Question

I am doing a 4 function calculator app in java on android studio which calculates math expressions (add, subtract, multiply, and divide).

I ONLY NEED THE CODE THE EQUAL BUTTON.

When the user clicks the equal button, it should take the entire sequence of numbers and operators on the screen, such as 1+2*5 , and save them into a String [I am pretty sure this can be done using the toString() call]. Then, the String should be split using StringTokenizer, with the delimiters being "+-/*" . After that to display the correct answer, I am pretty sure Double.parseDouble needs to be called to covert the String tokens to double values. After that, the correct operation should be done to get the correct answer.

Note -

- The calcuator should use order of operations (PEMDAS)

- It should be able to continue from a previous answer (Ex: If you type 1+5 and press equals, the calculator will display 4. If you then multiply by 2, the current result will simply be multiplied by 2)

Answer 1

Solution:

We are using

text view - To show users the numbers which they have been written and also used to display their calculation result which we are going to do.

buttons - 0-9 number have a different buttons, in addition we are adding 4 buttons for addition, subtraction, multiplication and division. other than this we require a button to calculate the operation, a button for decimals and a button for clearing the display.

Requirements needed:

1. Edit text : To display the numbers and the result.

2. Button : number (0-9) , operational button (+,-,*,/,=) , decimal button (.) , clear display button

User Interface for Android Simple Calculator App

<?xml version="1.0" encoding="utf-8"?>
<android.support.constraint.ConstraintLayout 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"
    tools:context="com.example.a84.calculator.MainActivity">

    <RelativeLayout
        android:layout_width="368dp"
        android:layout_height="495dp"
        android:layout_marginBottom="8dp"
        android:layout_marginEnd="8dp"
        android:layout_marginTop="8dp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toTopOf="parent">

        <Button
            android:id="@+id/btn_1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentLeft="true"
            android:layout_alignParentStart="true"
            android:layout_below="@+id/edText1"
            android:layout_marginTop="60dp"
            android:onClick="PressOne"
            android:text="1"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_0"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/btn_8"
            android:layout_toEndOf="@+id/btn_7"
            android:layout_toRightOf="@+id/btn_7"
            android:text="0"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_9"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/btn_6"
            android:layout_toEndOf="@+id/btn_5"
            android:layout_toRightOf="@+id/btn_5"
            android:text="9"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_8"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/btn_5"
            android:layout_toEndOf="@+id/btn_7"
            android:layout_toRightOf="@+id/btn_7"
            android:text="8"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_7"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/btn_4"
            android:layout_alignStart="@+id/btn_4"
            android:layout_below="@+id/btn_4"
            android:text="7"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_6"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignBaseline="@+id/btn_5"
            android:layout_alignBottom="@+id/btn_5"
            android:layout_toEndOf="@+id/btn_5"
            android:layout_toRightOf="@+id/btn_5"
            android:text="6"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_5"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/btn_2"
            android:layout_toEndOf="@+id/btn_4"
            android:layout_toRightOf="@+id/btn_4"
            android:text="5"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_4"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/btn_1"
            android:layout_alignStart="@+id/btn_1"
            android:layout_below="@+id/btn_1"
            android:text="4"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_3"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignBaseline="@+id/btn_2"
            android:layout_alignBottom="@+id/btn_2"
            android:layout_toEndOf="@+id/btn_2"
            android:layout_toRightOf="@+id/btn_2"
            android:text="3"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_2"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignBaseline="@+id/btn_1"
            android:layout_alignBottom="@+id/btn_1"
            android:layout_toEndOf="@+id/btn_1"
            android:layout_toRightOf="@+id/btn_1"
            android:text="2"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_Add"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_above="@+id/btn_6"
            android:layout_alignParentEnd="true"
            android:layout_alignParentRight="true"
            android:backgroundTint="@android:color/darker_gray"
            android:text="+"
            android:textColor="@android:color/background_light"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_Sub"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/btn_Add"
            android:layout_alignStart="@+id/btn_Add"
            android:layout_below="@+id/btn_Add"
            android:backgroundTint="@android:color/darker_gray"
            android:text="-"
            android:textColor="@android:color/background_light"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_Mul"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/btn_Sub"
            android:layout_alignStart="@+id/btn_Sub"
            android:layout_below="@+id/btn_6"
            android:backgroundTint="@android:color/darker_gray"
            android:text="*"
            android:textColor="@android:color/background_light"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_Div"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignLeft="@+id/btn_Mul"
            android:layout_alignStart="@+id/btn_Mul"
            android:layout_below="@+id/btn_9"
            android:backgroundTint="@android:color/darker_gray"
            android:text="/"
            android:textColor="@android:color/background_light"
            android:textSize="18sp" />

        <EditText
            android:id="@+id/edText1"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentEnd="true"
            android:layout_alignParentLeft="true"
            android:layout_alignParentRight="true"
            android:layout_alignParentStart="true"
            android:layout_alignParentTop="true"
            android:layout_marginTop="22dp"
            android:ems="10"
            android:inputType="textPersonName"
            android:textAlignment="textEnd"
            android:textSize="24sp" />

        <Button
            android:id="@+id/btn_calc"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/btn_0"
            android:layout_toEndOf="@+id/btn_0"
            android:layout_toRightOf="@+id/btn_0"
            android:backgroundTint="@android:color/holo_green_light"
            android:text="="
            android:textColor="@android:color/background_light"
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_dec"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_below="@+id/btn_7"
            android:layout_toLeftOf="@+id/btn_8"
            android:layout_toStartOf="@+id/btn_8"
            android:text="."
            android:textSize="18sp" />

        <Button
            android:id="@+id/btn_clear"
            android:layout_width="wrap_content"
            android:layout_height="wrap_content"
            android:layout_alignParentEnd="true"
            android:layout_alignParentRight="true"
            android:layout_below="@+id/btn_Div"
            android:backgroundTint="@android:color/holo_blue_dark"
            android:text="clear"
            android:textColor="@android:color/background_light"
            android:textSize="18sp" />

    </RelativeLayout>
</android.support.constraint.ConstraintLayout>

Full Code for the Functionality of Android Calculator App

(Note: Remember to change your package name to what you are using)

package com.example.a84.calculator;

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

public class MainActivity extends AppCompatActivity {

    Button btn_1,btn_2,btn_3,btn_4,btn_5,btn_6,btn_7,btn_8,btn_9,btn_0,btn_Add,btn_Sub,btn_Mul,btn_Div,btn_calc,btn_dec,btn_clear;
    EditText ed1;

    float Value1, Value2;
    boolean mAddition, mSubtract, mMultiplication, mDivision ;

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

        btn_0 = (Button) findViewById(R.id.btn_0);
        btn_1 = (Button) findViewById(R.id.btn_1);
        btn_2 = (Button) findViewById(R.id.btn_2);
        btn_3 = (Button) findViewById(R.id.btn_3);
        btn_4 = (Button) findViewById(R.id.btn_4);
        btn_5 = (Button) findViewById(R.id.btn_5);
        btn_6 = (Button) findViewById(R.id.btn_6);
        btn_7 = (Button) findViewById(R.id.btn_7);
        btn_8 = (Button) findViewById(R.id.btn_8);
        btn_9 = (Button) findViewById(R.id.btn_9);
        btn_Add = (Button) findViewById(R.id.btn_Add);
        btn_Div = (Button) findViewById(R.id.btn_Div);
        btn_Sub = (Button) findViewById(R.id.btn_Sub);
        btn_Mul = (Button) findViewById(R.id.btn_Mul);
        btn_calc = (Button) findViewById(R.id.btn_calc);
        btn_dec = (Button) findViewById(R.id.btn_dec);
        btn_clear = (Button) findViewById(R.id.btn_clear);
        ed1 = (EditText) findViewById(R.id.edText1);

        btn_0.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"0");
            }
        });

        btn_1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"1");
            }
        });

        btn_2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"2");
            }
        });

        btn_3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"3");
            }
        });

        btn_4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"4");
            }
        });

        btn_5.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"5");
            }
        });

        btn_6.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"6");
            }
        });

        btn_7.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"7");
            }
        });

        btn_8.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"8");
            }
        });

        btn_9.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+"9");
            }
        });

        btn_dec.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText(ed1.getText()+".");
            }
        });

        btn_Add.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                if (ed1 == null){
                    ed1.setText("");
                }else {
                    Value1 = Float.parseFloat(ed1.getText() + "");
                    mAddition = true;
                    ed1.setText(null);
                }
            }
        });

        btn_Sub.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Value1 = Float.parseFloat(ed1.getText() + "");
                mSubtract = true ;
                ed1.setText(null);
            }
        });

        btn_Mul.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Value1 = Float.parseFloat(ed1.getText() + "");
                mMultiplication = true ;
                ed1.setText(null);
            }
        });

        btn_Div.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Value1 = Float.parseFloat(ed1.getText()+"");
                mDivision = true ;
                ed1.setText(null);
            }
        });

        btn_calc.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Value2 = Float.parseFloat(ed1.getText() + "");

                if (mAddition == true){

                    ed1.setText(Value1 + Value2 +"");
                    mAddition=false;
                }


                if (mSubtract == true){
                    ed1.setText(Value1 - Value2 +"");
                    mSubtract=false;
                }

                if (mMultiplication == true){
                    ed1.setText(Value1 * Value2 + "");
                    mMultiplication=false;
                }

                if (mDivision == true){
                    ed1.setText(Value1 / Value2+"");
                    mDivision=false;
                }
            }
        });

        btn_clear.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                ed1.setText("");
            }
        });
    }

}