Android：将值传递给另一个活动的函数，然后传递给Sqlite数据库[复制]

Question

这个问题在这里已有答案：

• What is a NullPointerException, and how do I fix it? 12个答案

自从我开始使用android studio已经有一个月了，此时我已经陷入了这样的境地：我必须将<EditText/>中的一组MainActivity值传递给UpdateActivity，然后将这些值发送到Sqlite数据库进行更新表。

我所取得的成就：

成功地将值从MainActivity的<EditText/>发送到UpdateActivity的<EditText/>

我是如何从MainActivity那样做的：

public class MainActivity extends AppCompatActivity {
    DatabaseHelper myDb;
    EditText editName, editSurname, editMarks;
    Button btnUpdate;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    myDb = new DatabaseHelper(this);

    editName = (EditText) findViewById(R.id.UpdName);
    editSurname = (EditText) findViewById(R.id.editTextSurname);
    editMarks = (EditText) findViewById(R.id.UpdMarks);
    btnUpdate = (Button)findViewById(R.id.btnUpdate);

    btnUpdate.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            String MainName = editName.getText().toString();
            String MainSurname = editSurname.getText().toString();
            String MainMarks = editMarks.getText().toString();

            Intent UpdateAct = new Intent(MainActivity.this, UpdateActivity.class);
            UpdateAct.putExtra("Name", MainName);
            UpdateAct.putExtra("SurName",MainSurname );
            UpdateAct.putExtra("Marks", MainMarks);
            startActivity(UpdateAct);
        }
    });
}

然后在UpdateActivity内：

public class UpdateActivity extends AppCompatActivity {
DatabaseHelper myDb;
Button btnUpd;
EditText editTextId, editUpdtName, editUpdSurname, editUpdMarks;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_update);

    Intent getData = getIntent();
    String Name = getData.getStringExtra("Name");
    String SurName = getData.getStringExtra("SurName");
    String Marks = getData.getStringExtra("Marks");

    editTextId = (EditText)findViewById(R.id.editTextID);
    editUpdtName = (EditText)findViewById(R.id.UpdName);
    editUpdSurname = (EditText)findViewById(R.id.UpdSurname);
    editUpdMarks = (EditText)findViewById(R.id.UpdMarks);
    btnUpd = (Button)findViewById(R.Id.btnUpd);

    editUpdtName.setText(Name);
    editUpdSurname.setText(SurName);
    editUpdMarks.setText(Marks);
}

我坚持的是将这些值发送到一个名为updateData();的函数。为此我试图将MainActivity中的值直接用于UpdateActivity但失败了，然后我尝试投射那些<EditText/>并在函数内使用它但又失败了。

在我的UpdateActivity里面：

..//    
        editUpdtName.setText(Name);
        editUpdSurname.setText(SurName);
        editUpdMarks.setText(Marks);
        //updateData(Name, SurName, Marks);
        updateData();
    }

//function definiton
//   public void updateData(final String Name, final String SurName, final String Marks)
    public void updateData()
    {
        btnUpd.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
//             boolean isUpdated =   myDb.updateData(editTextId.getText().toString(), Name,SurName, Marks);
                boolean isUpdated =   myDb.updateData(editTextId.getText().toString(), editUpdtName.getText().toString(),
                        editUpdSurname.getText().toString(), editUpdMarks.getText().toString());
            if(isUpdated == true){
                Toast.makeText(UpdateActivity.this, "Data Updated Successfully!", Toast.LENGTH_SHORT).show();
            }
            else{
                    Toast.makeText(UpdateActivity.this, "Data NOT Updated!", Toast.LENGTH_SHORT).show();
        }

    }
        });
    }
}

每次调用函数updateData();时应用程序崩溃，以确保这是我甚至评论过的真正原因，并且还尝试将其放入处理程序中以在3000ms之后运行它，因为它已经在时间段之后崩溃了。

通过初始化db来解决它。

myDb = new DatabaseHelper(this);

Answer 1

你忘了初始化myDb。在你的onCreate()方法中初始化它：

myDb = new DatabaseHelper(this);

Answer 2

为什么不将值传递给updateData()？

editUpdtName.setText(Name);
editUpdSurname.setText(SurName);
editUpdMarks.setText(Marks);
updateData(Name, SurName, Marks);

Answer 3

你有没有初始化btnUpd？我在你的onCreate（）中看不到这个

btnUpd = (Button) findViewById(...)