onClick（）方法中的空指针异常

Question

我有一些问题解决onClick函数上的空点异常 - 我会把代码放在这里，希望有人能发现我不能发现的东西。

主要课程：

package org.matthewdann.mathsapp;

import android.content.Intent;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;

public class MainActivity extends AppCompatActivity {
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        //insert buttons to get different activities
        //SUVAT
        Button buttonSUVAT = (Button) findViewById(R.id.buttonSUVAT);
        buttonSUVAT.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(MainActivity.this, suvatSolver.class));
            }
        });
        //Temp Converter
        Button tempConvertButton = (Button) findViewById(R.id.temp_convert_button);
        tempConvertButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                startActivity(new Intent(MainActivity.this, TempConverter.class));
            }

        });
    }
        @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }
    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();
        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }
        return super.onOptionsItemSelected(item);
    }
}

清单：

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="org.matthewdann.mathsapp">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity
            android:name="org.matthewdann.mathsapp.MainActivity"
            android:label="@string/app_name"
            android:theme="@style/AppTheme.NoActionBar">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name="org.matthewdann.mathsapp.suvatSolver"
            android:label="@string/title_activity_suvat_solver"
            android:theme="@style/AppTheme.NoActionBar" />
        <activity
            android:name="org.matthewdann.mathsapp.TempConverter"
            android:label="@string/title_activity_temp_converter"
            android:theme="@style/AppTheme.NoActionBar">

        </activity>
    </application>

</manifest>

logcat的：

03-10 21:32:27.061 11535-11535/org.matthewdann.mathsapp D/AndroidRuntime: Shutting down VM
03-10 21:32:27.062 11535-11535/org.matthewdann.mathsapp E/AndroidRuntime: FATAL EXCEPTION: main
                                                                          Process: org.matthewdann.mathsapp, PID: 11535
                                                                          java.lang.RuntimeException: Unable to start activity ComponentInfo{org.matthewdann.mathsapp/org.matthewdann.mathsapp.TempConverter}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
                                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
                                                                              at android.app.ActivityThread.-wrap11(ActivityThread.java)
                                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
                                                                              at android.os.Handler.dispatchMessage(Handler.java:102)
                                                                              at android.os.Looper.loop(Looper.java:148)
                                                                              at android.app.ActivityThread.main(ActivityThread.java:5417)
                                                                              at java.lang.reflect.Method.invoke(Native Method)
                                                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                           Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                                                                              at org.matthewdann.mathsapp.TempConverter.onCreate(TempConverter.java:42)
                                                                              at android.app.Activity.performCreate(Activity.java:6251)
                                                                              at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
                                                                              at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
                                                                              at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476) 
                                                                              at android.app.ActivityThread.-wrap11(ActivityThread.java) 
                                                                              at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344) 
                                                                              at android.os.Handler.dispatchMessage(Handler.java:102) 
                                                                              at android.os.Looper.loop(Looper.java:148) 
                                                                              at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                                              at java.lang.reflect.Method.invoke(Native Method) 
                                                                              at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726) 
                                                                              at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616) 

编辑：content_main.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"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="org.matthewdann.mathsapp.MainActivity"
    tools:showIn="@layout/activity_main">
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceLarge"
        android:text="@string/app_name"
        android:id="@+id/mainPageHeader"
        android:layout_below="@+id/adView1"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/buttonSUVAT"
        android:id="@+id/buttonSUVAT"
        android:layout_below="@+id/mainPageHeader"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_alignParentRight="true"
        android:layout_alignParentEnd="true" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="@string/temp_convert_button"
        android:id="@+id/temp_convert_button"
        android:layout_below="@+id/buttonSUVAT"
        android:layout_alignRight="@+id/buttonSUVAT"
        android:layout_alignEnd="@+id/buttonSUVAT"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true" />
</RelativeLayout>

编辑2 - tempConverter：

package org.matthewdann.mathsapp;

import android.content.Context;
import android.os.Bundle;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.inputmethod.InputMethodManager;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;
import java.text.DecimalFormat;

public class TempConverter extends AppCompatActivity {
    private EditText tempEditText;
    private TextView showTempTextView;
    DecimalFormat round = new DecimalFormat("0.0");
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        //define Text and button variables
        tempEditText = (EditText) findViewById(R.id.TempInput);
        Button cButton = (Button) findViewById(R.id.buttonC);
        Button fButton = (Button) findViewById(R.id.buttonF);
        showTempTextView = (TextView) findViewById(R.id.Output);

        //button event listeners

        fButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //hide keyboard
                InputMethodManager inputManager = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
                inputManager.hideSoftInputFromWindow((null == getCurrentFocus()) ? null : getCurrentFocus().getWindowToken(), InputMethodManager.HIDE_NOT_ALWAYS);
                //error if empty
                String editTextVal = tempEditText.getText().toString();
                if (editTextVal.isEmpty()) {
                    Toast.makeText(getApplicationContext(), "Enter a value", Toast.LENGTH_LONG).show();
                } else {
                    //something has been entered - parse into decimal
                    double doubleEditText = Double.parseDouble(editTextVal);
                    //convert
                    double convertedVal1 = convertToCelsius(doubleEditText);
                    double convertedVal2 = (273.15 + convertedVal1);
                    if (0 < convertedVal2) {
                        //parse into string for output
                        String stringResult = String.valueOf(round.format(convertedVal1) + " C ; " + round.format(convertedVal2) + " K ");
                        showTempTextView.setText(stringResult);
                    } else {
                        String stringResult = String.valueOf("Error!");
                        showTempTextView.setText(stringResult);
                        Toast.makeText(getApplicationContext(), "Value is too small!", Toast.LENGTH_LONG).show();
                    }
                }
            }
        });
        cButton.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                //convert to f
                InputMethodManager inputManager = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);
                inputManager.hideSoftInputFromWindow((null == getCurrentFocus()) ? null : getCurrentFocus().getWindowToken(), InputMethodManager.HIDE_NOT_ALWAYS);
                String editTextVal = tempEditText.getText().toString();
                if (editTextVal.isEmpty()) {
                    Toast.makeText(getApplicationContext(), "Edit a value", Toast.LENGTH_LONG).show();
                } else {
                    //something entered
                    double doubleEditText = Double.parseDouble(editTextVal);
                    double convertedVal = convertToF(doubleEditText);
                    double convertedVal2 = (273.15 + doubleEditText);

                    if (0 < convertedVal2) {
                        String stringResult = String.valueOf(round.format(convertedVal) + " F ; " + round.format(convertedVal2) + " K ");
                        showTempTextView.setText(stringResult);
                    } else {
                        Toast.makeText(getApplicationContext(), "Value is too small!", Toast.LENGTH_LONG).show();
                        String stringResult = String.valueOf("Error!");
                        showTempTextView.setText(stringResult);
                    }
                }
            }
        });
    }

    public double convertToCelsius (double farVal) {
        double resultCel;
        resultCel = (farVal - 32) * 5/9;
        return resultCel;
    }
    public double convertToF (double celVar) {
        double resultF;
        resultF = (celVar * 9/5) + 32;
        return resultF;
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }
}

Answer 1

首先检查是否

R.id.temp_convert_button 

与

在同一XML中

R.layout.activity_main.

你可能会发现findViewById（）返回null。  试试

Button tempConvertButton = (Button) findViewById(R.id.temp_convert_button)
Log.i("test"," is null " +(null == tempConvertButton));

Answer 2

问题不在于onClick方法，它是您注册监听器的地方！按钮对象为null，如果在尚未加载所需的XML文件时尝试使用findViewById()，则最有可能发生这种情况。尝试将监听器注册移至onCreateView()

Answer 3

我认为你正在调用错误的布局。 在您的代码中，您要求activity_main布局，但您的布局文件似乎是content_main.xml。

尝试将setContentView(R.layout.activity_main);更改为setContentView(R.layout.content_main);

或者只需将content_main.xml文件重命名为activity_main.xml

编辑：

在第二次编辑之后，您似乎正在为两个活动使用相同的布局。在某些情况下，您可能会提到错误的布局。

Answer 4

所以我有一个setContentView(R.layout.activity_main);的流氓实例  在应用程序中 - 修复似乎解决了这个问题！