error : "Attempt to invoke virtual method 'void android.widget.LinearLayout.addView(android.view.View)' on a null object reference"

so what should ı do to fix it if someone help me ı'll be so gratefull here is my codes

cardbg.xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="75dp"
    android:background="@color/purple_500"
    android:id="@ id/cardbg">

</LinearLayout>

activit_ymain.xml

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayoutxmlns: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=".MainActivity">

<androidx.core.widget.NestedScrollView
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    tools:ignore="MissingConstraints">

<LinearLayout
    android:id="@ id/main"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent"
    android:orientation="vertical" />

</androidx.core.widget.NestedScrollView>

</androidx.constraintlayout.widget.ConstraintLayout>

MainActivity.java

package com.thehuman.todo;

import androidx.appcompat.app.AppCompatActivity;

import android.annotation.SuppressLint;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.LinearLayout;

public class MainActivity extends AppCompatActivity {
    LinearLayout main;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        main = findViewById(R.id.main);
        card first = new card(MainActivity.this);
        main.addView(first);
        }

}

class card extends LinearLayout{
    card self;
    LinearLayout bg,main;
    Button clon,delete;
    @SuppressLint("ResourceAsColor")
    public card(Context context) {
        super(context);
        self=this;
        main = findViewById(R.id.main);
        bg = findViewById(R.id.cardbg);
        clon = new Button(context);
        clon.setBackgroundColor(R.color.white);
        clon.setPadding(20,20,20,20);
        clon.setTextColor(R.color.teal_200);
        clon.setText("clon me");
        delete = new Button(context);
        delete.setBackgroundColor(R.color.black);
        delete.setTextColor(R.color.teal_200);
        delete.setPadding(20,20,20,20);
        delete.setText("delete me");
        bg.addView(clon);
        bg.addView(delete);
        clon.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {

                 card cloncard = new card(context);
                 main.addView(cloncard);

        }
    });
        delete.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                main.removeView(self);

        }
    });

}

}

CodePudding user response：

I suppose you are getting null object reference error in class Card. Because you are using bg = findViewById(R.id.cardbg);. You can not find cardbg.xml using findViewById because cardbg.xml have not been attached to you activity layout. You have to inflate your xml by yourself using an inflater.

Use

bg = LayoutInflater.from(context).inflate(R.layout.cardbg, null).findViewById(R.id.cardbg);