Android Studio 4.0.1 GridLayout使应用程序崩溃

问题描述

我正在学习如何获取网格并从网格中查找一些元素，但是当我想通过id查找网格时，我的应用程序崩溃了，所以我决定创建一个可以正确查找网格的测试应用程序从一开始，这个应用程序也崩溃了。请注意，在testapp中，我引入了旧版库以确保我不会错过任何内容。
这是视图的xml代码

<?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"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.498"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.264" />

    <android.support.v7.widget.GridLayout
        android:id="@+id/grid"
        android:layout_width="409dp"
        android:layout_height="354dp"
        android:layout_marginEnd="1dp"
        android:layout_marginStart="1dp"
        android:layout_marginTop="1dp"
        app:columnCount="3"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/textView"
        app:rowCount="3">

    </android.support.v7.widget.GridLayout>

</android.support.constraint.ConstraintLayout>

这是在调用“ GridLayout测试”时发生崩溃的mainActivity

package com.example.test2;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.GridLayout;

public class MainActivity extends AppCompatActivity {

    GridLayout test =(GridLayout) findViewById(R.id.grid);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }
}

这是我收到的错误消息

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.test2,PID: 11311
    java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.test2/com.example.test2.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:2679)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856)
        at android.app.ActivityThread.-wrap11(UnkNown Source:0)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:164)
        at android.app.ActivityThread.main(ActivityThread.java:6494)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.Window$Callback android.view.Window.getCallback()' on a null object reference
        at android.support.v7.app.AppCompatDelegateImpl.<init>(AppCompatDelegateImpl.java:249)
        at android.support.v7.app.AppCompatDelegate.create(AppCompatDelegate.java:182)
        at android.support.v7.app.AppCompatActivity.getDelegate(AppCompatActivity.java:520)
        at android.support.v7.app.AppCompatActivity.findViewById(AppCompatActivity.java:191)
        at com.example.test2.MainActivity.<init>(MainActivity.java:9)
        at java.lang.class.newInstance(Native Method)
        at android.app.Instrumentation.newActivity(Instrumentation.java:1174)
        at android.app.ActivityThread.performlaunchActivity(ActivityThread.java:2669)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2856) 
        at android.app.ActivityThread.-wrap11(UnkNown Source:0) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1589) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:164) 
        at android.app.ActivityThread.main(ActivityThread.java:6494) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)

感谢您提供有关解决此问题的任何帮助，并详细解释了发生此问题的原因，谢谢。

解决方法

从以下位置更改代码：

GridLayout test =(GridLayout) findViewById(R.id.grid);

收件人：

androidx.gridlayout.widget.GridLayout gridLayout = findViewById(R.id.gridLayout);

您试图在活动了解其xml布局之前获取ID。您需要将get id行放在如下的onCreate方法中

public class MainActivity extends AppCompatActivity {

    private GridLayout test;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        test =(GridLayout) findViewById(R.id.grid);
   }
}

android-gridlayout grid-layout java java