尝试在零对象引用的无线电组上调用虚拟方法

Question

我添加了这个无线电组，其中是radiobuttons，尽管现在我得到了整个无线电组为null？一个非常有趣的是，当我在API为22的设备上运行该错误时，我的应用不会检测到此错误，但是当我在28API模拟器上重新运行它时，此错误确实会出现。 这是代码：

radioGroup = findViewById(R.id.radioGroup2);
    Rd1= findViewById(R.id.radioButton5);
    Rd2= findViewById(R.id.radioButton6);
    radioGroup.setOnCheckedChangeListener( new RadioGroup.OnCheckedChangeListener() {
        @Override
        public void onCheckedChanged(RadioGroup group, int checkedId) {
            if (Rd1.isChecked()) {
                attemptLogin();
                Intent intent = new Intent(LoginActivity.this, BottomActivity.class);
                startActivity(intent);
            }else {
                if(Rd2.isChecked()) {
                    attemptLogin();
                    Intent intent = new Intent(LoginActivity.this, LoginActivity2.class);
                    startActivity(intent);
                }
            }
        }
    } );

这是XML代码，我尝试在我的登录活动中添加radiogroup视图，以便将两个按钮彼此放置，尽管一旦我这样做，我立即就会遇到此错误。

<RadioGroup
            android:id="@+id/radioGroup2"
            android:layout_width="361dp"
            android:layout_height="125dp"
            android:layout_gravity="bottom"
            android:background="@color/licolor"
            android:gravity="center"
            android:layout_marginTop="20dp"
            android:orientation="vertical"
            tools:ignore="UselessParent">
            <RadioButton
                android:id="@+id/radioButton5"
                android:layout_width="match_parent"
                android:layout_height="45dp"
                android:layout_marginTop="16dp"
                android:background="@drawable/mybutton"
                android:gravity="center"
                android:text="@string/register"
                android:textColor="#fafafa"
                android:textStyle="bold"
                android:textSize="19sp"
                android:button="@null"/>
            <RadioButton
                android:id="@+id/radioButton6"
                android:layout_width="match_parent"
                android:layout_height="45dp"
                android:layout_marginTop="16dp"
                android:background="@drawable/mybutton"
                android:gravity="center"
                android:text="@string/login"
                android:textColor="#fafafa"
                android:textStyle="bold"
                android:textSize="19sp"
                android:button="@null"/>
        </RadioGroup>

您有以下错误：

Attempt to invoke virtual method 'void android.widget.RadioGroup.setOnCheckedChangeListener(android.widget.RadioGroup$OnCheckedChangeListener)' on a null object reference

Answer 1

您的radiogroup具有ID =" radiogroup2"

android:id="@+id/radioGroup2"

在代码中，您正在尝试找到" radiogroup"

radioGroup = findViewById( R.id.radioGroup );

只需将其更改为：

radioGroup = findViewById( R.id.radioGroup2 );

Answer 2

//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">
<RadioGroup
    android:id="@+id/radioGroup"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:orientation="vertical"
     >
    <RadioButton
        android:id="@+id/radioButton1"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Radio Button 1"/>
    <RadioButton
        android:id="@+id/radioButton2"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Radio Button 2"/>
    <RadioButton
        android:id="@+id/radioButton3"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Radio Button 3"/>
</RadioGroup>

//活动

RadioGroup radioGroup;
RadioButton RD1;
RadioButton RD2;
RadioButton RD3;
    radioGroup = (RadioGroup)findViewById(R.id.radioGroup);
    RD1 = (RadioButton)findViewById(R.id.radioButton1);
    RD2 = (RadioButton)findViewById(R.id.radioButton2);
    RD3 = (RadioButton)findViewById(R.id.radioButton3);
           radioGroup.setOnCheckedChangeListener(new RadioGroup.OnCheckedChangeListener() {
        @Override
        public void onCheckedChanged(RadioGroup radioGroup, int checkedButtonId) {
            // This will get the radiobutton that has changed in its check state
            RadioButton checkedRadioButton = (RadioButton)radioGroup.findViewById(checkedButtonId);
            // This puts the value (true/false) into the variable
            boolean isChecked = checkedRadioButton.isChecked();
            // If the radiobutton that has changed in check state is now checked...
            if (isChecked)
            {
                switch (checkedButtonId)
                {
                    case R.id.radioButton1:
                        Log.i("test" , "Checked Button 1 ");
                        break;
                    case R.id.radioButton2:
                        Log.i("test" , "Checked Button 2 ");
                        break;
                    case R.id.radioButton3:
                        Log.i("test" , "Checked Button 3 ");
                        break;
                }
            }
        }
    });

，您有Gradle文件：

apply plugin: 'com.android.application'
android {
compileSdkVersion 28
defaultConfig {
    applicationId "com.example.shrinkio"
    minSdkVersion 19
    targetSdkVersion 28
    versionCode 1
    versionName "1.0"
    testInstrumentationRunner 
"android.support.test.runner.AndroidJUnitRunner"
    vectorDrawables.useSupportLibrary = true
}
buildTypes {
    release {
        minifyEnabled false
        proguardFiles getDefaultProguardFile('proguard-android-optimize.txt'), 'proguard-rules.pro'
    }
}

}

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'com.android.support:appcompat-v7:28.0.0'
implementation 'com.android.support.constraint:constraint-layout:1.1.3'
implementation 'com.android.support:design:28.0.0'
implementation 'com.android.support:support-vector-drawable:28.0.0'
implementation 'com.android.support:support-v4:28.0.0'
implementation 'com.android.support:recyclerview-v7:28.0.0'
implementation 'android.arch.lifecycle:extensions:1.1.1'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:1.0.2'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:3.0.2'
implementation 'com.google.firebase:firebase-core:16.0.7'
implementation 'com.google.firebase:firebase-auth:16.1.0'
implementation "com.google.android.gms:play-services-gcm:16.1.0"
implementation 'com.android.support:cardview-v7:28.0.0'

}

apply plugin: 'com.google.gms.google-services'