Question

在我的应用程序中，我希望默认情况下音乐将会运行。但是如果点击about.java中的开关那么它应该关闭。所以我做了一个全局变量。但问题是当我点击开关关闭时，它会出错。

MainActivity.java

package com.example.asifsabir.breathholdingchallenge;

import android.content.Intent;
import android.graphics.Color;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.os.CountDownTimer;

import android.support.v7.app.AppCompatActivity;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import java.util.concurrent.TimeUnit;

public class MainActivity extends AppCompatActivity {
    TextView tv1;
    private android.support.v7.widget.Toolbar toolbar;
    EditText et1, et2;
    Button bt1, bt2;
    public int min = 0;
    public int sec = 0;
    public int second = 0;
    public CountDownTimer myC;
    public MediaPlayer mySong;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        toolbar = (android.support.v7.widget.Toolbar) findViewById(R.id.app_bar);
        setSupportActionBar(toolbar);
        toolbar.setTitle("Hold Breath Challenge");
        toolbar.setSubtitleTextColor(R.color.colortitle);
        toolbar.setPopupTheme(R.style.AppTheme);
        toolbar.inflateMenu(R.menu.main_menu);
        toolbar.setTitleTextColor(Color.WHITE);
        tv1 = (TextView) findViewById(R.id.textView1);
        et1 = (EditText) findViewById(R.id.et1);
        et2 = (EditText) findViewById(R.id.et2);
        bt1 = (Button) findViewById(R.id.bt1);
        bt2 = (Button) findViewById(R.id.bt2);
        mySong = MediaPlayer.create(getApplicationContext(), R.raw.heartbeat);
        mySong.setLooping(true);
        MyApplication mAxx = (MyApplication) getApplicationContext();
        if (mAxx.getGlobalVarValue())
            mySong.start();
        else
            mySong.pause();

        Toast.makeText(MainActivity.this, "You must click RESET before stating a new countdown!", Toast.LENGTH_LONG).show();
        bt1.setOnClickListener(
                new View.OnClickListener() {
                    @Override
                    public void onClick(View v) {


                        if ((!et1.getText().toString().isEmpty() && !et2.getText().toString().isEmpty()) ||
                                !et1.getText().toString().isEmpty() ||
                                !et2.getText().toString().isEmpty()) {
                            if (!et1.getText().toString().isEmpty())
                                min = Integer.parseInt(et1.getText().toString());
                            if (et1.getText().toString().isEmpty())
                                min = 0;

                            if (!et2.getText().toString().isEmpty())
                                second = Integer.parseInt(et2.getText().toString());
                            if (et2.getText().toString().isEmpty())
                                second = 0;

                            sec = min * 60 * 1000 + second * 1000;
                            myC = new CountDownTimer(sec, 1000) {


                                public void onTick(long millisUntilFinished) {
                                    tv1.setText("" + String.format("%02d:%02d",
                                            TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished),
                                            TimeUnit.MILLISECONDS.toSeconds(millisUntilFinished) - TimeUnit.MINUTES.toSeconds(
                                                    TimeUnit.MILLISECONDS.toMinutes(millisUntilFinished))));
                                }


                                public void onFinish() {
                                    tv1.setText("done!");
                                }
                            }.start();
                        }
                    }
                }
        );
        bt2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                myC.cancel();
                tv1.setText("Count");
            }
        });
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        getMenuInflater().inflate(R.menu.main_menu, 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.

        switch (item.getItemId()) {
            //noinspection SimplifiableIfStatement
            case R.id.about:
                startActivity(new Intent(getApplicationContext(), About.class));

                return true;


        }

        return super.onOptionsItemSelected(item);
    }

}

MyApplcation.java

package com.example.asifsabir.breathholdingchallenge;

import android.app.Application;

/**
 * Created by Asif Sabir on 29-Apr-16.
 */
public class MyApplication extends Application {
    public static boolean mGlobalVarValue=true;

    public boolean getGlobalVarValue() {
        return mGlobalVarValue;
    }

    public void setGlobalVarValue(boolean str) {
        mGlobalVarValue = str;
    }
}

About.java

package com.example.asifsabir.breathholdingchallenge;

import android.graphics.Color;
import android.media.MediaPlayer;
import android.os.Bundle;
import android.support.v4.app.NavUtils;
import android.support.v7.app.AppCompatActivity;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.CompoundButton;
import android.widget.Switch;
import android.widget.Toast;

/**
 * Created by Asif Sabir on 20-Apr-16.
 */

public class About extends AppCompatActivity {
    android.support.v7.widget.Toolbar t1;
    public static MyApplication myA;
    Switch sw1;
    public MyApplication mApp = (MyApplication) getApplication();

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.about);
        sw1 = (Switch) findViewById(R.id.sw1);
        t1 = (android.support.v7.widget.Toolbar) findViewById(R.id.about_bar);
        t1.setTitleTextColor(Color.WHITE);
        setSupportActionBar(t1);
        getSupportActionBar().setTitle("About");
        getSupportActionBar().setDisplayHomeAsUpEnabled(true);
        getSupportActionBar().setHomeButtonEnabled(true);
        sw1.setOnCheckedChangeListener(new CompoundButton.OnCheckedChangeListener() {
            @Override
            public void onCheckedChanged(CompoundButton buttonView, boolean isChecked) {
                if (isChecked) {
                    mApp.setGlobalVarValue(true);
                } else {
                    mApp.setGlobalVarValue(false);
                }
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {

        return super.onCreateOptionsMenu(menu);
    }

    @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.

        switch (item.getItemId()) {
            //noinspection SimplifiableIfStatement
            case R.id.home:
                NavUtils.navigateUpFromSameTask(this);


                return true;

        }

        return super.onOptionsItemSelected(item);
    }
}

错误信息是：

java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.asifsabir.breathholdingchallenge.MyApplication.setGlobalVarValue(boolean)' on a null object reference
                                                                                                at com.example.asifsabir.breathholdingchallenge.About$1.onCheckedChanged(About.java:41)

Answer 1

如果你的意图（基于你的评论）真的要做到以下几点：

如何在包含布尔值的设置中使用开关，并根据该布尔值更改其他活动？

然后可能的解决方案是：

使用SharedPreferences存储旨在“全局”的布尔值。
创建可用于访问/修改全局变量的设置Activity or Fragment。
您需要访问该值的任何地方，请为更改注册listener。请参阅文档here中的示例。

android全局变量错误

1 个答案: