Meta Android Developer Professional Certificate

 

What you'll learn

Gain the skills required for an entry-level career as an Android developer. 

Learn how to create applications for Android including how to build and manage the lifecycle of a mobile app using Android Studio.

Learn coding in Kotlin and the programming fundamentals for how to create the user interface (UI) and best practices for design.   

Create cross-platform mobile applications using React Native. Demonstrate your new skills by creating a job-ready portfolio you can show during interviews.

Join Free: Meta Android Developer Professional Certificate

Prepare for a career in Android Development

Receive professional-level training from Meta

Demonstrate your proficiency in portfolio-ready projects

Earn an employer-recognized certificate from Meta

Qualify for in-demand job titles: Android Developer, Mobile Applications Developer, Mobile Developer

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Login with Retrofit in android Studio with SourceCode | Login and Registration form in android using JSON example

 Build.gradle File:-  

implementation 'com.google.android.material:material:1.3.0'
implementation 'com.squareup.retrofit2:retrofit:2.8.1'
implementation 'com.squareup.retrofit2:converter-gson:2.8.1'
implementation 'com.squareup.okhttp3:logging-interceptor:4.6.0'

 Activity-main 

<?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:orientation="vertical"
    android:background="@color/colorWhite"
    android:layout_height="match_parent"
    tools:context=".MainActivity">
<TextView
    android:layout_width="wrap_content"
    android:text="Login to continue"
    android:layout_marginTop="100dp"
    android:textSize="24sp"
    android:layout_gravity="center_horizontal"
    android:textColor="@color/colorPrimaryDark"
    android:layout_height="wrap_content" />

    <com.google.android.material.textfield.TextInputLayout
        android:layout_width="match_parent"
        android:layout_marginTop="30dp"
        android:layout_marginStart="30dp"
        android:layout_marginEnd="30dp"
        android:hint="Username"
        android:layout_height="wrap_content">

        <com.google.android.material.textfield.TextInputEditText
            android:layout_width="match_parent"
            android:inputType="text"
            android:id="@+id/edUsername"
            android:layout_height="wrap_content" />

    </com.google.android.material.textfield.TextInputLayout>
    <com.google.android.material.textfield.TextInputLayout
        android:layout_width="match_parent"
        android:layout_marginTop="30dp"
        android:hint="Password"
        android:layout_marginStart="30dp"
        android:layout_marginEnd="30dp"
        android:layout_height="wrap_content">
        <com.google.android.material.textfield.TextInputEditText
            android:layout_width="match_parent"
            android:inputType="textPassword"
            android:id="@+id/edPassword"
            android:layout_height="wrap_content" />
    </com.google.android.material.textfield.TextInputLayout>
    <Button
        android:layout_width="match_parent"
        android:layout_margin="30dp"
        android:text="Login"
        android:id="@+id/btnLogin"
        android:background="@color/colorPrimary"
        android:textColor="@color/colorWhite"
        android:layout_height="wrap_content" />
</LinearLayout> 

 Java File :-

1. Api Client 

import okhttp3.OkHttpClient;
import okhttp3.logging.HttpLoggingInterceptor;
import retrofit2.Retrofit;
import retrofit2.converter.gson.GsonConverterFactory;

public class ApiClient {

    private static Retrofit getRetrofit(){

        HttpLoggingInterceptor httpLoggingInterceptor = new HttpLoggingInterceptor();
        httpLoggingInterceptor.setLevel(HttpLoggingInterceptor.Level.BODY);

        OkHttpClient okHttpClient = new OkHttpClient.Builder().addInterceptor(httpLoggingInterceptor).build();

        Retrofit retrofit = new Retrofit.Builder()
                .addConverterFactory(GsonConverterFactory.create())
                .baseUrl("http://api.irawen.net/")  //Change server URL
                .client(okHttpClient)
                .build();

        return retrofit;
    }


    public static UserService getUserService(){
        UserService userService = getRetrofit().create(UserService.class);

        return userService;
    }

}

2. Login Request :-

public class LoginRequest {

    private String username;
    private String password;

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }

    public String getPassword() {
        return password;
    }

    public void setPassword(String password) {
        this.password = password;
    }
}

3. Login response :- 

public class LoginResponse {


    private int user_id;
    private String email;
    private String username;

    public int getUser_id() {
        return user_id;
    }

    public void setUser_id(int user_id) {
        this.user_id = user_id;
    }

    public String getEmail() {
        return email;
    }

    public void setEmail(String email) {
        this.email = email;
    }

    public String getUsername() {
        return username;
    }

    public void setUsername(String username) {
        this.username = username;
    }
}

4. Main Activity :-

 import android.content.Intent;
import android.os.Bundle;
import android.os.Handler;
import android.text.TextUtils;
import android.view.View;
import android.widget.Button;
import android.widget.Toast;

import com.google.android.material.textfield.TextInputEditText;

import androidx.appcompat.app.AppCompatActivity;
import retrofit2.Call;
import retrofit2.Callback;
import retrofit2.Response;

public class MainActivity extends AppCompatActivity {


    TextInputEditText username, password;
    Button btnLogin;

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

        username = findViewById(R.id.edUsername);
        password = findViewById(R.id.edPassword);
        btnLogin = findViewById(R.id.btnLogin);

        btnLogin.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {

                if(TextUtils.isEmpty(username.getText().toString()) || TextUtils.isEmpty(password.getText().toString())){
                    Toast.makeText(MainActivity.this,"Username / Password Required", Toast.LENGTH_LONG).show();
                }else{
                    //proceed to login
                    login();
                }

            }
        });
   }


   public void login(){
      LoginRequest loginRequest = new LoginRequest();
      loginRequest.setUsername(username.getText().toString());
      loginRequest.setPassword(password.getText().toString());

      Call<LoginResponse> loginResponseCall = ApiClient.getUserService().userLogin(loginRequest);
      loginResponseCall.enqueue(new Callback<LoginResponse>() {
          @Override
          public void onResponse(Call<LoginResponse> call, Response<LoginResponse> response) {

              if(response.isSuccessful()){
                  Toast.makeText(MainActivity.this,"Login Successful", Toast.LENGTH_LONG).show();
                  LoginResponse loginResponse = response.body();

                  new Handler().postDelayed(new Runnable() {
                      @Override
                      public void run() {

                        startActivity(new Intent(MainActivity.this,DashboardActivity.class).putExtra("data",loginResponse.getUsername()));
                      }
                  },700);

              }else{
                  Toast.makeText(MainActivity.this,"Login Failed", Toast.LENGTH_LONG).show();

              }

          }

          @Override
          public void onFailure(Call<LoginResponse> call, Throwable t) {
              Toast.makeText(MainActivity.this,"Throwable "+t.getLocalizedMessage(), Toast.LENGTH_LONG).show();

          }
      });


   }

}

5. User Service :-

import retrofit2.Call;
import retrofit2.http.Body;
import retrofit2.http.POST;

public interface UserService {


    @POST("authenticate/")
 Call<LoginResponse> userLogin(@Body LoginRequest loginRequest);


}

Read More

Calculator in android studio with Source Code | Calculator in Android studio | Clcoding

 Activity Main :

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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"
    android:background="#000"
    tools:context=".MainActivity">


    <Button
        android:id="@+id/button1"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="1"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.043"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.817" />

    <Button
        android:id="@+id/button2"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="2"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.352"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.817" />

    <Button
        android:id="@+id/button3"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="3"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.672"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.817" />

    <Button
        android:id="@+id/button4"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="4"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.043"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.677" />

    <Button
        android:id="@+id/button5"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="5"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.352"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.677" />

    <Button
        android:id="@+id/button6"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="6"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.672"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.677" />

    <Button
        android:id="@+id/button7"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="7"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.043"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.541" />

    <Button
        android:id="@+id/button8"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="8"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.352"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.541" />

    <Button
        android:id="@+id/button9"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="9"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.672"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.541" />

    <Button
        android:id="@+id/button0"
        android:layout_width="185dp"
        android:layout_height="80dp"
        android:background="@drawable/new_but"
        android:paddingRight="95sp"
        android:text="0"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.061"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.957" />

    <Button
        android:id="@+id/button_equal"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/arithmatic_button"
        android:text="="
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.99"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.958" />

    <Button
        android:id="@+id/button_multi"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/arithmatic_button"
        android:text="×"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.99"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.541" />

    <Button
        android:id="@+id/button_divide"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/arithmatic_button"
        android:text="/"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.987"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.406" />

    <Button
        android:id="@+id/button_add"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/arithmatic_button"
        android:text="+"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.99"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.817" />

    <Button
        android:id="@+id/button_sub"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/arithmatic_button"
        android:text="-"
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.99"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.677" />

    <Button
        android:id="@+id/button_clear"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/button"
        android:text="AC"
        android:textColor="#060606"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.043"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.406" />

    <Button
        android:id="@+id/button_para1"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/button"
        android:text="%"
        android:textColor="#060606"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.672"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.406" />

    <Button
        android:id="@+id/button_para2"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/button"
        android:text="+/-"
        android:textColor="#060606"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.349"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.406" />

    <Button
        android:id="@+id/button_dot"
        android:layout_width="80dp"
        android:layout_height="80dp"
        android:background="@drawable/circle_button"
        android:text="."
        android:textColor="#ffff"
        android:textSize="28sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.671"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.958" />

    <TextView
        android:id="@+id/input"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@android:color/background_light"
        android:textSize="68sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.909"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.252" />

    <TextView
        android:id="@+id/output"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textColor="@android:color/background_light"
        android:textSize="36sp"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.909"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.051" />

</androidx.constraintlayout.widget.ConstraintLayout>

Main Activity :

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.util.TypedValue;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    private Button b1;
    private Button b2;
    private Button b3;
    private Button b4;
    private Button b5;
    private Button b6;
    private Button b7;
    private Button b8;
    private Button b9;
    private Button b0;
    private Button b_equal;
    private Button b_multi;
    private Button b_divide;
    private Button b_add;
    private Button b_sub;
    private Button b_clear;
    private Button b_dot;
    private Button b_para1;
    private Button b_para2;
    private TextView t1;
    private TextView t2;
    private final char ADDITION = '+';
    private final char SUBTRACTION = '-';
    private final char MULTIPLICATION = '*';
    private final char DIVISION = '/';
    private final char EQU = '=';
    private final char EXTRA = '@';
    private final char MODULUS = '%';
    private char ACTION;
    private double val1 = Double.NaN;
    private double val2;

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

        b1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "1");
            }
        });

        b2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "2");
            }
        });

        b3.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "3");
            }
        });

        b4.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "4");
            }
        });

        b5.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "5");
            }
        });

        b6.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "6");
            }
        });

        b7.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "7");
            }
        });

        b8.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "8");
            }
        });

        b9.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "9");
            }
        });

        b0.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                ifErrorOnOutput();
                exceedLength();
                t1.setText(t1.getText().toString() + "0");
            }
        });

        b_dot.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                exceedLength();
                t1.setText(t1.getText().toString() + ".");
            }
        });

        b_para1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (t1.getText().length() > 0) {
                    ACTION = MODULUS;
                    operation();
                    if (!ifReallyDecimal()) {
                        t2.setText(val1 + "%");
                    } else {
                        t2.setText((int) val1 + "%");
                    }
                    t1.setText(null);
                } else {
                    t2.setText("Error");
                }
            }
        });

        b_add.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (t1.getText().length() > 0) {
                    ACTION = ADDITION;
                    operation();
                    if (!ifReallyDecimal()) {
                        t2.setText(val1 + "+");
                    } else {
                        t2.setText((int) val1 + "+");
                    }
                    t1.setText(null);
                } else {
                    t2.setText("Error");
                }
            }
        });

        b_sub.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (t1.getText().length() > 0) {
                    ACTION = SUBTRACTION;
                    operation();
                    if (t1.getText().length() > 0)
                        if (!ifReallyDecimal()) {
                            t2.setText(val1 + "-");
                        } else {
                            t2.setText((int) val1 + "-");
                        }
                    t1.setText(null);
                } else {
                    t2.setText("Error");
                }
            }
        });

        b_multi.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (t1.getText().length() > 0) {
                    ACTION = MULTIPLICATION;
                    operation();
                    if (!ifReallyDecimal()) {
                        t2.setText(val1 + "×");
                    } else {
                        t2.setText((int) val1 + "×");
                    }
                    t1.setText(null);
                } else {
                    t2.setText("Error");
                }
            }
        });

        b_divide.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (t1.getText().length() > 0) {
                    ACTION = DIVISION;
                    operation();
                    if (ifReallyDecimal()) {
                        t2.setText((int) val1 + "/");
                    } else {
                        t2.setText(val1 + "/");
                    }
                    t1.setText(null);
                } else {
                    t2.setText("Error");
                }
            }
        });

        b_para2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (!t2.getText().toString().isEmpty() || !t1.getText().toString().isEmpty()) {
                    val1 = Double.parseDouble(t1.getText().toString());
                    ACTION = EXTRA;
                    t2.setText("-" + t1.getText().toString());
                    t1.setText("");
                } else {
                    t2.setText("Error");
                }
            }
        });

        b_equal.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (t1.getText().length() > 0) {
                    operation();
                    ACTION = EQU;
                    if (!ifReallyDecimal()) {
                        t2.setText(/*t2.getText().toString() + String.valueOf(val2) + "=" + */String.valueOf(val1));
                    } else {
                        t2.setText(/*t2.getText().toString() + String.valueOf(val2) + "=" + */String.valueOf((int) val1));
                    }
                    t1.setText(null);
                } else {
                    t2.setText("Error");
                }
            }
        });

        b_clear.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                if (t1.getText().length() > 0) {
                    CharSequence name = t1.getText().toString();
                    t1.setText(name.subSequence(0, name.length() - 1));
                } else {
                    val1 = Double.NaN;
                    val2 = Double.NaN;
                    t1.setText("");
                    t2.setText("");
                }
            }
        });

        // Empty text views on long click.
        b_clear.setOnLongClickListener(new View.OnLongClickListener() {
            @Override
            public boolean onLongClick(View view) {
                val1 = Double.NaN;
                val2 = Double.NaN;
                t1.setText("");
                t2.setText("");
                return true;
            }
        });
    }

    private void viewSetup() {
        b1 = findViewById(R.id.button1);
        b2 = findViewById(R.id.button2);
        b3 = findViewById(R.id.button3);
        b4 = findViewById(R.id.button4);
        b5 = findViewById(R.id.button5);
        b6 = findViewById(R.id.button6);
        b7 = findViewById(R.id.button7);
        b8 = findViewById(R.id.button8);
        b9 = findViewById(R.id.button9);
        b0 = findViewById(R.id.button0);
        b_equal = findViewById(R.id.button_equal);
        b_multi = findViewById(R.id.button_multi);
        b_divide = findViewById(R.id.button_divide);
        b_add = findViewById(R.id.button_add);
        b_sub = findViewById(R.id.button_sub);
        b_clear = findViewById(R.id.button_clear);
        b_dot = findViewById(R.id.button_dot);
        b_para1 = findViewById(R.id.button_para1);
        b_para2 = findViewById(R.id.button_para2);
        t1 = findViewById(R.id.input);
        t2 = findViewById(R.id.output);
    }

    private void operation() {
        if (!Double.isNaN(val1)) {
            if (t2.getText().toString().charAt(0) == '-') {
                val1 = (-1) * val1;
            }
            val2 = Double.parseDouble(t1.getText().toString());

            switch (ACTION) {
                case ADDITION:
                    val1 = val1 + val2;
                    break;
                case SUBTRACTION:
                    val1 = val1 - val2;
                    break;
                case MULTIPLICATION:
                    val1 = val1 * val2;
                    break;
                case DIVISION:
                    val1 = val1 / val2;
                    break;
                case EXTRA:
                    val1 = (-1) * val1;
                    break;
                case MODULUS:
                    val1 = val1 % val2;
                    break;
                case EQU:
                    break;
            }
        } else {
            val1 = Double.parseDouble(t1.getText().toString());
        }
    }

    // Remove error message that is already written there.
    private void ifErrorOnOutput() {
        if (t2.getText().toString().equals("Error")) {
            t2.setText("");
        }
    }

    // Whether value if a double or not
    private boolean ifReallyDecimal() {
        return val1 == (int) val1;
    }

    private void noOperation() {
        String inputExpression = t2.getText().toString();
        if (!inputExpression.isEmpty() && !inputExpression.equals("Error")) {
            if (inputExpression.contains("-")) {
                inputExpression = inputExpression.replace("-", "");
                t2.setText("");
                val1 = Double.parseDouble(inputExpression);
            }
            if (inputExpression.contains("+")) {
                inputExpression = inputExpression.replace("+", "");
                t2.setText("");
                val1 = Double.parseDouble(inputExpression);
            }
            if (inputExpression.contains("/")) {
                inputExpression = inputExpression.replace("/", "");
                t2.setText("");
                val1 = Double.parseDouble(inputExpression);
            }
            if (inputExpression.contains("%")) {
                inputExpression = inputExpression.replace("%", "");
                t2.setText("");
                val1 = Double.parseDouble(inputExpression);
            }
            if (inputExpression.contains("×")) {
                inputExpression = inputExpression.replace("×", "");
                t2.setText("");
                val1 = Double.parseDouble(inputExpression);
            }
        }
    }

    // Make text small if too many digits.
    private void exceedLength() {
        if (t1.getText().toString().length() > 10) {
            t1.setTextSize(TypedValue.COMPLEX_UNIT_SP, 20);
        }
    }
} 

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