MainActivity.java
package com.bandiapps.graphicfuture;
import androidx.appcompat.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Toast;
public class MainActivity extends AppCompatActivity {
private CustomView cv;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
cv = findViewById(R.id.cus_1);
cv.setWidth();
}
}
CustomView.java
package com.bandiapps.graphicfuture;
import android.content.Context;
import android.graphics.Canvas;
import android.graphics.Paint;
import android.util.AttributeSet;
import android.view.View;
public class CustomView extends View {
int width,height;
private Paint paint;
public CustomView(Context context, AttributeSet attrs) {
super(context);
paint = new Paint();
}
@Override
protected void onSizeChanged(int w, int h, int oldw, int oldh) {
super.onSizeChanged(w, h, oldw, oldh);
width = w;
height = h;
}
@Override
protected void onDraw(Canvas canvas) {
super.onDraw(canvas);
paint.setStyle(Paint.Style.STROKE);
paint.setARGB(255,0,255,0);
canvas.drawRect(1,1,width -1 ,height -1 ,paint);
}
public void setWidth(){
paint.setStrokeWidth(10);
}
}
activity_main.xml
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout 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=".MainActivity">
<com.bandiapps.graphicfuture.CustomView
android:id="@ id/cus_1"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentEnd="true"
android:layout_alignParentStart="true"
android:layout_alignParentTop="true"
android:layout_alignParentBottom="true"
android:layout_margin="100dp"
/>
</RelativeLayout>
when I going to run that code in my device it keep closing. Log cat shows error like this.
E FATAL EXCEPTION: main
Process: com.bandiapps.graphicfuture, PID: 25635
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.bandiapps.graphicfuture/com.bandiapps.graphicfuture.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.bandiapps.graphicfuture.CustomView.setMessage()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3846)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4022)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2336)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:246)
at android.app.ActivityThread.main(ActivityThread.java:8653)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void com.bandiapps.graphicfuture.CustomView.setMessage()' on a null object reference
at com.bandiapps.graphicfuture.MainActivity.onCreate(MainActivity.java:19)
at android.app.Activity.performCreate(Activity.java:8207)
at android.app.Activity.performCreate(Activity.java:8191)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3819)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:4022)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2336)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:246)
at android.app.ActivityThread.main(ActivityThread.java:8653)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:602)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1130)
I use physical device to implement this code. I'm beginner android developer also. I follow different kind of tutorials but I could not be able to solve it.I want to use this custom component same as normal widgets like buttons,Edit Text.I also want to place this code using xml also. I cannot able to change it. because my target project is already developed using xml layouts. How can I solve it ?
CodePudding user response：
The problem is that you are not passing the attrs to the super. This causes it to not have an id
Change your constructor to
public CustomView(Context context, AttributeSet attrs) {
super(context, attrs);
paint = new Paint();
}