android findViewById

群居的山羊 2016-10-11 12:33:09

在制作注册界面时发现一个奇怪的问题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, "请输入", 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;
}

}

这么写的程序一打开就会停止运行
但是把findViewById放到onCreate方法中就没有问题了

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, "请输入", 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;
}

}
所以我想问一下findViewById使用时有什么限制么？

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