Question

我有一个导航抽屉，当我点击其中的一个项目时，它应该打开另一个活动。所以我做了一个开关，并没有得到任何错误。仍然当我启动应用程序时它仍然崩溃。需要任何帮助我能得到!!! Java在下面

package com.gfo.enexis;

import android.app.AlertDialog;
import android.content.DialogInterface;
import android.content.Intent;
import android.support.annotation.IdRes;
import android.support.design.widget.NavigationView;
import android.support.v4.widget.DrawerLayout;
import android.support.v7.app.ActionBarDrawerToggle;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.support.v7.widget.Toolbar;
import android.util.Log;
import android.view.KeyEvent;
import android.view.MenuItem;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;

import org.w3c.dom.Text;

import java.util.ArrayList;
import java.util.List;

import static android.R.attr.id;
import static android.R.attr.start;
import static android.R.id.edit;
import static android.R.id.list;
import static com.gfo.enexis.R.id.button;
import static com.gfo.enexis.R.id.lv1;
import static com.gfo.enexis.R.id.nav_acount;

public class MainActivity extends AppCompatActivity {

private DrawerLayout mDrawerLayout;
private ActionBarDrawerToggle mToggle;

private Toolbar mToolbar;

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

    mToolbar = (Toolbar) findViewById(R.id.nav_action);
    setSupportActionBar(mToolbar);

    mDrawerLayout = (DrawerLayout) findViewById(R.id.drawerLayout);
    mToggle = new ActionBarDrawerToggle(this, mDrawerLayout, R.string.open, R.string.close);

    mDrawerLayout.addDrawerListener(mToggle);
    mToggle.syncState();

    getSupportActionBar().setDisplayHomeAsUpEnabled(true);

    registerClickCallBack();
    OnNavigationItemSelected();
}

@Override
public boolean onOptionsItemSelected(MenuItem item) {

    if (mToggle.onOptionsItemSelected(item)) {
        return true;
    }
    return super.onOptionsItemSelected(item);
}


public boolean onKeyDown(int keyCode, KeyEvent event) {
    if (keyCode == KeyEvent.KEYCODE_BACK) {
        exitByBackKey();

        return true;
    }
    return super.onKeyDown(keyCode, event);
}

protected void exitByBackKey() {

    AlertDialog alertbox = new AlertDialog.Builder(this)
            .setMessage("Weet u zeker dat u wilt uitloggen?")
            .setPositiveButton("Log uit", new DialogInterface.OnClickListener() {

                // do something when the button is clicked
                public void onClick(DialogInterface arg0, int arg1) {

                    finish();
                    //close();


                }
            })
            .setNegativeButton("Nee", new DialogInterface.OnClickListener() {

                // do something when the button is clicked
                public void onClick(DialogInterface arg0, int arg1) {
                }
            })
            .show();

}

EditText NotitieEt;
ListView ListNote;
ArrayList<String> lv = new ArrayList<>();


public void NoteAdd(View view) {
    ListNote = (ListView) findViewById(lv1);
    NotitieEt = (EditText) findViewById(R.id.notitie);

    String note1 = NotitieEt.getText().toString();
    if (lv.contains(note1)) {
        Toast.makeText(this, "Notitie bestaat al", Toast.LENGTH_LONG).show();
    } else if (note1 == null || note1.trim().equals("")) {
        Toast.makeText(this, "Imput can't be empty", Toast.LENGTH_LONG).show();
    } else {
        lv.add(note1);
        ArrayAdapter<String> adapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, lv);
        ListNote.setAdapter(adapter);
        ((EditText) findViewById(R.id.notitie)).setText("");
    }


}

private void registerClickCallBack() {
    ListNote = (ListView) findViewById(lv1);
    ListNote.setOnItemClickListener(new AdapterView.OnItemClickListener() {
        @Override
        public void onItemClick(AdapterView<?> paret, View viewClicked, int position, long id) {
            lv.remove(position);
            ArrayAdapter<String> adapter = new ArrayAdapter<>(MainActivity.this, android.R.layout.simple_list_item_1, lv);
            ListNote.setAdapter(adapter);
        }
    });
}

MenuItem item;


public boolean OnNavigationItemSelected() {
    int id = item.getItemId();

    switch (id) {

        case R.id.nav_acount:
            Intent i = new Intent(MainActivity.this, DashBoard1.class);
            startActivity(i);
            break;
        case R.id.nav_dashboard:
            Intent o = new Intent(MainActivity.this, Dashboard2.class);
            startActivity(o);
    }
    return true;
}

}

这些是我得到的错误：

java.lang.RuntimeException：无法启动活动ComponentInfo {com.gfo.enexis / com.gfo.enexis.MainActivity}：java.lang.NullPointerException：尝试调用接口方法'int android.view.MenuItem.getItemId （）'在空对象引用

上

和

引起：java.lang.NullPointerException：尝试在空对象引用上调用接口方法'int android.view.MenuItem.getItemId（）'

Answer 1

我认为您的问题是您尚未在onCreate（）中注册导航抽屉。使用此：

NavigationView navigationView = (NavigationView) findViewById(R.id.yourIdHere);
navigationView.setNavigationItemSelectedListener(this);

此外，要处理导航抽屉点击，请覆盖方法：

onNavigationItemSelected(MenuItem item)

Answer 2

您没有初始化

MenuItem item;

所以它是NULL！我认为你正在实施＆＃34; OnNavigationItemSelected＆＃34;以错误的方式。你为什么要打电话＆＃34; OnNavigationItemSelected（）;＆＃34; in＆＃34; onCreate（）＆＃34;方法

只需按照官方教程，您就可以了：

https://developer.android.com/training/implementing-navigation/nav-drawer.html

单击导航栏中的项目时打开其他活动

2 个答案: