因為你是用文字直接相加

那樣等同於把文字串起來

要拿到正確的結果

請用數字相加

編譯器比你還聰明,
他看到 String + int 就會自動轉成兩個 String 相加,
不過也有些程式語言不允許這樣,
一定要你轉型才能相加.

然後...
其實我不知道你的問題是什麼,
也許你也不知道...

依照你的說明來說
我猜想是想做飲料店的手機版本的pos簡易系統

功能應該是選擇飲料 選擇大小 選擇杯數 選擇加料
總金額公式應該是 strTotlePrice = 飲料價格* 杯數 + 加料

int TotalPrice = counter * count;
String strTotalPrice = String.valueOf( TotalPrice );
total_price.setText( strTotalPrice );

我猜想你的總金額應該是 int TotalPrice
counter是飲料價格 count是杯數

switch (v.getId()) {
               case R.id.jelly: {
               total_price.setText( strTotalPrice + 10);
               break;}
               case R.id.aloe: {
               total_price.setText( strTotalPrice + 10);
               break; }
               case R.id.redbeans: {
               total_price.setText( strTotalPrice + 10 );
               break; }
               case R.id.pearl: {
               total_price.setText( strTotalPrice + 10 );
               break;}
               case R.id.jade: {
               total_price.setText( strTotalPrice + "10" );
               break; }
               case R.id.Coconut: {
               total_price.setText( strTotalPrice + "10" );
               break;}
               case R.id.pudding: {
               total_price.setText( strTotalPrice + "10" );
               break; }
               case R.id.Perilla: {
               total_price.setText( strTotalPrice + "10" );
               break;}
                   }  }     

這段我猜想每個case是各種不同的加料 每種都是10元
所以你要加總的金額 應該是TotalPrice + 10
最後要輸出時 再將TotalPrice 轉成string型態

package test.com.testapp;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.CheckBox;
import android.widget.CompoundButton;
import android.widget.RadioGroup;
import android.widget.TextView;

public class PosActivity extends AppCompatActivity {
    private CheckBox chkperilla;
    private CheckBox chkjella;
    private CheckBox chkredbeans;
    private CheckBox chkaloe;
    private CheckBox chkpearl;
    private CheckBox chkcoconut;
    private CheckBox chkjade;
    private CheckBox chkpudding;
    private TextView txtCnt;
    private TextView txtMOA;
    int iUnitPrice = 0; /* 單價 */
    int iAddOn = 0;     /* 加料的單價，每一種加 10 元 */
    int iCount = 0;     /* 杯數 */

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_pos);

        chkperilla = findViewById(R.id.chkperilla);
        chkjella = findViewById(R.id.chkjella);
        chkredbeans = findViewById(R.id.chkredbeans);
        chkaloe = findViewById(R.id.chkaloe);
        chkpearl = findViewById(R.id.chkpearl);
        chkcoconut = findViewById(R.id.chkcoconut);
        chkjade = findViewById(R.id.chkjade);
        chkpudding = findViewById(R.id.chkpudding);

        txtCnt = findViewById(R.id.txtCount);
        txtMOA = findViewById(R.id.txtMOA);

        Button.OnClickListener chooseSize = new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                switch (view.getId()) {
                    case R.id.medium : { iUnitPrice = 45; break; }
                    case R.id.large : { iUnitPrice = 80; break; }
                    case R.id.bottled : { iUnitPrice = 100; break; }
                }
                showMOA(caculateMOA());
                Log.d("KKK", Integer.toString(iUnitPrice));
            }
        };

        Button btn = findViewById(R.id.medium);
        btn.setOnClickListener(chooseSize);
        btn = findViewById(R.id.large);
        btn.setOnClickListener(chooseSize);
        btn = findViewById(R.id.bottled);
        btn.setOnClickListener(chooseSize);

        Button.OnClickListener chooseCount = new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                switch (view.getId()) {
                    case R.id.less : {
                        if(iCount>0) {
                            iCount--;
                        }
                        break;
                    }
                    case R.id.plus : {
                        iCount++;
                        break;
                    }
                }
                showCount(iCount);
                showMOA(caculateMOA());
                Log.d("KKK", Integer.toString(iCount));
            }
        };

        btn = findViewById(R.id.less);
        btn.setOnClickListener(chooseCount);
        btn = findViewById(R.id.plus);
        btn.setOnClickListener(chooseCount);

        chkperilla.setOnCheckedChangeListener(new BonusOnCheckedChangeListener());
        chkjella.setOnCheckedChangeListener(new BonusOnCheckedChangeListener());
        chkredbeans.setOnCheckedChangeListener(new BonusOnCheckedChangeListener());
        chkaloe.setOnCheckedChangeListener(new BonusOnCheckedChangeListener());
        chkpearl.setOnCheckedChangeListener(new BonusOnCheckedChangeListener());
        chkcoconut.setOnCheckedChangeListener(new BonusOnCheckedChangeListener());
        chkjade.setOnCheckedChangeListener(new BonusOnCheckedChangeListener());
        chkpudding.setOnCheckedChangeListener(new BonusOnCheckedChangeListener());
    }

    private class BonusOnCheckedChangeListener implements CompoundButton.OnCheckedChangeListener{
        @Override
        public void onCheckedChanged(CompoundButton button,boolean isChecked){
            if(button.isChecked()){
                iAddOn += 10;
            } else {
                iAddOn -= 10;
            }
            showMOA(caculateMOA());
            Log.d("KKK", Integer.toString(iAddOn));
        }
    }

    private void showCount(int cnt) {
        txtCnt.setText(Integer.toString(cnt));
    }

    private void showMOA(int moa) {
        txtMOA.setText(Integer.toString(moa));
    }

    private int caculateMOA() {
        int iRet = (iUnitPrice+iAddOn) * iCount;
        return iRet;
    }
}

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:orientation="vertical"
    tools:context=".PosActivity">

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <TextView
            android:id="@+id/textView2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="金額"
            android:textSize="50sp" />

        <TextView
            android:id="@+id/txtMOA"
            android:layout_width="wrap_content"
            android:layout_height="100dp"
            android:layout_weight="1"
            android:text="0"
            android:textAlignment="viewEnd"
            android:textSize="80sp" />

    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <Button
            android:id="@+id/medium"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Medium"
            android:textSize="20sp" />

        <Button
            android:id="@+id/large"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Large"
            android:textSize="20sp" />

        <Button
            android:id="@+id/bottled"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Bottle"
            android:textSize="20sp" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <TextView
            android:id="@+id/textView3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="杯數"
            android:textSize="50sp" />

        <TextView
            android:id="@+id/txtCount"
            android:layout_width="wrap_content"
            android:layout_height="100dp"
            android:layout_weight="1"
            android:text="0"
            android:textAlignment="viewEnd"
            android:textSize="80sp" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <Button
            android:id="@+id/less"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="-"
            android:textSize="50sp" />

        <Button
            android:id="@+id/plus"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="+"
            android:textSize="50sp" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <CheckBox
            android:id="@+id/chkperilla"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Perilla" />

        <CheckBox
            android:id="@+id/chkjella"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Jella" />

        <CheckBox
            android:id="@+id/chkredbeans"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="RedBean" />

        <CheckBox
            android:id="@+id/chkaloe"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Aloe" />
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:orientation="horizontal">

        <CheckBox
            android:id="@+id/chkpearl"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Pearl" />

        <CheckBox
            android:id="@+id/chkcoconut"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Coconut" />

        <CheckBox
            android:id="@+id/chkjade"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Jade" />

        <CheckBox
            android:id="@+id/chkpudding"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_weight="1"
            android:text="Pudding" />
    </LinearLayout>

</LinearLayout>