Java 为Android应用程序编程计数器

我的目标是编程一个计数器应用程序。在智能手机的中间有一张图片，图片下面是从100开始的倒计时。

每次单击图片时，图片应闪烁，计数器应减小，直到单击100次并达到0

我有一点代码，但每次我在VM上运行项目时，应用程序都会崩溃

package com.syntaix.appleclicker;

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.ImageView;
import android.widget.TextView;

public class MainActivity extends ActionBarActivity {

    int counter = 100;

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

        final TextView text = (TextView) findViewById(R.id.TextView01);

        final ImageView image = (ImageView) findViewById(R.id.appel);
        image.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                counter--;
                text.setText("" + counter);
            }
        });


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

    @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;
        }
    }

}

以下是LogCat屏幕截图：

---

创建片段时，您可以在片段中找到视图。但是您正在尝试在将片段附加到活动之前获取视图。您需要将一些代码移动到片段中，如下所示

MainActivity.java

  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    if (savedInstanceState == null) {
        getSupportFragmentManager().beginTransaction()
                .add(R.id.container, new PlaceholderFragment()).commit();
    }
  }

占位符片段.java

  public static class PlaceholderFragment extends Fragment {

    int counter = 100;

    public PlaceholderFragment() {
    }

    @Override
    public View onCreateView(LayoutInflater inflater, ViewGroup container,
                             Bundle savedInstanceState) {
        View rootView = inflater.inflate(R.layout.fragment_main, container,
                false);
        final TextView text = (TextView) rootView.findViewById(R.id.TextView01);

        final ImageView image = (ImageView) rootView.findViewById(R.id.appel);
        image.setOnClickListener(new View.OnClickListener() {

            @Override
            public void onClick(View v) {
                counter--;
                text.setText("" + counter);
            }
        });
        return rootView;
    }
}

---

请发布崩溃期间写入的logcat输出。某个地方应该有异常。我在描述的末尾编辑了onCreate（）中的某个地方有一个NullPointerException。我建议在

image.setOnClickListener（）

行设置断点，并确保

text

和

image

都不为空。事实上，我非常确定

image

为空。您确定图像视图的id在xml布局中是

R.id.appel

吗？问题应该在布局中

占位符片段的用途是什么，在
R.layout中您在哪里定义了
TextView
和
ImageView
。活动\u main
或
R.layout.fragment\u main
？