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);
	}
}