泛型的好处

• 类型安全
• 消除强制转换

package generics;

/**
 * Created by lipei on 2017/5/1.
 */
public class NoGenericsDemo {

    public static void main(String[] args) {

        Wrapper w1 = new Wrapper(123);
        Wrapper w2 = new Wrapper("1234");
        Wrapper w3 = new Wrapper(Long.valueOf("1234567890"));

        System.out.println(w1);
        System.out.println(w2);
        System.out.println(w3);

        //放进去没有错误，取出也没有问题，但是运行时就会出问题
        String w11 = (String) w1.getContent();
        System.out.println("w11 =" + w11);

    }
}

class  Wrapper{
    public Object content;

    public Wrapper(Object content) {
        this.content = content;
    }

    public Object getContent() {
        return content;
    }

    public void setContent(Object content) {
        this.content = content;
    }

    @Override
    public String toString() {
        return "Wrapper{" +
                "content=" + content +
                '}';
    }
}

什么是泛型

• 实现了参数化类型的概念，是代码可以应 用于多种类型
• 可以用在类、接口和方法的创建中，分别 称为泛型类、泛型接口、泛型方法
• 当创建类型化参数时，编译器会负责转换 操作

通过泛型优化

package generics;

/**
 * Created by lipei on 2017/5/1.
 */
public class GenericsDemo {

    public static void main(String[] args) {

        Wrapper02<Integer>w1 = new Wrapper02(123);
        Wrapper02 w2 = new Wrapper02("1234");
        Wrapper02 w3 = new Wrapper02(Long.valueOf("1234567890"));

        System.out.println(w1);
        System.out.println(w2);
        System.out.println(w3);

        //在类型初始化的时候已经告知是什么类型，故运行时不会出错
        Integer w11 =   w1.getContent();
        System.out.println("w11 =" + w11);

    }
}

class  Wrapper02<T>{
    public T content;

    public Wrapper02(T content) {
        this.content = content;
    }

    public T getContent() {
        return content;
    }

    public void setContent(T content) {
        this.content = content;
    }

    @Override
    public String toString() {
        return "Wrapper{" +
                "content=" + content +
                '}';
    }
}

声明与使用泛型接口

package generics;

/**
 * Created by lipei on 2017/5/1.
 */
public interface GenericsInterface<T> {
    public T update (T t);
}


class  test01 implements  GenericsInterface <String>{
    @Override
    public String update(String s) {
        return null;
    }
}


class  test02 implements  GenericsInterface<Integer>{

    @Override
    public Integer update(Integer integer) {
        return null;
    }
}