英文:
Getting Null Object reference on Edittext
问题
我知道这个问题看起来与SO上的其他问题相似，但我已经尝试了其他相似问题中的所有建议，但没有取得任何成功。
我收到了以下错误：
2020-08-03 20:41:21.372 8916-8916/com.example.spacing E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.spacing, PID: 8916
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.spacing/com.example.spacing.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2843)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
...
在这段代码上出现问题：
package com.example.spacing;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class MainActivity extends AppCompatActivity {
public int firstValue;
EditText first;
String firstString;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
first = (EditText)findViewById(R.id.firstNum);
firstString = first.getText().toString();
Button Go = (Button) findViewById(R.id.goButton);
Go.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
firstValue = Integer.parseInt(firstString);
System.out.print(firstValue);
}
});
}
}
这是XML布局：
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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">
<!-- 其他视图元素 -->
</androidx.constraintlayout.widget.ConstraintLayout>
我尝试过的建议：
- 我尝试将EditText的初始化放在on-create之前和之后。
- 我确保所有ID都是正确的，引用了正确的元素。
还有其他几种方法，我无法回忆起来，但当我尝试构建时，它们都没有起作用。
任何帮助都将不胜感激。
英文:
I know that this question looks similar to others on SO but i have tried all the suggestions in the other similar questions and haven't had any success.
I'm getting this error:
2020-08-03 20:41:21.372 8916-8916/com.example.spacing E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.spacing, PID: 8916
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.spacing/com.example.spacing.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2843)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:193)
at android.app.ActivityThread.main(ActivityThread.java:6669)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:159)
at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:157)
at android.content.Context.obtainStyledAttributes(Context.java:675)
at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
at com.example.spacing.MainActivity.<init>(MainActivity.java:15)
at java.lang.Class.newInstance(Native Method)
at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:69)
at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:41)
at android.app.Instrumentation.newActivity(Instrumentation.java:1215)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2831)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3048) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:78) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:108) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:68) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1808) 
at android.os.Handler.dispatchMessage(Handler.java:106) 
at android.os.Looper.loop(Looper.java:193) 
at android.app.ActivityThread.main(ActivityThread.java:6669) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 
2020-08-03 20:41:21.411 8916-8916/com.example.spacing I/Process: Sending signal. PID: 8916 SIG: 9
on this code:
package com.example.spacing;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class MainActivity extends AppCompatActivity {
public int firstValue;
EditText first;
String firstString;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
first = (EditText)findViewById(R.id.firstNum);
firstString = first.getText().toString();
Button Go = (Button) findViewById(R.id.goButton);
Go.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
firstValue = Integer.parseInt(firstString);
System.out.print(firstValue);
}
});
}
}
Here is the XML:
<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.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">
<Button
android:id="@+id/goButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/buttonName"
app:layout_constraintBottom_toBottomOf="parent"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.498"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.898" />
<TextView
android:id="@+id/FirstAttribute"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/spacedMaterial"
app:layout_constraintBottom_toTopOf="@+id/goButton"
app:layout_constraintEnd_toStartOf="@+id/firstNum"
app:layout_constraintHorizontal_bias="0.457"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.043" />
<EditText
android:id="@+id/firstNum"
android:layout_width="165dp"
android:layout_height="40dp"
android:ems="10"
android:hint="@string/inMM"
android:importantForAutofill="no"
android:inputType="number"
app:layout_constraintBottom_toBottomOf="@+id/FirstAttribute"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.934"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="@+id/FirstAttribute"
app:layout_constraintVertical_bias="0.476" />
<TextView
android:id="@+id/secondAttribute"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/amount"
app:layout_constraintBottom_toTopOf="@+id/goButton"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.073"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.131" />
<EditText
android:id="@+id/secondNumber"
android:layout_width="165dp"
android:layout_height="40dp"
android:ems="10"
android:hint="@string/number"
android:importantForAutofill="no"
android:inputType="number"
app:layout_constraintBottom_toBottomOf="@+id/secondAttribute"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.934"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="@+id/secondAttribute" />
<TextView
android:id="@+id/thirdAttribute"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:text="@string/areaWidth"
app:layout_constraintBottom_toTopOf="@+id/goButton"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.068"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="parent"
app:layout_constraintVertical_bias="0.218" />
<EditText
android:id="@+id/thirdNumber"
android:layout_height="40dp"
android:layout_width="165dp"
android:ems="10"
android:hint="@string/inMM"
android:inputType="number"
app:layout_constraintBottom_toBottomOf="@+id/thirdAttribute"
app:layout_constraintEnd_toEndOf="parent"
app:layout_constraintHorizontal_bias="0.934"
app:layout_constraintStart_toStartOf="parent"
app:layout_constraintTop_toTopOf="@+id/thirdAttribute"
app:layout_constraintVertical_bias="0.476"
android:importantForAutofill="no" />
</androidx.constraintlayout.widget.ConstraintLayout>
Suggestions i have tried:
I have tried changing the init for the edit text to before and after on-create.
I have made sure that all ids are correct and are referencing the right thing.
And about 5 other things that i cant recall but none of them worked when i tried to build.
Any help is appreciated.
答案1
得分: 0
问题是：
您试图在单击监听器之外初始化“firstString”变量，并且在创建Activity时立即进行初始化，这意味着该变量将始终是一个空对象，因为您实际上没有在EditText中写入任何内容。
只需在ClickListener内部初始化String！
package com.example.spacing;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class MainActivity extends AppCompatActivity {
public int firstValue;
EditText first;
String firstString;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
first = (EditText)findViewById(R.id.firstNum);
Button Go = (Button) findViewById(R.id.goButton);
Go.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
firstString = first.getText().toString();
firstValue = Integer.parseInt(firstString);
System.out.print(firstValue);
}
});
}
}
英文:
Problem is:
you are trying to initialize the "firstString" variable OUTSIDE of the click listener and as soon as the Activity is created, which means the variable will always be a null object because you did not actually write something in the EditText.
Simply initialize the String inside the ClickListener!
` package com.example.spacing;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;
import android.content.Context;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
public class MainActivity extends AppCompatActivity {
public int firstValue;
EditText first;
String firstString;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
first = (EditText)findViewById(R.id.firstNum);
Button Go = (Button) findViewById(R.id.goButton);
Go.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
firstString = first.getText().toString();
firstValue = Integer.parseInt(firstString);
System.out.print(firstValue);
}
});
}
}`
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