java - 在 NavigationView 中获取 NullPointerException

Question

我正在尝试制作一个抽屉菜单，但我有 NullPointer，但我认为一切都是正确的。

我将在这里发布我的所有代码:

package wagner.com.meuartesanato.home;

import android.content.Intent;
import android.net.Uri;
import android.support.annotation.NonNull;
import android.support.design.widget.NavigationView;
import android.support.v4.app.Fragment;
import android.support.v4.app.FragmentManager;
import android.support.v4.view.GravityCompat;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.MenuItem;
import android.view.View;
import android.widget.Button;
import android.widget.FrameLayout;
import android.widget.ImageView;
import android.widget.TextView;
import android.widget.Toast;

import wagner.com.meuartesanato.R;

public class MainActivity extends AppCompatActivity implements NavigationView.OnNavigationItemSelectedListener {

    private DrawerLayout drawer;

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

        showHome(null);

        TextView textMenu = (TextView) findViewById(R.id.text_menu);
        TextView textHome = (TextView) findViewById(R.id.text_home);
        TextView textSimulation = (TextView) findViewById(R.id.text_simulation);

        drawer = (DrawerLayout) findViewById(R.id.drawer_layout);

        loadDrawer();
    }

    public void showMenu(View v){
        //drawer.openDrawer(GravityCompat.END);
    }

    public void showHome(View v){
        //loadFragment(Home1Fragment.class);
    }

    public void showSimulation(View v){
        Toast.makeText(this,"Mostrar simulação",Toast.LENGTH_SHORT).show();
    }

    private void loadDrawer(){

        NavigationView navigationView = (NavigationView) findViewById(R.id.nav_view);
        navigationView.setNavigationItemSelectedListener(this);
        View view = navigationView.getHeaderView(0);


        Button about = (Button)view.findViewById(R.id.button_about);
        Button howToUse = (Button)view.findViewById(R.id.button_how_to_use);
        Button addAccount = (Button)view.findViewById(R.id.button_add_account);
        Button changeRegister = (Button)view.findViewById(R.id.button_change_register);
        Button changePassword = (Button)view.findViewById(R.id.button_change_password);
        Button help = (Button)view.findViewById(R.id.button_help);
        Button exit = (Button)view.findViewById(R.id.button_exit);

        about.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //Mostra tela sobre o app
                //loadFragment(SobreFragment.class);
                drawer.closeDrawer(GravityCompat.END);
            }
        });

        howToUse.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(MainActivity.this,"Mostrar como usar",Toast.LENGTH_SHORT).show();
                drawer.closeDrawer(GravityCompat.END);
            }
        });


        addAccount.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(MainActivity.this,"Mostrar algo",Toast.LENGTH_SHORT).show();
                drawer.closeDrawer(GravityCompat.END);

            }
        });


        changeRegister.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(MainActivity.this,"Alterar cadastro",Toast.LENGTH_SHORT).show();
                drawer.closeDrawer(GravityCompat.END);
            }
        });


        changePassword.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(MainActivity.this,"Alterar cadastro",Toast.LENGTH_SHORT).show();
                drawer.closeDrawer(GravityCompat.END);
            }
        });


        help.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Toast.makeText(MainActivity.this,"Help",Toast.LENGTH_SHORT).show();
                drawer.closeDrawer(GravityCompat.END);
            }
        });


        exit.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                //fecha a activity, voltando pra tela de login
                finish();

            }
        });

    }

    private void loadFragment(Class fragmentClass){
        try {
            Fragment fragment = (Fragment) fragmentClass.newInstance();
            FragmentManager fragmentManager = getSupportFragmentManager();
            fragmentManager.beginTransaction().replace(R.id.fragment_holder, fragment).commit();
        }catch (Exception e) {
            e.printStackTrace();
        }
    }

    @Override
    public boolean onNavigationItemSelected(@NonNull MenuItem item) {
        drawer.closeDrawer(GravityCompat.END);
        return false;
    }

    @Override
    public void onBackPressed() {
        if (drawer.isDrawerOpen(GravityCompat.END)) {
            drawer.closeDrawer(GravityCompat.END);
        } else {
            drawer.openDrawer(GravityCompat.END);
        }
    }
}

还有我的 xml 文件:

activity_home.xml

<?xml version="1.0" encoding="utf-8"?>
<android.support.v4.widget.DrawerLayout 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:id="@+id/drawer_layout"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:openDrawer="start">

    <include
        layout="@layout/activity_main"
        android:layout_width="match_parent"
        android:layout_height="match_parent" />

    <android.support.design.widget.NavigationView
        android:id="@+id/nav_view"
        android:layout_width="wrap_content"
        android:layout_height="match_parent"
        android:layout_gravity="end"
        app:headerLayout="@layout/nav_header_main"
        android:fitsSystemWindows="true"
        android:background="@color/colorWhite"/>

</android.support.v4.widget.DrawerLayout>

activity_main.xml

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:id="@+id/activity_main"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingTop="10dp"
    android:background="@drawable/degrade"
    tools:context="wagner.com.meuartesanato.home.MainActivity">

    <LinearLayout
        android:baselineAligned="false"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:id="@+id/top_bar">

        <LinearLayout
            android:onClick="showMenu"
            android:layout_width="0dp"
            android:layout_weight="3"
            android:gravity="center_horizontal"
            android:orientation="vertical"
            android:layout_gravity="center_vertical"
            android:layout_height="wrap_content">
            <ImageView
                android:layout_width="60dp"
                android:layout_height="60dp"
                android:src="@mipmap/menu"/>
            <TextView
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:id="@+id/text_menu"
                android:layout_marginTop="5dp"
                android:textSize="12sp"
                android:textColor="#FFF"
                android:text="Menu"/>
        </LinearLayout>

        <LinearLayout
            android:layout_width="0dp"
            android:layout_height="wrap_content"
            android:layout_weight="3"
            android:onClick="showHome"
            android:gravity="center_horizontal"
            android:orientation="vertical"
            android:layout_gravity="center_vertical">
            <ImageView
                android:layout_width="60dp"
                android:layout_height="60dp"
                android:src="@mipmap/home"/>
            <TextView
                android:id="@+id/text_home"
                android:layout_marginTop="5dp"
                android:textSize="12sp"
                android:layout_width="wrap_content"
                android:layout_height="wrap_content"
                android:textColor="#FFF"
                android:text="Home"
                />
        </LinearLayout>

        <LinearLayout
            android:onClick="showSimulation"
            android:layout_width="0dp"
            android:layout_weight="3"
            android:gravity="center_horizontal"
            android:orientation="vertical"
            android:layout_gravity="center_vertical"
            android:layout_height="wrap_content">
            <ImageView
                android:layout_width="60dp"
                android:layout_height="60dp"
                android:src="@mipmap/simulacao"
                />
            <TextView
                android:id="@+id/text_simulation"
                android:layout_marginTop="5dp"
                android:layout_width="wrap_content"
                android:textSize="12sp"
                android:layout_height="wrap_content"
                android:textColor="#FFF"
                android:text="Simulação"
                />
        </LinearLayout>
    </LinearLayout>

    <LinearLayout
        android:layout_width="match_parent"
        android:layout_height="match_parent"
        android:orientation="vertical"
        android:layout_below="@id/top_bar">
        <View
            android:layout_width="match_parent"
            android:layout_height="10dp"
            android:background="@drawable/shadow"/>

        <FrameLayout
            android:id="@+id/fragment_holder"
            android:layout_width="match_parent"
            android:layout_height="match_parent"
            android:background="#FFF"
            android:layout_marginBottom="@dimen/activity_vertical_margin">
        </FrameLayout>

    </LinearLayout>
</RelativeLayout>

nav_header_main.xml

<?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:orientation="vertical"
    android:background="#FFF"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:gravity="center_horizontal">

    <ImageView
        android:id="@+id/drawer_avatar"
        android:layout_marginTop="20dp"
        android:layout_width="115dp"
        android:layout_height="115dp" />
    <TextView
        android:id="@+id/full_name"
        android:layout_width="180dp"
        android:gravity="center"
        android:textColor="@color/colorDarkGray"
        android:layout_height="wrap_content"
        android:textSize="20sp"
        android:layout_marginBottom="20dp"
        android:layout_marginTop="10dp"
        />

    <Button
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@color/colorLightGray"
        android:text="Sobre o App"
        android:id="@+id/button_about"
        android:textColor="@color/colorPurple"
        android:textSize="@dimen/normal_text"
        android:textAllCaps="false"
        />


    <Button
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@color/colorWhite"
        android:text="Como usar o App"
        android:id="@+id/button_how_to_use"
        android:textColor="@color/colorPurple"
        android:textSize="@dimen/normal_text"
        android:textAllCaps="false"
        />


    <Button
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@color/colorLightGray"
        android:text="Adicionar Conta Bancaria?"
        android:id="@+id/button_add_account"
        android:textColor="@color/colorPurple"
        android:textSize="@dimen/normal_text"
        android:textAllCaps="false"
        />


    <Button
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@color/colorWhite"
        android:text="Alterar Cadastro?"
        android:id="@+id/button_change_register"
        android:textColor="@color/colorPurple"
        android:textSize="@dimen/normal_text"
        android:textAllCaps="false"
        />

    <Button
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@color/colorLightGray"
        android:text="Alterar Senha?"
        android:id="@+id/button_change_password"
        android:textColor="@color/colorPurple"
        android:textSize="@dimen/normal_text"
        android:textAllCaps="false"
        />



    <Button
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@color/colorWhite"
        android:text="Ajuda"
        android:id="@+id/button_help"
        android:textColor="@color/colorPurple"
        android:textSize="@dimen/normal_text"
        android:textAllCaps="false"
        />
    <Button
        android:layout_width="match_parent"
        android:layout_height="50dp"
        android:background="@color/colorLightGray"
        android:text="Sair"
        android:id="@+id/button_exit"
        android:textColor="@color/colorPurple"
        android:textSize="@dimen/normal_text"
        android:textAllCaps="false"
        />

    <TextView
        android:layout_marginTop="50dp"
        android:layout_marginBottom="20dp"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Copyright"
        android:textColor="@color/colorGray"
        />

</LinearLayout>

错误日志是:

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: wagner.com.meuartesanato, PID: 9112
    java.lang.RuntimeException: Unable to start activity ComponentInfo{wagner.com.meuartesanato/wagner.com.meuartesanato.home.MainActivity}:
    java.lang.NullPointerException: Attempt to invoke virtual method 'void
    android.support.design.widget.NavigationView.setNavigationItemSelectedListener(android.support.design.widget.NavigationView$OnNavigationItemSelectedListener)'
    on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
    at android.app.ActivityThread.-wrap11(ActivityThread.java)
    at android.app.ActivityThread$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$MethodAndArgsCaller.run(ZygoteInit.java:726)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
    Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void
    android.support.design.widget.NavigationView.setNavigationItemSelectedListener(android.support.design.widget.NavigationView$OnNavigationItemSelectedListener)'
    on a null object reference
    at wagner.com.meuartesanato.home.MainActivity.loadDrawer(MainActivity.java:59)
    at wagner.com.meuartesanato.home.MainActivity.onCreate(MainActivity.java:41)
    at android.app.Activity.performCreate(Activity.java:6237)
    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)

我真的不知道为什么会出现空指针异常。

谢谢您的解答!

Answer 1

堆栈跟踪清楚地表明了缺少的内容。通过仔细检查代码，您实际上可以发现编程错误。无论如何，请修改 onCreate 方法中的以下行。您夸大了错误的布局

现在是什么 - setContentView(R.layout.activity_main);

您应该将其更改为 - setContentView(R.layout.activity_home);