.wildcard.Dog;
public class WildcardDemo {
    /**
     * 通常在方法参数中才会使用通配符
     */
    public void printAnimalSleep(List<? extends Animal> list){
        for (int i = 0; i < list.size(); i++) {
            System.err.println(i+"----"+list.get(i).sleep());
        }
    }
    public static void main(String[] args) {
        //？为无界通配符    extends为上界，super为下界
        //通配符？在修饰一个变量的时候用到
        List<?> list = new ArrayList<String>();
        //使用过通配符修饰的集合变量，只能add(null)，因为这个集合中包含的类型已经确定，只是类型信息已经丢失了，add(Object)也不行
        list.add(null);
        //不加通配符修改变量，可以add任何类型，但不建议这样做
        List list2 = new ArrayList<String>();
        list2.add(1);
        list2.add("hello");
        //测试通配符
        WildcardDemo wildcardDemo = new WildcardDemo();
        List<Cat> cats = new ArrayList<Cat>();
        cats.add(new Cat());
        cats.add(new Cat());
        cats.add(new Cat());
        List<Dog> dogs = new ArrayList<Dog>();
        dogs.add(new Dog());
        dogs.add(new Dog());
        dogs.add(new Dog());
        List<Apple> apples = new ArrayList<Apple>();
        wildcardDemo.printAnimalSleep(cats);
        wildcardDemo.printAnimalSleep(dogs);
        //编译不通过
        //wildcardDemo.printAnimalSleep(apples);
    }
}