如何制作一个打开另一个活动的按钮

Question

I've created a activity called Activity1 and I want to have a button in its layout to launch the MainActivity. But when I do it, it says cannot resolve setOnClickListener. Here is my Activity1:

package br.raphael.detector;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.widget.Button;

public class Activity1 extends Activity {



    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_1);
    }

    Button Bdetectar = (Button) findViewById(R.id.buttondetectar);
    Bdetectar.setOnClickListener(new OnClickListener(){

        public void onClick(View v){

            Intent intent = new Intent (v.getContext(), MainActivity.class);
            startActivityForResult(intent, 0);
        }
    });

}

The beggining of the MainActivity:

package br.raphael.detector;

import java.io.File;
import java.io.IOException;
import java.text.SimpleDateFormat;
import java.util.ArrayList;
import java.util.Date;
import java.util.List;
import java.util.ListIterator;
import java.util.Set;

import org.opencv.android.OpenCVLoader;
import org.opencv.android.Utils;
import org.opencv.core.Core;
import org.opencv.core.Mat;
import org.opencv.core.Point;
import org.opencv.core.Scalar;
import org.opencv.imgproc.Imgproc;

import android.R.integer;
import android.annotation.SuppressLint;
import android.app.Activity;
import android.graphics.Bitmap;
import android.graphics.Bitmap.Config;
import android.graphics.BitmapFactory;
import android.graphics.BitmapFactory.Options;
import android.hardware.Camera.Size;
import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;
import android.view.MotionEvent;
import android.view.SubMenu;
import android.view.SurfaceView;
import android.view.View;
import android.view.View.OnClickListener;
import android.view.View.OnTouchListener;
import android.view.WindowManager;
import android.widget.AdapterView;
import android.widget.AdapterView.OnItemClickListener;
import android.widget.Button;
import android.widget.LinearLayout;
import android.widget.ListView;
import android.widget.TextView;
import android.widget.Toast;
import br.raphael.extended.AdapterTwoLinesImage;
import br.raphael.extended.ViewExtended;
import br.raphael.extended.CameraBridgeViewBaseExtended.CvCameraViewFrame;
import br.raphael.extended.CameraBridgeViewBaseExtended.CvCameraViewListener2;


public class MainActivity extends Activity implements CvCameraViewListener2, OnTouchListener, OnClickListener, OnItemClickListener {


    //Variaveis Basicas - Basic Variables
    private static final String TAG = "Main Activity: ";
    private ViewExtended CameraView;
    private Handler mainHandler;
    //Loading para FullDetection
    private LinearLayout Loading;
    //Botoes - Buttons
    private Button FullDetection;
    private Button CleanList;
    //Menu de Opcoes - Options Menu
    private List<Size> mResolutionList;
    private MenuItem[] mEffectMenuItems;
    private SubMenu mColorEffectsMenu;
    private MenuItem[] mResolutionMenuItems;
    private SubMenu mResolutionMenu;
    private SubMenu Mode;
    private MenuItem[] ModeItens;
    private SubMenu Debug;
    private MenuItem[] DebugItens;
    //Personagens(Pre-Carregamento) - Characters(Pre-Load)
    private SimpsonDetector previousDetector;
    private Mat homer;
    private Mat homer2;
    private Mat marge;
    private List<Mat> preLoadedImages;
    private boolean[] DetectedChars;
    //Controle de Thread
    private boolean[] threadControl;
    //Outros
    //Others
    private TextView Status;
    private String ModeValue;
    private int teste1 = 1, teste2 = 1;
    private boolean debug;
    //Lista de Objetos
    //Object List
    private ListView DetectedObj;
    private List<String> Objetos;
    private ArrayList<ObjectList> personal;
    private boolean[] objControl;



    public MainActivity() {
        Log.i(TAG, "Instantiated new " + this.getClass());
    }

    /**
     * Chamado quando a Activity  criada.
     * Called when activity is created
     * */
    @Override
    public void onCreate(Bundle savedInstanceState) {
        Log.i(TAG, "called onCreate");
        super.onCreate(savedInstanceState);
        getWindow().addFlags(WindowManager.LayoutParams.FLAG_KEEP_SCREEN_ON);

        setContentView(R.layout.activity_main);

        CameraView = (ViewExtended) findViewById(R.id.ExtendedSurfaceView);

        CameraView.setVisibility(SurfaceView.VISIBLE);

        CameraView.setCvCameraViewListener(this);

        Loading = (LinearLayout) findViewById(R.id.Loading);
        Loading.setVisibility(View.GONE);

        Status = (TextView) findViewById(R.id.text_status);
        Status.setOnClickListener(this);

        FullDetection = (Button) findViewById(R.id.fullDetection);
        FullDetection.setOnClickListener(this);

        CleanList = (Button) findViewById(R.id.CleanList);
        CleanList.setOnClickListener(this);

        DetectedObj = (ListView) findViewById(R.id.DetectedObjects);
        DetectedObj.setOnItemClickListener(this);
        personal = new ArrayList<ObjectList>();
        objControl = new boolean[30];

        debug = false;

And the AndroidManifest:

<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="br.raphael.detector"
    android:versionCode="1"
    android:versionName="0.1" >

    <uses-sdk
        android:minSdkVersion="15"
        android:targetSdkVersion="21" />

    <uses-permission android:name="android.permission.CAMERA" />

    <uses-feature
        android:name="android.hardware.camera"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.autofocus"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.front"
        android:required="false" />
    <uses-feature
        android:name="android.hardware.camera.front.autofocus"
        android:required="false" />

    <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

    <android:uses-permission android:name="android.permission.READ_PHONE_STATE" />
    <android:uses-permission android:name="android.permission.READ_EXTERNAL_STORAGE" />

    <application
        android:allowBackup="true"
        android:hardwareAccelerated="true"
        android:icon="@drawable/ic_donuts"
        android:label="@string/app_name"
        android:largeHeap="true"
        android:theme="@android:style/Theme.Holo.NoActionBar.Fullscreen" >
        <activity
            android:name=".Activity1"
            android:label="@string/title_activity_1" >
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
        <activity
            android:name=".MainActivity"
            android:label="@string/app_name"
            android:screenOrientation="landscape" >
        </activity>
    </application>

</manifest>

Answer 1

You closed the onCreate() too early, so you're out of the scope of the function.
Move the button declartion and setOnClickListener into onCreate() and it will work.

package br.raphael.detector;

import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.widget.Button;

public class Activity1 extends Activity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_1);

        Button Bdetectar = (Button) findViewById(R.id.buttondetectar);
        Bdetectar.setOnClickListener(new OnClickListener() {
            public void onClick(View v){

                Intent intent = new Intent (v.getContext(), MainActivity.class);
                startActivityForResult(intent, 0);
            }
        });
    }
}

Answer 2

Instead of setOnClickListener set in the xml of your Button:

android:onClick="onClick"

like:

<Button android:id="@+id/mybutton"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Click me!"
        android:onClick="onClick" />

Now you can simply use the public void onClick(View v) in your Activity without the Listener.