Android studio Błąd

Question

Witam, napisałem aplikacje z przełączaniem się między activity. Aplikacja się kompiluje ale przy naciśnięciu przycisku wywala błąd:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.mathtool, PID: 22050
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.mathtool/com.example.mathtool.PrzekatnaProstokata}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
       

KOD podemnu

import androidx.appcompat.app.AppCompatActivity;

import android.annotation.SuppressLint;
import android.content.Intent;
import android.os.Bundle;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.view.View;
import android.widget.Toast;
public class PrzekatnaProstokata extends AppCompatActivity {

    private Button buttonpowrotprostokatprzekatna;
    private Button Result_Rectangle_Diagonal;
    private EditText bok_a_rectangle;
    private EditText bok_b_rectangle;
    private TextView result_rectangle_diagonal;

    @SuppressLint("MissingInflatedId")
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_przekatna_prostokata);

        bok_a_rectangle = (EditText) findViewById(R.id.bokaprzekatnaprostokata);
        bok_b_rectangle = (EditText) findViewById(R.id.bokbprzekatnaprostokata2);

        getSupportActionBar().setTitle("Przekątna prostokąta");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        buttonpowrotprostokatprzekatna = (Button) findViewById(R.id.buttonpowrotprostokatprzekatna);
        buttonpowrotprostokatprzekatna.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                openMenuProstokat();
            }
        });

        Result_Rectangle_Diagonal.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if((bok_a_rectangle.getText().length()>0) && (bok_b_rectangle.getText().length()>0))
                {
                    double oper1 = Double.parseDouble(bok_a_rectangle.getText().toString());
                    double oper2 = Double.parseDouble(bok_b_rectangle.getText().toString());
                    double result = Math.sqrt(Math.pow(oper1,2) + Math.pow(oper2,2));
                    result_rectangle_diagonal.setText(Double.toString(result));
                }
                else{
                    Toast toast= Toast.makeText(PrzekatnaProstokata.this,"Wprowadź poprawne dane !!!",Toast.LENGTH_LONG);
                    toast.show();
                }

            }
        });
    }


    public void openMenuProstokat() {
        Intent intent = new Intent(this, ProstokatMenu.class);
        startActivity(intent);
    }
}

XML:

<?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="@drawable/istockphoto_185005145_612x612"
    tools:context=".PrzekatnaProstokata">

    <TextView
        android:id="@+id/textView5"
        android:layout_width="409dp"
        android:layout_height="50dp"
        android:layout_marginStart="4dp"
        android:background="#A8175B"
        android:text="    Przekątna prostokąta"
        android:textSize="35dp"
        app:layout_constraintStart_toStartOf="parent"
        tools:ignore="MissingConstraints"
        tools:layout_editor_absoluteY="5dp" />

    <ImageView
        android:id="@+id/imageView10"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="20dp"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView5"
        app:srcCompat="@drawable/prostokat" />

    <Button
        android:id="@+id/buttonpowrotprostokatprzekatna"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="688dp"
        android:text="Powrót"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.178"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:ignore="MissingConstraints" />

    <TextView
        android:id="@+id/textView3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="52dp"
        android:background="#459348"
        android:text="d = "
        android:textSize="30dp"
        app:layout_constraintTop_toBottomOf="@+id/imageView10"
        tools:ignore="MissingConstraints"
        tools:layout_editor_absoluteX="56dp" />

    <TextView
        android:id="@+id/textView4"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginStart="12dp"
        android:layout_marginTop="52dp"
        android:background="#671F6C"
        android:text="4 * a "
        android:textSize="30dp"
        app:layout_constraintStart_toEndOf="@+id/textView3"
        app:layout_constraintTop_toBottomOf="@+id/imageView10"
        tools:ignore="MissingConstraints" />

    <TextView
        android:id="@+id/textView14"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:background="#3F51B5"
        android:text="Bok a:  "
        android:textSize="25dp"
        app:layout_constraintTop_toBottomOf="@+id/textView4"
        tools:ignore="MissingConstraints"
        tools:layout_editor_absoluteX="154dp" />

    <TextView
        android:id="@+id/textView15"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_marginTop="16dp"
        android:background="#3F51B5"
        android:text="Bok b:  "
        android:textSize="25dp"
        app:layout_constraintTop_toBottomOf="@+id/bokaprzekatnaprostokata"
        tools:ignore="MissingConstraints"
        tools:layout_editor_absoluteX="154dp" />

    <EditText
        android:id="@+id/bokaprzekatnaprostokata"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="12dp"
        android:ems="10"
        android:gravity="center_horizontal"
        android:inputType="number"
        android:textSize="18sp"
        android:visibility="visible"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.0"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView14"
        tools:ignore="MissingConstraints" />

    <EditText
        android:id="@+id/bokbprzekatnaprostokata2"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_marginTop="12dp"
        android:layout_marginEnd="4dp"
        android:ems="10"
        android:gravity="center_horizontal"
        android:inputType="number"
        android:textSize="18sp"
        android:visibility="visible"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView15"
        tools:ignore="MissingConstraints" />

    <Button
        android:id="@+id/button_result_diagonal_rectangle"
        android:layout_width="110dp"
        android:layout_height="43dp"
        android:layout_marginTop="24dp"
        android:background="@color/red"
        android:text="Calc"
        app:layout_constraintTop_toBottomOf="@+id/textView1"
        tools:ignore="MissingConstraints"
        tools:layout_editor_absoluteX="225dp" />

    <TextView
        android:id="@+id/textView1"
        android:layout_width="332dp"
        android:layout_height="50dp"
        android:layout_marginTop="40dp"
        android:fontFamily="serif"
        android:gravity="center_horizontal"
        android:text="result"
        android:textColorLink="?android:attr/editTextColor"
        android:textSize="27sp"
        android:textStyle="bold|italic"
        android:visibility="visible"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintHorizontal_bias="0.493"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/bokbprzekatnaprostokata2"
        tools:textStyle="bold|italic" />

</androidx.constraintlayout.widget.ConstraintLayout>

Kod menu:
 

import androidx.appcompat.app.AppCompatActivity;

import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;

public class ProstokatMenu extends AppCompatActivity {

    private Button buttonpowrotprostokat1;
    private Button buttonpolerectangle;
    private Button buttonprzekatnarectangle;
    private Button buttonobwodrectangle;

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


        getSupportActionBar().setTitle("Pole prostokąta");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);

        buttonpolerectangle = (Button) findViewById(R.id.buttonpolerectangle);
        buttonpolerectangle.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                openPoleProstokat();
            }
        });

        buttonprzekatnarectangle = (Button) findViewById(R.id.buttonprzekatnarectangle);
        buttonprzekatnarectangle.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                openPrzekatnaProstokat();
            }
        });

        buttonobwodrectangle = (Button) findViewById(R.id.buttonobwodrectangle);
        buttonobwodrectangle.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                openObwodProstokat();
            }
        });

        buttonpowrotprostokat1 = (Button) findViewById(R.id.buttonpowrotprostokat1);
        buttonpowrotprostokat1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                openObwodProstokat();
            }
        });
    }

    public void openPoleProstokat() {
        Intent intent = new Intent(this, PoleProstokat.class);
        startActivity(intent);
    }
    public void openPrzekatnaProstokat() {
        Intent intent = new Intent(this, PrzekatnaProstokata.class);
        startActivity(intent);
    }
    public void openObwodProstokat() {
        Intent intent = new Intent(this, ObwodProstokata.class);
        startActivity(intent);
    }
    public void openBackMenuProstokat() {
        Intent intent = new Intent(this, ProstokatMenu.class);
        startActivity(intent);
    }
}

Proszę o pomoc co jest źle ?

Answer 1

Prawdopodobnie błąd wynika z tego, że nie inicjalizujesz obiektu Result_Rectangle_Diagonal w swoim kodzie. Zmodyfikuj swój kod następująco:

Result_Rectangle_Diagonal = (Button) findViewById(R.id.Result_Rectangle_Diagonal);

Zamień "Result_Rectangle_Diagonal" na id przycisku, który masz w swoim pliku XML.

Comments

działa, dzięki, mój błąd. Pozdrawiam :)