尝试在空对象引用上调用虚拟方法“void android.widget.TextView.setBackgroundColor(int)”

Question

This code is inside a while loop that moves the cursor in a database for each loop. I want that if daysLeftInt is under 6, then the id (R.id.leftColor) should be the color of the variable redColor.

            if(daysLeftInt < 6) {
                Context context = this;
                int redColor = ContextCompat.getColor(context, R.color.red);
                TextView rectangleView = (TextView)findViewById(R.id.leftColor);
                rectangleView.setBackgroundColor(redColor);

            }

I get this error message:

Attempt to invoke virtual method 'void android.widget.TextView.setBackgroundColor(int)' on a null object reference

This is how my layouts is setup:

Do anyone know how i will get the result that I want?

activity_main.xml.

<?xml version="1.0" encoding="utf-8"?>
    <android.support.v4.widget.DrawerLayout 
        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:id="@+id/drawer_layout"
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:background="@drawable/bg"
        android:fitsSystemWindows="true"
        tools:openDrawer="start">


        <include
            layout="@layout/app_bar_main"
            android:layout_width="match_parent"
            android:layout_height="match_parent" />

        <android.support.design.widget.NavigationView
            android:id="@+id/nav_view"
            android:layout_width="wrap_content"
            android:layout_height="match_parent"
            android:layout_gravity="start"
            android:fitsSystemWindows="true"
            app:headerLayout="@layout/nav_header_main"
            app:menu="@menu/activity_main_drawer" />

    </android.support.v4.widget.DrawerLayout>

activity_main.xml includes this file called app_bar_main.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout 
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.mysubscriptions.alexklug.mysubscriptions.MainActivity">

<android.support.design.widget.AppBarLayout
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:theme="@style/AppTheme.AppBarOverlay">

    <android.support.v7.widget.Toolbar
        android:id="@+id/toolbar"
        android:layout_width="match_parent"
        android:layout_height="?attr/actionBarSize"
        android:background="?attr/colorPrimary"
        app:popupTheme="@style/AppTheme.PopupOverlay" />

</android.support.design.widget.AppBarLayout>

<include layout="@layout/content_main" />
<include layout="@layout/content_subscription" />

<android.support.design.widget.FloatingActionButton
    android:id="@+id/fab"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_gravity="bottom|end"
    android:layout_margin="16dp"
    app:srcCompat="@android:drawable/ic_menu_send"
    android:onClick="openSubscriptionActivity"
    />

</android.support.design.widget.CoordinatorLayout>

app_bar_main.xml include the layout file content_main.xml which is this file:

<?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"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.mysubscriptions.alexklug.mysubscriptions.MainActivity"
tools:showIn="@layout/app_bar_main">



<ListView
    android:id="@+id/subscriptionsList"
    android:layout_width="368dp"
    android:textAllCaps="true"
    android:layout_alignParentStart="true"
    android:layout_alignParentTop="true"
    android:layout_height="wrap_content"
    tools:layout_editor_absoluteY="0dp"
    tools:layout_editor_absoluteX="8dp">
</ListView>

Inside the ListView an ArrayAdapter helps displaying the last file called list_item.xml. The first TextView is the view with the id of leftColor.

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:orientation="horizontal"
    android:layout_width="match_parent"
    android:layout_height="wrap_content">

<LinearLayout
    android:layout_width="match_parent"
    android:layout_height="50dp"
    android:orientation="horizontal">

    <TextView

        android:id="@+id/leftColor"
        android:layout_width="3dp"
        android:layout_height="wrap_content"
        android:layout_weight="0.01"
        />


    <ImageView
        android:id="@+id/subscription_image"
        android:layout_height="wrap_content"
        android:layout_width="0dp"
        android:layout_weight="1"
        android:src="@mipmap/ic_netflix"
        android:padding="10dp"/>


    <TextView
        android:id="@+id/subscription_name"
        android:layout_weight="2"
        android:layout_width="0dp"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        android:textColor="#000000"
        android:textSize="15sp"
        android:textStyle="bold"
        tools:text="First"
        android:ellipsize="end"
        android:maxLines="1"

        />


    <TextView
        android:id="@+id/subscription_price"
        android:layout_height="wrap_content"
        android:layout_marginTop="8dp"
        tools:text="Second"
        android:textSize="12sp"
        android:layout_width="0dp"
        android:layout_weight=".7"
        android:gravity="center"
        />


    <TextView
        android:id="@+id/subscription_days_left"
        android:layout_height="wrap_content"
        android:layout_marginTop="12dp"
        android:textSize="24sp"
        android:textColor="#000000"
        tools:text="12"
        android:layout_width="0dp"
        android:layout_weight=".7"
        android:textStyle="bold"
        android:gravity="center"
        />


    <View
        android:id="@+id/myRectangleView"
        android:layout_width="7dp"
        android:layout_height="7dp"
        android:layout_marginTop="24dp"
        android:layout_marginRight="20dp"
        android:layout_marginLeft="20dp"
        android:background="@drawable/circle" />


    </LinearLayout>
</LinearLayout>

Answer 1

This is happening because " R.id.leftColor " doesn't match with the resource in the XML file. In this case, what you can do is hold 'ctrl' and hover the mouse over " R.id.leftColor " in java code and click the link it points to which appears after hovering.

1. If it takes it to the resource in the layout XML file, then it's alright, just perform a

Build->Clean Project

Build->Rebuild Project

2. Else, if it takes to an id in the R.java file, like this

public static final int leftColor=0x7f0d008c;

then you might have mistakenly renamed the leftColor in the layout XML file to something else. Correct that to whatever you have given in the java code and it should work fine.

Answer 2

It's android studios fault, just put in a delay of about 50 milliseconds. Example:

 Handler handler = new Handler();
        handler.postDelayed(new Runnable() {
            @Override
            public void run() {
                listView.getChildAt(0).setBackgroundColor(Color.BLUE);
                listView.getChildAt(1).setBackgroundColor(Color.WHITE);
                listView.getChildAt(2).setBackgroundColor(Color.RED);
                listView.getChildAt(3).setBackgroundColor(Color.BLACK);
            }
        }, 50);