IllegalStateException and NullPointer Exception

Question

我正在尝试运行以下活动，但一旦我单击提交按钮，我的应用程序就会崩溃并出现给定的异常。

我已经将声明和初始化分开了。

我的投诉.java文件如下

package com.example.android.complaintbox_ritesltd;
import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import androidx.appcompat.app.AppCompatActivity;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;

public class Complaint extends AppCompatActivity {
    private FirebaseDatabase mFirebaseDatabase;
    private DatabaseReference mComplaintsDatabaseRegerence;
    private EditText ct;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        mFirebaseDatabase=FirebaseDatabase.getInstance();
        mComplaintsDatabaseRegerence=mFirebaseDatabase.getReference().child("Complaints");
        ct=findViewById(R.id.comp_box);

        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_complaint);
    }
    public void submitButton(View view) {
        //TODO: Send Complaint to the database
       ComplaintDetails complaintDetails=new ComplaintDetails(ct.getText().toString());
       mComplaintsDatabaseRegerence.push().setValue(complaintDetails);

        //Clears the input box
        ct.setText("");
    }
}

activity_complaint.xml如下

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".Complaint"
    android:orientation="vertical"
    android:background="#E3F2FD"
    >

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:textColor="@color/colorPrimary"
        android:text="Write Your Complaint Here!"
        android:textSize="30dp"
        android:padding="20dp"
        android:layout_marginTop="30dp"
        android:textStyle="bold"
        />
    <EditText
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:padding="30dp"
        android:hint="Complaint!"
        android:id="@+id/comp_box"
        android:layout_marginTop="60dp"
        android:inputType="text"
        android:background="@drawable/white_round_corner_box"
        android:layout_margin="14dp"
        />
    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Submit"
        android:layout_margin="14dp"
        android:padding="14dp"
        android:textAllCaps="true"
        android:background="@drawable/rounded_corners"
        android:textColor="@android:color/white"
        android:layout_marginTop="90dp"
        android:id="@+id/submit_button"
        android:onClick="submitButton"
        />
</LinearLayout>

错误是：

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.android.complaintbox_ritesltd, PID: 3227
    java.lang.IllegalStateException: Could not execute method for android:onClick
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:390)
        at android.view.View.performClick(View.java:6597)
        at android.view.View.performClickInternal(View.java:6574)
        at android.view.View.access$3100(View.java:778)
        at android.view.View$PerformClick.run(View.java:25885)
        at android.os.Handler.handleCallback(Handler.java:873)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        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$MethodAndArgsCaller.run(RuntimeInit.java:493)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858)
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invoke(Native Method)
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385)
        at android.view.View.performClick(View.java:6597) 
        at android.view.View.performClickInternal(View.java:6574) 
        at android.view.View.access$3100(View.java:778) 
        at android.view.View$PerformClick.run(View.java:25885) 
        at android.os.Handler.handleCallback(Handler.java:873) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        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$MethodAndArgsCaller.run(RuntimeInit.java:493) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:858) 
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.text.Editable android.widget.EditText.getText()' on a null object reference
        at com.example.android.complaintbox_ritesltd.Complaint.submitButton(Complaint.java:42)
        at java.lang.reflect.Method.invoke(Native Method) 
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:385) 
        at android.view.View.performClick(View.java:6597) 
        at android.view.View.performClickInternal(View.java:6574) 
        at android.view.View.access$3100(View.java:778) 
        at android.view.View$PerformClick.run(View.java:25885) 
        at android.os.Handler.handleCallback(Handler.java:873) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        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)

Answer 1

public class Complaint extends AppCompatActivity {
    private FirebaseDatabase mFirebaseDatabase;
    private DatabaseReference mComplaintsDatabaseRegerence;
    private EditText ct;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_complaint);
        mFirebaseDatabase=FirebaseDatabase.getInstance();
        mComplaintsDatabaseRegerence=mFirebaseDatabase.getReference().child("Complaints");
        ct=findViewById(R.id.comp_box); // It needs to be below your setContentView.
    }
}

原因 ：

原因onCreate需要知道您用于类的布局是什么。

然后，您可以分配您的Edittext否则它会导致崩溃并出现异常nullpointer。