主要活动.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();
    }
}

自定义视图.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);
    }
}

活动主文件.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>

当我要在我设备中运行代码时，它一直关闭。日志猫显示这样的错误。

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) 

我使用物理设备来实现此代码。我也是初学者android开发人员。我遵循不同类型的教程，但我不能解决它。我想使用此自定义组件一样，如按钮，编辑文本正常的小部件。我也想把此代码也使用xml。我不能改变它。因为我的目标项目已经开发使用xml布局。我该如何解决它？