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();
}
}
自定義視圖.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>
當我要在我的設備中運行該代碼時，它會一直關閉。日志貓顯示這樣的錯誤。
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)
我使用物理設備來實作此代碼。我也是初學者的android開發人員。我遵循不同型別的教程，但我無法解決它。我想使用這個自定義組件與按鈕等普通小部件相同，編輯文本。我也想使用 xml 放置此代碼。我無法改變它。因為我的目標專案已經使用 xml 布局開發。我該如何解決？
uj5u.com熱心網友回復：
問題是您沒有attrs將super. 這導致它沒有id
將您的建構式更改為
public CustomView(Context context, AttributeSet attrs) {
super(context, attrs);
paint = new Paint();
}
轉載請註明出處，本文鏈接：https://www.uj5u.com/ruanti/520730.html
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