儿子高考前创业是好是坏?

http://bbs.tianya.cn/post-free-6148377-1.shtml

package com.example.genericparadigmdemo.paradigm.array;

public class A {
}

package com.example.genericparadigmdemo.paradigm.array;

public class B extends A{
}

package com.example.genericparadigmdemo.paradigm.array;

public class C extends B {
}

package com.example.genericparadigmdemo.paradigm.array;

public class ArrayUtil {
    public static <T> T getMiddle(T... a){
        return a[a.length/2];
    }

    public static void main(String[] args) {
        String middleStr = ArrayUtil.<String>getMiddle("你好", "在家", "无故");
        System.out.println("middleStr:"+middleStr);

        Integer middleInte = ArrayUtil.getMiddle(1, 2, 3);
        System.out.println("middleInte:"+middleInte);

    }
}

package com.example.genericparadigmdemo.paradigm.array;

public class Father {

    public B f1(B obj){
        System.out.println("Father.f1()");
        return new B();
    }

}

package com.example.genericparadigmdemo.paradigm.array;

public class Son extends Father {

    /**
     * 返回值可以是B也可以是C
     * @param obj
     * @return
     */
    @Override
    public C f1(B obj){
        System.out.println("Son.f1()");
        return new C();
    }

}

package com.example.genericparadigmdemo.paradigm.array;

import java.util.ArrayList;

/**
 * 其中B类继承于A类
 */
public class TestAB {
    public static void main(String[] args) {

        ArrayList<B> list1 = new ArrayList<>();
        list1.add(new B());

        /**
         * 编译error 需要A,but found B
         */
        //ArrayList<A> list2 = list1;

        /**
         * 协变,指定了上界
         */
        ArrayList<? extends A> list3 = new ArrayList<B>();

        /**
         * 逆变,指定了下界
         */
        ArrayList<? super B> list4 = new ArrayList<A>();
    }

}

package com.example.genericparadigmdemo.paradigm.array;

import java.util.ArrayList;
import java.util.Collections;

public class TestArrayAndList {

    public static void main(String[] args) {
        /**
         * compile error
         * 需要Abut 声明传入B
         */
        //B[] r1 = test(new ArrayList<B>());

        /**
         * 尽管接受类型是A[] but传入还是B的,编译异常
         */
        //A[] r2 = test(new ArrayList<B>());

        /**
         * 如果test接受的是Object,则以上包括这个就不会
         * 编译异常了
         */
        //Object[] r3 = test(new ArrayList<Object>());

        A[] r4 = test(new ArrayList<A>());

        Object[] r5 = test(new ArrayList<A>());

    }

    public static A[] test(ArrayList<A> list){
        return new A[1];
    }
}

package com.example.genericparadigmdemo.paradigm.array;

public class TestFatherSon {

    public static void main(String[] args) {
        Father foo = new Son();
        foo.f1(new B());
    }

}