android-Android findview byid

Android findview byid

在制作注冊界面時發現一個奇怪的問題public class MainActivity extends AppCompatActivity {
    EditText a =(EditText) findViewById(R.id.xingming);
    EditText b =(EditText) findViewById(R.id.mima);
    Button zhuce = (Button) findViewById(R.id.zhuce);
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        a=(EditText) findViewById(R.id.xingming);
       b =(EditText) findViewById(R.id.mima);
        zhuce = (Button) findViewById(R.id.zhuce);

            zhuce.setOnClickListener(new View.OnClickListener() {

                @Override

                public void onClick(View v) {
                    if(input()) {
                        Intent intent = new Intent();
                        intent.setClass(MainActivity.this, nnn.class);
                        startActivity(intent);
                        finish();
                    }
                        else {
                        Toast.makeText(MainActivity.this, "請輸入", Toast.LENGTH_SHORT).show();
                    }
                }
            });
        }
    private boolean input(){
        String d=a.getText().toString();
        String x=b.getText().toString();
        if(d.equals("")||x.equals(""))
            return false;
        else
            return true;
    }

    }

這麼寫的程序一打開就會停止運行
但是把findViewById放到onCreate方法中就沒有問題了

public class MainActivity extends AppCompatActivity {
    EditText a;
    EditText b;
    Button zhuce;
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        a=(EditText) findViewById(R.id.xingming);
       b =(EditText) findViewById(R.id.mima);
        zhuce = (Button) findViewById(R.id.zhuce);

            zhuce.setOnClickListener(new View.OnClickListener() {

                @Override

                public void onClick(View v) {
                    if(input()) {
                        Intent intent = new Intent();
                        intent.setClass(MainActivity.this, nnn.class);
                        startActivity(intent);
                        finish();
                    }
                        else {
                        Toast.makeText(MainActivity.this, "請輸入", Toast.LENGTH_SHORT).show();
                    }
                }
            });
        }
    private boolean input(){
        String d=a.getText().toString();
        String x=b.getText().toString();
        if(d.equals("")||x.equals(""))
            return false;
        else
            return true;
    }

    }
所以我想問一下findViewById使用時有什麼限制麼？

最佳回答：

要在setContentView之後，再找元素。你想想，布局文件還沒加載，你要它去哪裡找元素呢？