Question

When i try to run the app on my phone,it force closes when i try to do activity 3. Logcat says:

01-13 17:53:25.368: E/AndroidRuntime(3235): Caused by: java.lang.NullPointerException 01-13 17:53:25.368: E/AndroidRuntime(3235): at android.app.activity3.onCreate(activity3.java:18)

第18条

Button wg = (Button) findViewById(R.id.Back);

我的《活动守则》3.java:

package android.app;
import android.app.R;
import android.app.Activity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;


public class activity3 extends Activity{

    /** Called when the activity is first created. */
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main3);

        Button wg = (Button) findViewById(R.id.Back);
        wg.setOnClickListener(new View.OnClickListener() {
            public void onClick(View view) {
                Intent intent = new Intent();
                setResult(RESULT_OK, intent);
                finish();
            }

        });
    }
}

预收

Answer 1

你需要修改你的Xml:

<?xml version="1.0" encoding="utf-8"?>

<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="fill_parent"
    android:layout_height="fill_parent"
    >

    <!-- your textview -->
    <TextView
        android:id="@+id/text1"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/text1"
        android:textAppearance="?android:attr/textAppearanceMedium"
        />

    <!-- your back button -->
    <Button
        android:id="@+id/back"
        android:layout_width="fill_parent"
        android:layout_height="wrap_content"
        android:text="@string/Back"
        />

</RelativeLayout>

我做了些什么:

Removed text and orientation from your root RelativeLayout, these do nothing.
Modified your height and width, feel free to change these as you please.
Added a Button with id of R.id.back and text of @string/Back.

现在,你能够利用发现ById(R.id.back)来查阅你的子州,并点击听众。

Answer 2

我在xml中不见一顿。并且,一旦有这样的情况,它就需要:

android:id="@+id/Back"

Answer 3

之所以出现这种情况,是因为你看不到一个纽子。只有一份案文概览。你们必须建立托邦。你们只字不提,因此就没有。

你们可以找到一个 n子:here。

Answer 4

你没有用贴上Xml的“Back”的ton子,这就是为什么你在那里拿走。页: 1

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