Question

I need the java code for a 4-function calculator app on android studio (do this on android studio). Please make sure all the requirements shown below are followed (such as the error portion and etc). The topic of this app is to simply create a 4 function calculator which calculates math expressions (add, subtract, multiply, and divide numbers).

The requirements are the following :

- The only buttons needed are 0-9, *, /, +, -, a clear, and enter button

- Implement the onclicklistener on the main activity

- The calcuator should use order of operations (PEMDAS)

- It should be able to continue from a previous answer (Ex: If you type 2+6 the calculator will display 8. If you then multiple by 2 the current result will simply be multiplied by 2)

- It should read the entire sequence of numbers and operators and save them into a String. The information can be read out of the String using a StringTokenizer. The delimiters will be the operators on the calculator.

- The entire mathematical expression will be displayed on the screen before the equal button is clicked.

- The equation will simply be evaluated in the order that the user types in the numbers rather than the precedence

- If the user enters an equation with invalid syntax, the output should display “error”. The calculator should also display “error” for any mathematical calculations that are impossible.

Answer 1

1. TextView
2. Buttons

That’s it. We use the textView to show users the numbers they have written and also to display the result of the calculations we are going to do.

Each of the numbers will have a different button, from 0-9 and we will have four additional 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 to clear the display.

Requirements :

1.EditText: To display numbers and the result.
2. Button: Numbers 0-9
3. Button: Operational buttons (+, -, *, /, =)
4. Button: Decimal button
5. Button: Clear display button.

Algorithm :

Step 1: Creating objects for our elements.

Step 2: Fetching values from the elements into the working of our app.

Step 3: Changing the EditText value by pressing the buttons.

Step 4: The functional buttons.

Step 5: Calculating, the actual functionality.

Full Code for the User Interface for Android 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.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 Tutorial for the Functionality of Android Calculator App

Remember to change your package name to what you are using!

package com.example.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("");
            }
        });
    }

}

OUTPUT :

I have added all the requirements you want.If anything needed you can ask.I have wriitened in java only and i have checked the code.I have also included the algorthim to understand it clearly.