Android通过按钮启动活动？

Question

i keep getting a null pointer exception

I have a problem about starting a new activity via using button.

I already checked the previous questions related to mine problem, one of them is almost same but solutions didn't work out for me. So here is my problem.

whenever i start the app it crashes. i have a main class called MainActivity and new Activity called login

Main class

public class MainActivity extends Activity {

    Intent myIntent = new Intent(MainActivity.this, login.class);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button loginButton  = (Button) findViewById(R.id.button_login);


        loginButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {

                startActivity(myIntent);
            }
        });

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.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();
        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 {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
    }

}

logcat

02-18 10:27:42.520    1098-1098/com.example.app E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.app, PID: 1098
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.app/com.example.app.MainActivity}: java.lang.NullPointerException
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2195)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)
     Caused by: java.lang.NullPointerException
            at com.example.app.MainActivity.onCreate(MainActivity.java:30)
            at android.app.Activity.performCreate(Activity.java:5231)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1087)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2159)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2245)
            at android.app.ActivityThread.access$800(ActivityThread.java:135)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1196)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:136)
            at android.app.ActivityThread.main(ActivityThread.java:5017)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:515)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:779)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:595)
            at dalvik.system.NativeStart.main(Native Method)

thanks in advance!!!

EDIT---> MANIFEST

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

    <application
        android:allowBackup="true"
        android:icon="@drawable/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name="com.example.app.MainActivity"
            android:label="@string/app_name" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>

        <activity
            android:name="com.example.app.login"
            android:label="@string/title_activity_login" >
            <intent-filter>
                <action android:name="android.intent.action.LOGIN" />
                <category android:name="android.intent.category.DEFAULT" />
            </intent-filter>
        </activity>
    </application>

</manifest>

MainActivity Layout

<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"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    android:paddingBottom="@dimen/activity_vertical_margin"
    tools:context="com.example.app.MainActivity">

    //login Button
    <Button
        android:layout_width="120dp"
        android:layout_height="wrap_content"
        android:text="Login"
        android:id="@+id/button_login"
        android:layout_marginBottom="160dp"
        android:layout_alignParentBottom="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginLeft="23dp" />

    //high score button
    <Button
        android:layout_width="120dp"
        android:layout_height="wrap_content"
        android:text="high score"
        android:id="@+id/highScore"
        android:layout_alignTop="@+id/button_login"
        android:layout_toRightOf="@+id/button_login"
        android:layout_marginLeft="45dp" />

    //play Button
    <Button
        android:layout_width="150dp"
        android:layout_height="wrap_content"
        android:text="Play"
        android:id="@+id/Play"
        android:layout_above="@+id/button_login"
        android:layout_centerHorizontal="true"
        android:layout_marginBottom="55dp" />
</RelativeLayout>

Edited corrections with imports

package com.example.app;

import android.app.Activity;
import android.content.Intent;
import android.support.v7.app.ActionBarActivity;
import android.support.v4.app.Fragment;
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.ImageButton;

public class MainActivity extends Activity {


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

        Button loginButton  = (Button) findViewById(R.id.button_login);

//        if (savedInstanceState == null) {
//            getSupportFragmentManager().beginTransaction()
//                    .add(R.id.container, new PlaceholderFragment())
//                    .commit();
//        }

        loginButton.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                Intent myIntent = new Intent(MainActivity.this, login.class);
                startActivity(myIntent);
            }
        });

    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.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();
        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 {

        public PlaceholderFragment() {
        }

        @Override
        public View onCreateView(LayoutInflater inflater, ViewGroup container,
                Bundle savedInstanceState) {
            View rootView = inflater.inflate(R.layout.fragment_main, container, false);
            return rootView;
        }
    }

}

Answer 1

Move this inside the onCreate method :

Intent myIntent = new Intent(MainActivity.this, login.class);

By using that line outside the method, you are referring to the instance of MainActivity before it is fully created.

Since it looks like you are only using that intent once, you could even move that line inside the onClick method of the loginButton's OnClickListener, as suggested by InnocentKiller.

Answer 2

Remove this line from top of your screen

Intent myIntent = new Intent(MainActivity.this, login.class);

and put under the button click event. So basically your code will something like this for MainActivity.

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

    Button loginButton  = (Button) findViewById(R.id.button_login);


    loginButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {

    Intent myIntent = new Intent(MainActivity.this, login.class);
            startActivity(myIntent);
        }
    });
............
...........

}

Answer 3

You need an Intent!

try:

loginButton.setOnClickListener(new View.OnClickListener() {
        public void onClick(View v) {
            Intent myIntent = new Intent(MainActivity.this, Login.class);
            startActivity(myIntent);
        }
    });

Your login class should also be named "Login" to serve the name convention

And you need to add the new Login.class to your Manifest like

<activity
        android:name="com.example.yourapp.Login"
        android:label="@string/app_name" 

        <intent-filter>
            <action android:name="android.intent.action.VIEW" />

            <category android:name="android.intent.category.DEFAULT" />
        </intent-filter>
    </activity

Answer 4

Do you have the Login activity declared on your manifest as an activity

  <activity
        android:name="com.examlpe.app.loginActivity"/>