java - 切换 Activity Android Studio

Question

我厌倦了在 Android Studio 中尝试切换 Activity ，我知道这个问题可能太重复了，但是请帮助我，在我开始我的项目后，出现很多错误

，这是我的MainActivity

package com.example.satan.custommm;

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

public class MainActivity extends AppCompatActivity {

    private Button btt1;
    private EditText et1;
    private EditText et2;
    private TextView tv1;
    private TextView tv2;
    private TextView tv4;



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


        btt1 = (Button) findViewById(R.id.button);
        et1 = (EditText) findViewById(R.id.editText);
        et2 = (EditText) findViewById(R.id.editText2);
        tv1 = (TextView) findViewById(R.id.textView);
        tv2 = (TextView) findViewById(R.id.textView2);
        tv4 = (TextView) findViewById(R.id.textView4);

    }
    public void run() {


        btt1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {


                if (et1.getText().toString().equals("admin") && et2.getText().toString().equals("admin")) {

                    Intent i = new Intent(MainActivity.this, Main2Activity.class);
                    startActivity(i);



                } else {
                    tv4.setText("Wrong Username/Password!");

                }


            }
        });

    }

    @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);

    }
}

这是我的manifest.xml

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

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:theme="@style/AppTheme" >
        <activity
            android:name=".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=".Main2Activity"
            android:label="@string/app_name" >
        </activity>
    </application>

</manifest>

启动项目后，我收到此错误:

09-02 21:09:05.998  26875-26875/? I/art﹕ Not late-enabling -Xcheck:jni (already on)
09-02 21:09:06.049  26875-26882/? E/art﹕ Failed sending reply to debugger: Broken pipe
09-02 21:09:06.049  26875-26882/? I/art﹕ Debugger is no longer active
09-02 21:09:06.205  26875-26887/? I/art﹕ Background sticky concurrent mark sweep GC freed 2736(226KB) AllocSpace objects, 0(0B) LOS objects, 0% free, 13MB/13MB, paused 11.436ms total 32.643ms
09-02 21:09:06.339  26875-26875/? D/AndroidRuntime﹕ Shutting down VM
09-02 21:09:06.339  26875-26875/? E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.satan.custommm, PID: 26875
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.satan.custommm/com.example.satan.custommm.MainActivity}: 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:2325)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
            at com.example.satan.custommm.MainActivity.run(MainActivity.java:42)
            at com.example.satan.custommm.MainActivity.onCreate(MainActivity.java:28)
            at android.app.Activity.performCreate(Activity.java:5990)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1106)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2278)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2390)
            at android.app.ActivityThread.access$800(ActivityThread.java:151)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1303)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5257)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:903)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:698)

抱歉我的英语不好

Answer 1

你有一个java.lang.NullPointerException，因为你正在调用

btt1.setOnClickListener

在初始化btt1对象之前。然后，您将调用 null obj 上的方法 (btt1 = null)。

更改您的onCreate方法

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

        btt1 = (Button) findViewById(R.id.button);
        et1 = (EditText) findViewById(R.id.editText);
        et2 = (EditText) findViewById(R.id.editText2);
        tv1 = (TextView) findViewById(R.id.textView);

        tv2 = (TextView) findViewById(R.id.textView2);
        tv4 = (TextView) findViewById(R.id.textView4);


        // move this line
        run();
    }