Question

我只是编程的初学者。我的代码有问题。每当我尝试单击列表中的第二项时，应用程序突然崩溃。

这是我的代码：

public class MainActivity extends AppCompatActivity {

    ListView lvCategories;
    String[] categories = {
        "School", "Work", "Family Outings", "Friends Outings", "Groceries" , "Appointments", "Indoor Activities",
            "Outdoor Activities", "Games","Overseas Travelling"
    };

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        lvCategories = (ListView)findViewById(R.id.listViewCategories);

        final ArrayAdapter<String> adapter = new ArrayAdapter(this, android.R.layout.simple_list_item_1, categories);
        lvCategories.setAdapter(adapter);

        lvCategories.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                String category = (String) parent.getItemAtPosition(position);

                if (category == "School") {
                    Intent school = new Intent(view.getContext(), school_activity.class);
                    school.putExtra("school_category", "");
                    startActivity(school);
                }
                if (category == "Work") {
                    Intent work = new Intent(view.getContext(), work_activity.class);
                    work.putExtra("work_category", "");
                    startActivity(work);
                }
            }
        });
    }
}

我可以知道这是什么问题吗？

这是我在LogCat中发现的错误。

引起：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'void android.widget.EditText.setText（java.lang.CharSequence）'

这是我的work_activity

public class work_activity extends AppCompatActivity {

    Button btnSave;
    EditText etContent;
    TextView tvLastUpdated;

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

        btnSave = (Button)findViewById(R.id.buttonSave);
        etContent = (EditText)findViewById(R.id.editTextContent);
        tvLastUpdated = (TextView)findViewById(R.id.textViewLastUpdated);

        Intent intentReceived = getIntent();
        final String strContent = intentReceived.getStringExtra("work_category");
        etContent.setText(strContent);
        Calendar now = Calendar.getInstance();
        final String datetime = now.get(Calendar.DAY_OF_MONTH) + "/" +
                (now.get(Calendar.MONTH) + 1) + "/" +
                now.get(Calendar.YEAR) + " " +
                now.get(Calendar.HOUR_OF_DAY) + ":" +
                now.get(Calendar.MINUTE);
        tvLastUpdated.setText(datetime);

        btnSave.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                String strContentSaved = etContent.getText().toString();
                SharedPreferences preferences = getSharedPreferences("category_work_saved", MODE_PRIVATE);
                SharedPreferences.Editor prefEdit = preferences.edit();
                prefEdit.putString("contentSaved", strContentSaved);
                prefEdit.putString("date", datetime);
                prefEdit.commit();
                Toast.makeText(work_activity.this, "Content Saved Successfully", Toast.LENGTH_SHORT).show();
                finish();
            }
        });
    }

    @Override
    protected void onResume() {
        super.onResume();
        SharedPreferences preferences = getSharedPreferences("category_work_saved", MODE_PRIVATE);
        String contentSaved = preferences.getString("contentSaved", "");
        etContent.setText(contentSaved);
        String dateTime = preferences.getString("date","");
        tvLastUpdated.setText(dateTime);
    }
}

Answer 1

看起来您可能正在尝试设置文字

etContent.setText(strContent);

没有strContent值......是这样吗？您正在发送“work_category”，但它可能是空值“”

Answer 2

您的代码中存在一些严重问题。我在这里指出这些问题。

在您的MainActivity中，您错误地比较了String值。您需要使用equals()方法而不是==运算符比较来比较String。

是的，尝试向work_activity或school_activity发送内容。

lvCategories.setOnItemClickListener(new AdapterView.OnItemClickListener() {
    @Override
    public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
        String category = (String) parent.getItemAtPosition(position);

        if (category.equals("School")) {
            Intent school = new Intent(view.getContext(), school_activity.class);
            school.putExtra("school_category", "send something here");
            startActivity(school);
        }
        if (category.equals("Work")) {
            Intent work = new Intent(view.getContext(), work_activity.class);
            work.putExtra("work_category", "send something here");
            startActivity(work);
        }
    }
});

现在，在您的work_activity中，您需要检查收到的null的{{1}}值。

intent

检查String strContent = null; Intent intentReceived = getIntent(); // Check if the intent received is null if (intentReceived != null) strContent = intentReceived.getStringExtra("work_category"); // Now check if the string content is null if(strContent != null) etContent.setText(strContent);布局中EditText的ID是activity_work_activity的另一件事。 id需要匹配才能找到可能导致崩溃的editTextContent。

Answer 3

错误说，

引起：java.lang.NullPointerException：尝试在空对象引用上调用虚方法'void android.widget.EditText.setText（java.lang.CharSequence）'

这意味着null正在返回id。

我认为您忘了将editTextContent EditText添加到Editetext，或者您在布局activity_work_activity.xml中没有equals。

同样如评论中建议的cricket007，使用if (category.equals("School")){ ... }比较字符串如下，

<dependency>
    <groupId>org.opensaml</groupId>
    <artifactId>opensaml-saml-impl</artifactId>
    <version>3.2.0</version>
</dependency>

在onClick on 2nd item上等应用程序崩溃，依赖于listview

3 个答案: