NullPointerException（onClick事件）

Question

单击“SignUp”按钮时，NullPointerException出现问题。有人可以帮忙吗？ Thanx提前...

MainActivity.java：

public class MainActivity extends Activity {
    Button loginButton,signUpBtn;
    EditText txtUsername,txtPassword;
    private AppDataSouce datasource;


    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        loginButton = (Button) findViewById(R.id.btnLogin);
        txtUsername = (EditText) findViewById(R.id.txtUsername);
        txtPassword = (EditText) findViewById(R.id.txtPassword);

        signUpBtn=(Button) findViewById(R.id.signUpBtn);
        signUpBtn.setOnClickListener(new View.OnClickListener() {       
            @Override
            public void onClick(View v) {
                System.out.println("nulll1"+txtUsername.getText().toString());
                datasource.createUser(txtUsername.getText().toString(), txtPassword.getText().toString());
                System.out.println("nulll2" + txtUsername);
                Toast.makeText(MainActivity.this, "Registered Sucessfully", 3000).show();

                System.out.println("nulll3" + txtUsername.getText().toString());

                Intent listActivityIntent = new Intent(MainActivity.this,ShowListActivity.class);
                MainActivity.this.startActivity(listActivityIntent);

            }
        });
}

logcat的：

08-11 13:18:46.772: E/AndroidRuntime(14331):    at           com.example.androidlower.MainActivity$1.onClick(MainActivity.java:35)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at android.view.View.performClick(View.java:4204)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at android.view.View$PerformClick.run(View.java:17355)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at android.os.Handler.handleCallback(Handler.java:725)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at android.os.Handler.dispatchMessage(Handler.java:92)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at android.os.Looper.loop(Looper.java:137)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at android.app.ActivityThread.main(ActivityThread.java:5041)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at java.lang.reflect.Method.invokeNative(Native Method)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at java.lang.reflect.Method.invoke(Method.java:511)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:793)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:560)
08-11 13:18:46.772: E/AndroidRuntime(14331):    at dalvik.system.NativeStart.main(Native Method)

Answer 1

我没有看到你在创建AppDataSouce datasource;对象的任何地方 并且您在不创建对象的情况下使用datasource，因此它只是一个引用而不是对象。

datasource.createUser(txtUsername.getText().toString(), txtPassword.getText().toString());

因此它正在给予nullpointer exception.

Answer 2

在使用数据源之前，您缺少声明数据源！

datasource = new AppDataSouce(whatever);

Answer 3

datasource似乎无法在onCreate()中的任何位置进行初始化。如果一个对象没有初始化，那么打算如何使用它。

在设置clickListener之前，请创建此对象。

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

    datasource = new AppDataSouce();
     ....
}