为什么我的程序打不开？代码说没有错误，但是这两个类不会一起打开

Question

这两个 classes 来回发送方法和变量，我的代码说没有错误，但是程序打不开，我不明白为什么

这是第一个class，TopLevelActivity

package com.example.oife;

import android.media.Image;
import android.app.Activity;
import android.os.Bundle;
import android.view.LayoutInflater;
import android.view.Menu;
import android.view.MenuItem;
import android.view.ViewGroup;
import android.widget.Button;
import android.widget.ImageButton;
import android.widget.ShareActionProvider;
import android.widget.TextView;
import android.content.Intent;
import android.view.View;

import java.text.SimpleDateFormat;

public class TopLevelActivity extends Activity {

    TextView mText = (TextView) findViewById(R.id.calendarView);


    private ShareActionProvider shareActionProvider;

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

        long date = System.currentTimeMillis();

        TextView displayDate = (TextView) findViewById(R.id.displayDate);

        SimpleDateFormat sdf = new SimpleDateFormat("MMM dd, yyyy");
        String dateString = sdf.format(date);
        displayDate.setText(dateString);
    }
   public void onPostClick (View view) {

        Intent intent = new Intent(this, PostActivity.class);

        startActivity(intent);
    }

    //call onHomeClick() method when the button is clicked
    public void onHomeClick (View view){

        Intent intent = new Intent(this,TopLevelActivity.class);
        startActivity(intent);
    }

    //call onMoreClick() method when the button is clicked
    public void onMoreClick(View view){

        Intent intent = new Intent(this, MoreActivity.class);
        startActivity(intent);
    }



    public TextView getMainText(){
        return mText;
    }

    public void setMText(TextView t){
        mText.setText(t.getText());
    }

}

这是第二个class，PostActivity

这个 class 是访问另一个的大部分功能的地方

package com.example.oife;

import java.util.Calendar;
import android.app.Activity;
import android.app.DatePickerDialog;
import android.app.Dialog;
import android.os.Bundle;
import android.widget.Button;
import android.widget.DatePicker;
import android.widget.RadioGroup;
import android.widget.Toast;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.TextView;
import android.content.Intent;
import android.view.View;

public class PostActivity extends Activity {

    private TextView pDisplayDate;
    private Button pPickDate;
    private int pYear;
    private int pMonth;
    private int pDay;
    private TextView mainText;
    static final int DATE_DIALOG_ID = 0;
    private TextView name = (TextView) findViewById(R.id.name);
    private TextView day;

    TopLevelActivity top = new TopLevelActivity();




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

        /** Capture our View elements */
        pDisplayDate = (TextView) findViewById(R.id.displayDate);
        pPickDate = (Button) findViewById(R.id.pickDate);

        /** Listener for click event of the button */
        pPickDate.setOnClickListener(new View.OnClickListener() {
            public void onClick(View v) {
                showDialog(DATE_DIALOG_ID);
            }
        });

        /** Get the current date */
        final Calendar cal = Calendar.getInstance();
        pYear = cal.get(Calendar.YEAR);
        pMonth = cal.get(Calendar.MONTH);
        pDay = cal.get(Calendar.DAY_OF_MONTH);

        /** Display the current date in the TextView */
       // updateDisplay();

    }

   private void setMainText(){
        TextView d = (TextView) findViewById(R.id.description);

        mainText.setText(top.getMainText() + "/n/n" + name + "/n" + makeDay().getText() + "/n" + d.getText());

    }

    private TextView makeDay(){
        TextView ToD;
        RadioGroup t = (RadioGroup) findViewById(R.id.TOD);
        int id = t.getCheckedRadioButtonId();
        if(id == -1) {
            ToD = null;
        }
        else{
         ToD = (TextView) findViewById(id);
        }

        TextView wow;
        RadioGroup g = (RadioGroup) findViewById(R.id.WoW);
        id = g.getCheckedRadioButtonId();
        if (id == -1){
            //no item selected
        }
        else{
            wow = (TextView) findViewById(id);
        }

        TextView d = (TextView) findViewById(R.id.displayDate);

        String s = String.valueOf(d);
        String j;
        if (ToD.getText().equals("Morning")){
            j = "9:00";
        }
        else if(ToD.getText().equals("Midday")){
            j = "12:00";
        }
        else if (ToD.getText().equals("Evening")){
            j = "5:00";
        }
        else {
            j = "10:00";
        }

        TextView time = new TextView(this);

        time.setText( j + " " + s);

        return  time;

    }

    private DatePickerDialog.OnDateSetListener pDateSetListener =
            new DatePickerDialog.OnDateSetListener() {

                public void onDateSet(DatePicker view, int year,
                                      int monthOfYear, int dayOfMonth) {
                    pYear = year;
                    pMonth = monthOfYear;
                    pDay = dayOfMonth;
                    updateDisplay();
                    displayToast();
                }
            };

    private void updateDisplay() {
        pDisplayDate.setText(
                new StringBuilder()
                        // Month is 0 based so add 1
                        .append(pMonth + 1).append("/")
                        .append(pDay).append("/")
                        .append(pYear).append(" "));
    }

    private void updateMainDisplay(){

        setMainText();

        top.setMText(mainText);

    }

    private void displayToast() {
        Toast.makeText(this, new StringBuilder().append("Date choosen is ").append(pDisplayDate.getText()), Toast.LENGTH_SHORT).show();}



    /** Create a new dialog for date picker */
    @Override
    protected Dialog onCreateDialog(int id) {
        switch (id) {
            case DATE_DIALOG_ID:
                return new DatePickerDialog(this,
                        pDateSetListener,
                        pYear, pMonth, pDay);
        }
        return null;
    }


    @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_post, 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);
    }


    public void onCreateClick(View view){

        Intent intent = new Intent(this, TopLevelActivity.class);
        updateMainDisplay();
        startActivity(intent);

    }


}

Answer 1

在调用 setContentView:

之后调用这样的代码

Activity 1:

TextView mText = (TextView) findViewById(R.id.calendarView);

Activity 2:

private TextView name = (TextView) findViewById(R.id.name);

你在两个方面都做错了 Activitys。您在 Activitys 的成员部分调用 findViewById，此时 Activity 对布局文件一无所知。

那一定会导致 NullpointerException。

为什么我的程序打不开？代码说没有错误，但是这两个类不会一起打开

Why won't my program open? The code says there are no errors, but these two classes won't open together

java

android

runtime-error

function

android-activity

这个 class 是访问另一个的大部分功能的地方