Question

我创建的程序以“2000年1月7日”格式输入日期时计算星期几。当我点击按钮计算星期几时，为什么会出现NullPointerException异常？

以下是app的源代码：

public class MainActivity extends AppCompatActivity
{
   private int position;
private String value;
private Button button;
private EditText editText;
private TextView textView1,textView2;
private Spinner spinner_month,spinner_day;
private ArrayAdapter<CharSequence> adapter;

private boolean isValid;
private Date date;

private int inMonth, inDay, inYear;

    static boolean isDateValid(int month, int day, int year)
    {
        boolean validation = true;
        int[] daysInMonth = { 31, 28, 31, 30, 31, 30, 31, 31, 30, 31, 30, 31 };

        if (month < 1 || month > 12)
        {
            validation = false;
        }

        if (day < 1 || day > daysInMonth[month - 1])
        {
            validation = false;
        }

        if (year < 1700 || year > 3000)
        {
            validation = false;
        }
        return validation;
    }

    @Override
protected void onCreate(Bundle savedInstanceState)
{
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);

    button = (Button) findViewById(R.id.button_calculate);
    textView1 = (TextView) findViewById(R.id.textView_day);
    textView2 = (TextView) findViewById(R.id.textView_year);
    spinner_month = (Spinner) findViewById(R.id.spinner_month);
    spinner_day = (Spinner) findViewById(R.id.spinner_day);
    editText = (EditText) findViewById(R.id.editText_year);

    adapter = ArrayAdapter.createFromResource(this,R.array.months,android.R.layout.simple_spinner_item);
    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    spinner_month.setAdapter(adapter);

    adapter = ArrayAdapter.createFromResource(this,R.array.day,android.R.layout.simple_spinner_item);
    adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);
    spinner_day.setAdapter(adapter);

    if (spinner_month.getSelectedItemPosition() == 0)
        value = "1";
    if (spinner_month.getSelectedItemPosition() == 1)
        value = "2";
    if (spinner_month.getSelectedItemPosition() == 2)
        value = "3";
    if (spinner_month.getSelectedItemPosition() == 3)
        value = "4";
    if (spinner_month.getSelectedItemPosition() == 4)
        value = "5";
    if (spinner_month.getSelectedItemPosition() == 5)
        value = "6";
    if (spinner_month.getSelectedItemPosition() == 6)
        value = "7";
    if (spinner_month.getSelectedItemPosition() == 7)
        value = "8";
    if (spinner_month.getSelectedItemPosition() == 8)
        value = "9";
    if (spinner_month.getSelectedItemPosition() == 9)
        value = "10";
    if (spinner_month.getSelectedItemPosition() == 10)
        value = "11";
    if (spinner_month.getSelectedItemPosition() == 11)
        value = "12";

    button.setOnClickListener(new View.OnClickListener()
    {
        @Override
        public void onClick(View view)
        {
            String dayOfWeek;
            boolean isValid;
            Calendar c = Calendar.getInstance();
            SimpleDateFormat sdf = new SimpleDateFormat("yyyy/M/d");
            //SimpleDateFormat sdf = new SimpleDateFormat("yyyyMMdd_HHmmss");
            String strDate = sdf.format(c.getTime());
            do
            {
                try
                {
                    String getdate = editText.getText().toString()+ value +
                            spinner_day.getItemAtPosition(position).toString();
                    date = sdf.parse(getdate);
                }
                catch (ParseException ex)
                {
                    Toast.makeText(MainActivity.this,ex.getMessage(),Toast.LENGTH_SHORT);
                }
                isValid = isDateValid(date.getMonth(), date.getDay(), date.getYear());

                if (isValid == false)
                {
                    Toast.makeText(MainActivity.this, "Uneli ste pogresan datum", Toast.LENGTH_SHORT).show();
                }
            }
        while (isValid == false);

            dayOfWeek = zellerCalc(date.getMonth(), date.getDay(), date.getYear());

            textView1.setText(dayOfWeek);
        }
    });
}

logcat的：

02-07 12：32：16.349 3049-3049 / com.example.pavle.dayofborn E / AndroidRuntime：致命异常：主要过程：com.example.pavle.dayofborn，PID：3049 显示java.lang.NullPointerException 在 com.example.pavle.dayofborn.MainActivity $ 2.onClick（MainActivity.java:230）在android.view.View.performClick（View.java:4633）在android.view.View $ PerformClick.run（View.java:19270）在android.os.Handler.handleCallback（Handler.java:733）在android.os.Handler.dispatchMessage（Handler.java:95）在android.os.Looper.loop（Looper.java:136）在android.app.ActivityThread.main（ActivityThread.java:5476） at java.lang.reflect.Method.invokeNative（Native Method）在java.lang.reflect.Method.invoke（Method.java:515）在 com.android.internal.os.ZygoteInit $ MethodAndArgsCaller.run（ZygoteInit.java:1283）在com.android.internal.os.ZygoteInit.main（ZygoteInit.java:1099）在dalvik.system.NativeStart.main（本地方法）

//Line 230: isValid = isDateValid(date.getMonth(), date.getDay(), date.getYear());

Answer 1

你的问题是范围。

你有一个实例变量

private Date date;

但你在try catch

中声明了另一个

try
{
    String getdate = spinner.getItemAtPosition(position).toString() + value
                        + textView2.getText().toString();
    Date date = sdf.parse(getdate);
}
catch (ParseException ex)
{

}
isValid = isDateValid(date.getMonth(), date.getDay(), date.getYear());

因此，一旦你退出try catch，其中声明的那个（并且初始化）不存在，实例变量永远不会被声明，所以留在null。

您希望初始化实例1，因此请删除声明，如下所示：

date = sdf.parse(getdate);

PS：因为异常会跳过这个问题，你最终会遇到同样问题，你应该把isValid放在try catch中

boolean isValid = false;

...

try
{
    String getdate = spinner.getItemAtPosition(position).toString() + value
                        + textView2.getText().toString();
    date = sdf.parse(getdate);
    isValid = isDateValid(date.getMonth(), date.getDay(), date.getYear());
    if (isValid == false)
    {
                Toast.makeText(MainActivity.this, "Uneli ste pogresan datum", Toast.LENGTH_SHORT).show();
    }
}
catch (ParseException ex)
{

}

注意：请注意，Date getter几乎全部已弃用，您应该使用Calendar.setTime(Date date)来Calendar.get(int field)安全使用Date。然后，在验证中，检查您收到的值，日期不是1/1/0000的日/月/年数。月份和日期基于0，因此ask drivers [ ifelse temperature < 5 [set shape "car"] [ move-drivers ] ifelse temperature > 5 [set shape "bike"] [ move-drivers ] ]

的第一天/月为0

Answer 2

我认为你没有初始化变量日期。

试试这个

$query = <<<EOF
        LOAD DATA LOCAL INFILE '$file'
         INTO TABLE users
         FIELDS TERMINATED BY ','
         LINES TERMINATED BY '\n'
         IGNORE 1 LINES
        (name,mobile,email)
EOF;

祝你好运。

Android周日计算器

2 个答案: