android - FindViewById 给出错误

Question

我不明白为什么这段代码在我输入该行后就给出错误 EditText 输入=(EditText) findViewById(R.id.editText); 空白 Activity Hello World 效果很好!

这是java代码......

package com.example.abhishektiwari.deletethis;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.widget.EditText;


public class MainActivity extends ActionBarActivity {

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}
EditText input=(EditText) findViewById(R.id.editText);
}

这是 xml 代码..

<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:tools="http://schemas.android.com/tools"    android:layout_width="match_parent"
android:layout_height="match_parent" android:paddingLeft="@dimen/activity_horizontal_margin"
android:paddingRight="@dimen/activity_horizontal_margin"
android:paddingTop="@dimen/activity_vertical_margin"
android:paddingBottom="@dimen/activity_vertical_margin" tools:context=".MainActivity"
android:orientation="vertical">


<EditText
    android:layout_height="wrap_content"
    android:layout_width="fill_parent"
    android:id="@+id/editText"
    android:layout_alignParentTop="true"
    android:layout_alignParentEnd="true"
    android:hint="Type the mathematical expression here"
    />
<Button
    android:layout_alignParentEnd="true"
    android:layout_height="wrap_content"
    android:layout_width="fill_parent"
    android:text="Get SVG"
    android:id="@+id/button"

    />

<TextView
    android:layout_width="fill_parent"
    android:layout_height="wrap_content"
    android:text="Result"
    android:textSize="30dp"
    android:id="@+id/textView"
    />

Answer 1

改变

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
}
EditText input=(EditText) findViewById(R.id.editText);

至

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    EditText input=(EditText) findViewById(R.id.editText);
}

因为您无法访问成员函数之外的 View 。