Java泛型的应用

一、泛型类

package generics;
/**
 * 泛型类，格式：public class 类名<泛型类型1, ...>
 * @author zhongfg
 * @date 2015-06-16
 * @param <T>
 */
class Student<T> {
    
    private T num;

    public T getNum() {
        return num;
    }

    public void setNum(T num) {
        this.num = num;
    }
    
}

public class GenericsClass {
    public static void main(String[] args) {
        
        Student<String> s1 = new Student<String>();
        s1.setNum("001");
        String num1 = s1.getNum();
        System.out.println(num1);
        
        System.out.println("----------------");
        
        Student<Integer> s2 = new Student<Integer>();
        s2.setNum(2);
        int num2 = s2.getNum();
        System.out.println(num2);
    }
}

二、泛型方法

package generics;
/**
 * 泛型方法，public <泛型类型> 返回值  方法名(泛型类型)
 * @author zhongfg
 * @date 2015-06-16
 */
class Actor {
    
    public <T> void show(T t){
        
        System.out.println(t);
    }
    
}

public class GenericsMethod {

    public static void main(String[] args) {
        
        Actor a = new Actor();
        a.show("Angelababy");
        a.show(26);
    }
}

三、泛型接口

package generics;
/**
 * 泛型类，格式：public interface 接口名<泛型类型1, ...>
 * @author zhongfg
 * @date 2015-06-16
 * @param <T>
 */
interface Person<T> {

    public abstract void show(T t);
}

class PersonImpl<T> implements Person<T> {

    @Override
    public void show(T t) {
        // TODO Auto-generated method stub
        System.out.println(t);
    }
}

public class GenericsInterface {
    public static void main(String[] args) {
        
        Person<String> p1 = new PersonImpl<String>();
        p1.show("黄晓明");
        
        Person<Integer> p2 = new PersonImpl<Integer>();
        p2.show(38);
    }
}

四、泛型高级之通配符

package generics;

import java.util.ArrayList;
import java.util.Collection;

/**
 * 泛型高级（通配符）
 * ？：任意类型，如果没有明确指定，那么就是Object或者任意的Java类了
 * ？ extends E：向下限定，E及其子类
 * ? super E：向上限定，E及其父类
 * @author zhongfg
 * @date 2015-06-16
 */
class Animal {
    
}

class Dog extends Animal {
    
}

class Cat extends Animal {
    
}

public class GenericsWildcard {
    public static void main(String[] args) {
        
        //明确指明泛型的时候，前后必须一致
        Collection<Object> c1 = new ArrayList<Object>();
//        Collection<Object> c2 = new ArrayList<Animal>(); 报错，前后类型不一致
//        Collection<Object> c3 = new ArrayList<Dog>(); 同上
//        Collection<Object> c4 = new ArrayList<Cat>(); 同上
        
        //?表示任意类型都是可以的
        Collection<?> c5 = new ArrayList<Object>();
        Collection<?> c6 = new ArrayList<Animal>();
        Collection<?> c7 = new ArrayList<Dog>();
        Collection<?> c8 = new ArrayList<Cat>();
        
        //？ extends E：向下限定，E及其子类
//        Collection<? extends Animal> c9 = new ArrayList<Object>(); 报错，Object不是Animal子类
        Collection<? extends Animal> c10 = new ArrayList<Animal>();
        Collection<? extends Animal> c11 = new ArrayList<Dog>();
        Collection<? extends Animal> c12 = new ArrayList<Cat>();
        
        //? super E：向上限定，E及其父类
        Collection<? super Animal> c13 = new ArrayList<Object>(); 
        Collection<? super Animal> c14 = new ArrayList<Animal>();
//        Collection<? super Animal> c15 = new ArrayList<Dog>(); 报错，Dog不是Animal及其以上的类型
//        Collection<? super Animal> c16 = new ArrayList<Cat>(); 同上
    }
}