三位数的四则运算

先上图：

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:orientation="vertical"
    android:layout_height="match_parent"
    android:weightSum="1">

    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="TextView"
        />

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">
        <Button
            android:id="@+id/button4"
            android:layout_width="193dp"
            android:layout_height="wrap_content"
            android:text="显示" />

        <Button
            android:id="@+id/button3"
            android:layout_width="187dp"
            android:layout_height="wrap_content"
            android:text="下一题"
            android:layout_weight="0.01" />
    </LinearLayout>
</LinearLayout>

看到上面的图，大家肯定会说，这不算错了么，其实这是四舍五入的结果，

代码逻辑，为：

1. random生成随机数，包括数字a,b,c
2. 加减乘除的实现：以switch列出所有的加减乘除的组合，三位数的算数（包括2位数）总共20种，列出所有可能。
3. 显示答案：在同一个random下，例如显示a+b+c=?，然后在另一个方法中显示a+b+c=d。在点击下一题时，再次生成随机数，以此类推。

代码如下：

package com.example.myapplication;


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

import java.util.Random;

public class MainActivity extends AppCompatActivity {
    private int a, b, c, ysf;
    private TextView tv;
    private Button show, next;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        tv = (TextView) findViewById(R.id.tv);
        show = (Button) findViewById(R.id.show);
        next = (Button) findViewById(R.id.next);
        rd();

        show.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                answer(ysf,a,b,c);
            }
        });
        next.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                rd();
                show(ysf,a,b,c);
            }
        });
    }
    private void answer(int ty,int a,int b,int c){
        switch (ty) {
            //将所有的可能全列出来
            //两个数的算法
            case 0:
                tv.setText(a+"+"+b+"="+(a+b));
                break;
            case 1:
                tv.setText(a+"-"+b+"="+(a-b));
                break;
            case 2:
                tv.setText(a+"*"+b+"="+(a*b));
                break;
            case 3:
                tv.setText(a+"/"+b+"="+(a/b));
                break;
            //三个数的四则运算，共16种
            case 4:
                tv.setText(a + "+" + b + "-" + c + "=" + (a + b - c));
                break;
            case 5:
                tv.setText(a + "+" + b + "+" + c + "=" + (a + b + c));
                break;
            case 6:
                tv.setText(a + "+" + b + "*" + c + "=" + (a + b * c));
                break;
            case 7:
                tv.setText(a + "+" + b + "/" + c + "=" + (a + b / c));
                break;
            case 8:
                tv.setText(a+"-"+b+"-"+c+"="+(a-b-c));
                break;
            case 9:
                tv.setText(a+"-"+b+"+"+c+"="+(a-b+c));
                break;
            case 10:
                tv.setText(a+"-"+b+"*"+c+"="+(a-b*c));
                break;
            case 11:
                tv.setText(a+"-"+b+"/"+c+"="+(a-b/c));
                break;

            case 12:
                tv.setText(a+"*"+b+"*"+c+"="+(a*b*c));
                break;
            case 13:
                tv.setText(a+"*"+b+"+"+c+"="+(a*b+c));
                break;
            case 14:
                tv.setText(a+"*"+b+"-"+c+"="+(a*b-c));
                break;
            case 15:
                tv.setText(a+"*"+b+"/"+c+"="+(a*b/c));
                break;

            case 16:
                tv.setText(a+"/"+b+"/"+c+"="+(a/b/c));
                break;
            case 17:
                tv.setText(a+"/"+b+"+"+c+"="+(a/b+c));
                break;
            case 18:
                tv.setText(a+"/"+b+"-"+c+"="+(a/b-c));
                break;
            case 19:
                tv.setText(a+"/"+b+"*"+c+"="+(a/b*c));
        }
    }
    private void show(int ty,int a,int b,int c){
        switch (ty) {
            //将所有的可能全列出来
            //两个数的算法
            case 0:
                tv.setText(a+"+"+b+"="+"?");
                break;
            case 1:
                tv.setText(a+"-"+b+"="+"?");
                break;
            case 2:
                tv.setText(a+"*"+b+"="+"?");
                break;
            case 3:
                tv.setText(a+"/"+b+"="+"?");
                break;
            //三个数的四则运算，共16种
            case 4:
                tv.setText(a + "+" + b + "-" + c + "=" + "?");
                break;
            case 5:
                tv.setText(a + "+" + b + "+" + c + "=" + "?");
                break;
            case 6:
                tv.setText(a + "+" + b + "*" + c + "=" + "?");
                break;
            case 7:
                tv.setText(a + "+" + b + "/" + c + "=" + "?");
                break;
            case 8:
                tv.setText(a+"-"+b+"-"+c+"="+"?");
                break;
            case 9:
                tv.setText(a+"-"+b+"+"+c+"="+"?");
                break;
            case 10:
                tv.setText(a+"-"+b+"*"+c+"="+"?");
                break;
            case 11:
                tv.setText(a+"-"+b+"/"+c+"="+"?");
                break;

            case 12:
                tv.setText(a+"*"+b+"*"+c+"="+"?");
                break;
            case 13:
                tv.setText(a+"*"+b+"+"+c+"="+"?");
                break;
            case 14:
                tv.setText(a+"*"+b+"-"+c+"="+"?");
                break;
            case 15:
                tv.setText(a+"*"+b+"/"+c+"="+"?");
                break;

            case 16:
                tv.setText(a+"/"+b+"/"+c+"="+"?");
                break;
            case 17:
                tv.setText(a+"/"+b+"+"+c+"="+"?");
                break;
            case 18:
                tv.setText(a+"/"+b+"-"+c+"="+"?");
                break;
            case 19:
                tv.setText(a+"/"+b+"*"+c+"="+"?");
        }
    }

    private void rd() {
        Random random = new Random();
        random.nextInt();
        a = random.nextInt(100);
        b = random.nextInt(100);
        c = random.nextInt(100);
        ysf = random.nextInt(19);
    }
}