未知崩溃与简单的应用程序

Question

我的安卓应用因不明原因崩溃了。这是代码：

public class MainActivity extends ActionBarActivity {
    EditText x1 = (EditText)findViewById(R.id.idEdittext);
    TextView x2 = (TextView)findViewById(R.id.idtext);
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        Toast.makeText(this, "Welcome", Toast.LENGTH_LONG).show();
    }

启动时方法

@Override
protected void onStart() {
    super.onStart();
    Toast.makeText(this, "This is your first App :)) ", Toast.LENGTH_LONG).show();
}

这是单击按钮时执行按钮的方法

public void onClick() {
    String name = String.valueOf(x1.getText());
    Toast.makeText(this," Welcome " + name +"" +"to Android" , Toast.LENGTH_LONG).show();
    x2.setVisibility(View.INVISIBLE);
}

当我运行它时，它崩溃并说"不幸的是，您的应用程序已停止"

Answer 1

在创建活动之前，不应调用findViewById。

像这样更改代码：

public class MainActivity extends ActionBarActivity {
    EditText x1;
    TextView x2;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        x1 = (EditText)findViewById(R.id.idEdittext);
        x2 = (TextView)findViewById(R.id.idtext);
        Toast.makeText(this, "Welcome", Toast.LENGTH_LONG).show();
    }