从EditText获取字符串而没有可为空的错误

Question

我发现问题是EditText返回NullPointerException（remove(at:)）。现在，我已经在Google上搜索了几个小时的解决方案，并找到了一些解决方案，但它们并不适合我。有人能够编辑我的地雷代码，以便我从Edittext中正确地获取值作为字符串。这是我的代码：

XML

<?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">

<!-- Page Title -->
<TextView
    android:id="@+id/PageTitle"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:text="@string/test_title"
    android:textAlignment="center"
    android:textSize="24sp"
    android:textStyle="bold"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.5"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toTopOf="parent" />

<!-- Input 1 -->
<EditText
    android:id="@+id/input1"
    android:layout_width="140dp"
    android:layout_height="40dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:inputType="text"
    android:text=""
    android:hint="@string/hint_input"
    app:layout_constraintEnd_toStartOf="@+id/translation1"
    app:layout_constraintHorizontal_bias="0.5"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/PageTitle"
    android:imeOptions="actionNext"/>

<!-- Translation 1 -->
<TextView
    android:id="@+id/translation1"
    android:layout_width="140dp"
    android:layout_height="40dp"
    android:layout_marginEnd="8dp"
    android:layout_marginTop="8dp"
    android:gravity="center"
    android:text="@string/test"
    android:textAlignment="center"
    android:textSize="18sp"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintHorizontal_bias="0.5"
    app:layout_constraintStart_toEndOf="@+id/input1"
    app:layout_constraintTop_toBottomOf="@+id/PageTitle"/>

<!--Check button-->
<Button
    android:id="@+id/button"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="8dp"
    android:layout_marginEnd="8dp"
    android:layout_marginStart="8dp"
    android:layout_marginTop="8dp"
    android:onClick="Check"
    android:text="@string/check_btn"
    app:layout_constraintBottom_toBottomOf="parent"
    app:layout_constraintEnd_toEndOf="@+id/translation1"
    app:layout_constraintStart_toStartOf="@+id/input1"
    app:layout_constraintTop_toBottomOf="@+id/PageTitle" />

 </android.support.constraint.ConstraintLayout>

Java

package com.bnf.overhooring;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}

String right = getString(R.string.right);
String wrong = getString(R.string.wrong);
String translation1 = getString(R.string.translation1);

EditText input1 = (EditText) findViewById(R.id.input1);
String string1 = input1.getText().toString();

public void Check(View view) {
    if (string1.equals(translation1)) input1.setText(right);
    else input1.setText(wrong);
 }
}

堆栈跟踪

E/AndroidRuntime: FATAL EXCEPTION: main
              Process: com.bnf.overhooring, PID: 30985
              java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.bnf.overhooring/com.bnf.overhooring.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2849)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045)
                  at android.app.ActivityThread.-wrap14(ActivityThread.java)
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642)
                  at android.os.Handler.dispatchMessage(Handler.java:102)
                  at android.os.Looper.loop(Looper.java:154)
                  at android.app.ActivityThread.main(ActivityThread.java:6776)
                  at java.lang.reflect.Method.invoke(Native Method)
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496)
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386)
               Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.res.Resources android.content.Context.getResources()' on a null object reference
                  at android.content.ContextWrapper.getResources(ContextWrapper.java:86)
                  at android.view.ContextThemeWrapper.getResourcesInternal(ContextThemeWrapper.java:127)
                  at android.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:121)
                  at android.support.v7.app.AppCompatActivity.getResources(AppCompatActivity.java:542)
                  at android.content.Context.getString(Context.java:476)
                  at com.bnf.overhooring.MainActivity.<init>(MainActivity.java:16)
                  at java.lang.Class.newInstance(Native Method)
                  at android.app.Instrumentation.newActivity(Instrumentation.java:1086)
                  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2839)
                  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3045) 
                  at android.app.ActivityThread.-wrap14(ActivityThread.java) 
                  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1642) 
                  at android.os.Handler.dispatchMessage(Handler.java:102) 
                  at android.os.Looper.loop(Looper.java:154) 
                  at android.app.ActivityThread.main(ActivityThread.java:6776) 
                  at java.lang.reflect.Method.invoke(Native Method) 
                  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1496) 
                  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1386) 

Answer 1

您的代码经过简化，如下所示：

public class MainActivity extends AppCompatActivity {

    String right = getString(R.string.right);
    String wrong = getString(R.string.wrong);
    String translation1 = getString(R.string.translation1);
}

调用getString()时发生的事情确实有些复杂，但是要理解的重要一点是它需要一个Resources对象。在构造活动实例时（即正在尝试调用Resources时），该getString()对象不是立即“就绪”。

通常，要等到super.onCreate()被调用后才能尝试访问资源框架；这包括呼叫getString()。

您可以通过将代码更改为如下形式来解决它：

public class MainActivity extends AppCompatActivity {

    private String right;
    private String wrong;
    private String translation1;
    private EditText input1;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        this.right = getString(R.string.right);
        this.wrong = getString(R.string.wrong);
        this.translation1 = getString(R.string.translation1);
        this.input1 = (EditText) findViewById(R.id.input1);
    }

    public void Check(View view) {
        String string1 = input1.getText().toString();

        if (string1.equals(translation1)) {
            input1.setText(right);
        }
        else {
            input1.setText(wrong);
        }
    }
}

我们在这里要做的重要事情是将right，wrong，translation1和input1的分配移到我们的onCreate()方法中。我们还确保getString()调用发生在super.onCreate()之后，并且findViewById()调用发生在setContentView()之后。顺序很重要。

最后，在实际调用string1方法之前，没有理由为Check()分配值。这样一来，我们可以确保使用EditText中的最新值。

Answer 2

在生命周期中发生onCreate之前，您无法访问视图。当android初始化所有视图时，就可以访问它。

这对我来说很好

package com.example.casti.myapplication;


import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {

    String right = "right";
    String wrong = "wrong";
    String translation1 = "translation1";

    EditText input1;
    String string1;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        input1 = findViewById(R.id.input1);
        string1 = input1.getText().toString();
    }

    public void Check(View view) {
        if (string1.equals(translation1)) input1.setText(right);
        else input1.setText(wrong);
    }

}