第五天 实现点击item，切换Fragment

holder.itemView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Toast.makeText(mcontext,"第"+(position+1)+"条消息查看成功",Toast.LENGTH_SHORT).show();
                KeFragment keFragment=new KeFragment("");
                FragmentTransaction transaction=((StudentActivity)mcontext).getSupportFragmentManager().beginTransaction();
                transaction.replace(R.id.fragment_container,keFragment);
                transaction.addToBackStack(null);
                transaction.commit();

            }
        });