【提问】Android Studio随机加减游戏，初学者不太懂求大牛教

就是韩国教授让做一个小程序，初学者老师也没有教，不太会

加减,(1至99)随机生成
就好像是那种过关小游戏
答对下面就显示答对了
打错就显示错了

首先，你要了解android的随机数是怎么生成，其次，你要了解android中字符比较是用什么，至于加减都是数学问题，不难

xml：
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent">

    <LinearLayout
        android:id="@+id/ll_one"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <TextView
            android:id="@+id/tv_before"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginStart="20dp"
            android:layout_weight="1"
            android:gravity="center"
            android:textSize="24sp"
            tools:text="10" />

        <TextView
            android:id="@+id/tv_center"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:textSize="24sp"
            tools:text="+" />

        <TextView
            android:id="@+id/tv_after"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:textSize="24sp"
            tools:text="20" />

        <TextView
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:gravity="center"
            android:text="="
            android:textSize="24sp" />

        <EditText
            android:id="@+id/et_result"
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_marginEnd="30dp"
            android:layout_weight="1"
            android:digits="0123456789-"
            android:gravity="center"
            android:textSize="24sp"
            tools:text="30" />
    </LinearLayout>

    <TextView
        android:id="@+id/tv_opinion"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/ll_one"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="40dp"
        android:gravity="center"
        android:hint="正确或者错误" />

    <Button
        android:id="@+id/btn_next"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/tv_opinion"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="40dp"
        android:text="下一题"
        android:textSize="24sp" />

    <Button
        android:id="@+id/btn_sure"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_below="@+id/btn_next"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="40dp"
        android:text="确定"
        android:textSize="24sp" />
</RelativeLayout>
类：
package com.example.administrator.writeapp.shuzi_youxi.activity;

import android.content.SharedPreferences;
import android.os.Bundle;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import com.example.administrator.writeapp.R;

/**
 * Created by Administrator on 2019-9-27.
 */

public class ShuZiYouXiActivity extends AppCompatActivity {
    TextView tv_before;
    TextView tv_center;
    TextView tv_after;
    TextView tv_opinion;
    EditText et_result;
    Button btn_sure;
    Button btn_next;
    @Override
    protected void onCreate(@Nullable Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_youxi);

        initViews();
    }

    private void initViews() {
        tv_before = (TextView) findViewById(R.id.tv_before);
        tv_center = (TextView) findViewById(R.id.tv_center);
        tv_after = (TextView) findViewById(R.id.tv_after);
        tv_opinion = (TextView) findViewById(R.id.tv_opinion);
        et_result = (EditText) findViewById(R.id.et_result);
        btn_sure = (Button) findViewById(R.id.btn_sure);
        btn_next = (Button) findViewById(R.id.btn_next);
        setTextView();
        btn_sure.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                int youResult = Integer.valueOf(et_result.getText().toString());
                SharedPreferences spf = getSharedPreferences("result",MODE_PRIVATE);
                int ccResult = spf.getInt("jg",0);
                if (youResult == ccResult){
                    tv_opinion.setText("正确");
                }else{
                    tv_opinion.setText("错误");
                }
            }
        });
        btn_next.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                                        et_result.setText("");
                tv_opinion.setText("正确或错误");
                setTextView();
            }
        });
    }
    public void setTextView(){
        int before = (int) (Math.random()*99+1);
        int after = (int) (Math.random()*99+1);
        tv_before.setText(String.valueOf(before));
        tv_after.setText(String.valueOf(after));
        int center = (int) (Math.random()*4+1);
        int result = 0;
        switch (center){
            case 1:
               result = before + after;
                tv_center.setText("+");
                break;
            case 2:
                result = before - after;
                tv_center.setText("-");
                break;
            case 3:
                result = before * after;
                tv_center.setText("*");
                break;
            case 4:
                result = before / after;
                tv_center.setText("/");
                break;
            default:
        }
        SharedPreferences spf = getSharedPreferences("result",MODE_PRIVATE);
        SharedPreferences.Editor e = spf.edit();
        e.putInt("jg",result);
        e.apply();
    }

}