空指针预测 - 空对象参考

Question

Android Studio和当前正在创建规划器应用程序的新手，我在查看尝试查看客户活动时收到此错误。

错误：

E/AndroidRuntime: FATAL EXCEPTION: main
                  Process: com.example.user.planner, PID: 21140
                  java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.user.planner/com.example.user.planner.Customer}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2947)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3008)
                      at android.app.ActivityThread.-wrap14(ActivityThread.java)
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1650)
                      at android.os.Handler.dispatchMessage(Handler.java:102)
                      at android.os.Looper.loop(Looper.java:154)
                      at android.app.ActivityThread.main(ActivityThread.java:6688)
                      at java.lang.reflect.Method.invoke(Native Method)
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468)
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358)
                   Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                      at com.example.user.planner.Customer.AddData(Customer.java:68)
                      at com.example.user.planner.Customer.onCreate(Customer.java:44)
                      at android.app.Activity.performCreate(Activity.java:6912)
                      at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1126)
                      at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2900)
                      at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3008) 
                      at android.app.ActivityThread.-wrap14(ActivityThread.java) 
                      at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1650) 
                      at android.os.Handler.dispatchMessage(Handler.java:102) 
                      at android.os.Looper.loop(Looper.java:154) 
                      at android.app.ActivityThread.main(ActivityThread.java:6688) 
                      at java.lang.reflect.Method.invoke(Native Method) 
                      at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1468) 
                      at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1358) 

这是我的代码：

    package com.example.user.planner;

import android.support.design.widget.TabLayout;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;

import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.app.FragmentPagerAdapter;
import android.support.v4.view.ViewPager;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.view.ViewGroup;

import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

public class Customer extends AppCompatActivity {
    DatabaseHelper myDb;
    EditText editName, editSurname, editAddress, editContact;
    Button addDataBtn;

    private SectionsPagerAdapter mSectionsPagerAdapter;
    private ViewPager mViewPager;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_customer);
        myDb = new DatabaseHelper(this);

        editName = (EditText)findViewById(R.id.firstEntry);
        editSurname = (EditText)findViewById(R.id.surnameEntry);
        editAddress = (EditText)findViewById(R.id.addressEntry);
        editContact = (EditText)findViewById(R.id.contactEntry);
        addDataBtn = (Button)findViewById(R.id.addBtn);
        AddData();

        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);
        mSectionsPagerAdapter = new SectionsPagerAdapter(getSupportFragmentManager());

        mViewPager = (ViewPager) findViewById(R.id.container);
        mViewPager.setAdapter(mSectionsPagerAdapter);

        TabLayout tabLayout = (TabLayout) findViewById(R.id.tabs);
        tabLayout.setupWithViewPager(mViewPager);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });

    }

    public void AddData() {
        addDataBtn.setOnClickListener(
                new View.OnClickListener(){
                    public void onClick(View v) {
                     boolean isInserted = myDb.insertData(
                              editName.getText().toString(),
                              editSurname.getText().toString(),
                              editAddress.getText().toString(),
                              editContact.getText().toString());
                        if(isInserted =true)
                            Toast.makeText(Customer.this, "Data Inserted", Toast.LENGTH_LONG).show();
                        else
                            Toast.makeText(Customer.this, "Data not Inserted", Toast.LENGTH_LONG).show();
                    }
                }
        );
    }

    @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_customer, 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);
    }

    /**
     * A placeholder fragment containing a simple view.
     */
    public static class PlaceholderFragment extends Fragment {
        /**
         * The fragment argument representing the section number for this
         * fragment.
         */
        private static final String ARG_SECTION_NUMBER = "section_number";

        public PlaceholderFragment() {
        }

        /**
         * Returns a new instance of this fragment for the given section
         * number.
         */
        public static PlaceholderFragment newInstance(int sectionNumber) {
            PlaceholderFragment fragment = new PlaceholderFragment();
            Bundle args = new Bundle();
            args.putInt(ARG_SECTION_NUMBER, sectionNumber);
            fragment.setArguments(args);
            return fragment;
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                                 Bundle savedInstanceState) {

            if(getArguments().getInt(ARG_SECTION_NUMBER)==1)
            {
                View rootView = inflater.inflate(R.layout.fragment_sub_page01, container, false);
                return rootView;
            }
            else if(getArguments().getInt(ARG_SECTION_NUMBER)==2)
            {
                View rootView = inflater.inflate(R.layout.fragment_sub_page02, container, false);
                return rootView;
            }
            else
            {
                View rootView = inflater.inflate(R.layout.fragment_customer, container, false);
                TextView textView = (TextView) rootView.findViewById(R.id.section_label);
                textView.setText(getString(R.string.section_format, getArguments().getInt(ARG_SECTION_NUMBER)));
                return rootView;
            }

        }
    }

    /**
     * A {@link FragmentPagerAdapter} that returns a fragment corresponding to
     * one of the sections/tabs/pages.
     */
    public class SectionsPagerAdapter extends FragmentPagerAdapter {

        public SectionsPagerAdapter(FragmentManager fm) {
            super(fm);
        }

        @Override
        public Fragment getItem(int position) {
            // getItem is called to instantiate the fragment for the given page.
            // Return a PlaceholderFragment (defined as a static inner class below).
            return PlaceholderFragment.newInstance(position + 1);
        }

        @Override
        public int getCount() {
            // Show 3 total pages.
            return 2;
        }

        @Override
        public CharSequence getPageTitle(int position) {
            switch (position) {
                case 0:
                    return "Add Customer";
                case 1:
                    return "View Customer";
            }
            return null;
        }
    }
}

这是XML

<RelativeLayout 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="layout.SubPage01"
android:textAlignment="center">

<!-- TODO: Update blank fragment layout -->

<EditText
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:inputType="textPersonName"
    android:ems="10"
    android:id="@+id/firstEntry"
    android:hint="First Name"
    android:textAlignment="center" />

<EditText
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:inputType="textPersonName"
    android:ems="10"
    android:id="@+id/addressEntry"
    android:hint="Address"
    android:textAlignment="center"
    android:layout_below="@+id/surnameEntry"
    android:layout_alignParentStart="true" />

<EditText
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:inputType="textPersonName"
    android:ems="10"
    android:id="@+id/surnameEntry"
    android:hint="Surname"
    android:textAlignment="center"
    android:layout_below="@+id/firstEntry"
    android:layout_alignParentStart="true" />

<EditText
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:inputType="textPersonName"
    android:ems="10"
    android:id="@+id/contactEntry"
    android:hint="Contact"
    android:textAlignment="center"
    android:layout_below="@+id/addressEntry"
    android:layout_alignParentStart="true" />

<Button
    android:text="Add Customer"
    android:layout_width="match_parent"
    android:layout_height="wrap_content"
    android:id="@+id/addBtn"
    android:layout_marginBottom="81dp"
    android:layout_alignParentBottom="true"
    android:layout_alignParentStart="true" />

我猜这个错误来自AddData方法然而，我看不出有什么问题。或者我离开了很远。任何帮助都会很棒。

Answer 1

当您调用方法addDataBtn时，您的异常意味着null对象为public void AddData();

注意这一行：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.view.View.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
                  at com.example.user.planner.Customer.AddData(Customer.java:68)