有瑕疵的Android四则运算

java文件：

package com.example.dell_pc.homework;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

import java.util.Random;

public class MainActivity extends AppCompatActivity {
    private int a;
    private int b;
    private int c;
    private int d;
    private int sy;
    private TextView tv;
    private Button btn_1;
    private Button btn_2;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        //获取TextView，Button对象
        tv = (TextView) super.findViewById(R.id.textView5);
        btn_1 = (Button) super.findViewById(R.id.btn_1);
        btn_2 = (Button) super.findViewById(R.id.btn_2);
        random();
        view();

        btn_2.setOnClickListener(new Button.OnClickListener() {
            @Override

//算法
            public void onClick(View v) {

                switch (sy) {
                    case 1:
                        tv.setText(a + "+" + b + "-" + c + "*" +d+ "=?");
                        break;
                    case 2:
                        tv.setText(a + "-" + b + "*" + c + "/" + d+"=?");
                        break;
                    case 3:
                        tv.setText(a + "*" + b + "+" + c + "-" + d+"=?");
                        break;
                    case 4:
                        tv.setText(a + "/" + b + "-" + c + "+" + d+"=?");
                        break;
                    case 5:
                        tv.setText(a + "+" + b + "/" + c + "+" + d+"=?");
                        break;
                    case 6:
                        tv.setText(a + "+" + b + "/" + c + "+" + d+"=?");
                        break;
                    case 7:
                        tv.setText(a + "-" + b + "/" + c + "*" + d+"=?");
                        break;
                }
            }

        });
//事件监听
        btn_1.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                random();
                view();
            }
        });
    }

//生成100以内的随机数
    private void random() {
        a = new Random().nextInt(100);
        b = new Random().nextInt(100);
        c = new Random().nextInt(100);
        sy = new Random().nextInt(4);
    }

    private void view() {
        switch (sy) {
            case 1:
                tv.setText(a + "+" + b + "-" + c + "*" +d+ "="+(a+b-c*d));
                break;
            case 2:
                tv.setText(a + "-" + b + "*" + c + "/" + d+"="+(a-b*c/d));
                break;
            case 3:
                tv.setText(a + "*" + b + "+" + c + "-" + d+"="+(a*b+c-d));
                break;
            case 4:
                tv.setText(a + "/" + b + "-" + c + "+" + d+"="+(a/b-c+d));
                break;
            case 5:
                tv.setText(a + "+" + b + "/" + c + "+" + d+"="+(a+b/c+d));
                break;
            case 6:
                tv.setText(a + "+" + b + "/" + c + "+" + d+"="+(a+b/c+d));
                break;
            case 7:
                tv.setText(a + "-" + b + "/" + c + "*" + d+"="+(a-b/c*d));
                break;
        }
    }
}

布局文件：

<?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:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    tools:context="com.example.dell_pc.homework.MainActivity">


    <LinearLayout
        android:orientation="horizontal"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:layout_below="@+id/textView5"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginTop="108dp">

        <Button
            android:text="下一题"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/btn_1"
            android:layout_weight="1" />

        <Button
            android:text="答案"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:id="@+id/btn_2"
            android:layout_weight="1" />
    </LinearLayout>

    <TextView
        android:text="TextView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:id="@+id/textView5"
        android:layout_alignParentTop="true"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="38dp" />
</RelativeLayout>