Question

我正在尝试使用DataBase Sqlite在主页中显示图像。然后我在ImageView上获得空指针异常。

堆栈跟踪：

09-23 19:06:49.112: E/AndroidRuntime(979): FATAL EXCEPTION: main
09-23 19:06:49.112: E/AndroidRuntime(979): java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.loginregisterwithsqlite/com.example.loginregisterwithsqlite.Home}: java.lang.NullPointerException
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1647)
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:1663)
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.app.ActivityThread.access$1500(ActivityThread.java:117)
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.app.ActivityThread$H.handleMessage(ActivityThread.java:931)
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.os.Handler.dispatchMessage(Handler.java:99)
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.os.Looper.loop(Looper.java:123)
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.app.ActivityThread.main(ActivityThread.java:3683)
09-23 19:06:49.112: E/AndroidRuntime(979):  at java.lang.reflect.Method.invokeNative(Native Method)
09-23 19:06:49.112: E/AndroidRuntime(979):  at java.lang.reflect.Method.invoke(Method.java:507)
09-23 19:06:49.112: E/AndroidRuntime(979):  at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:839)
09-23 19:06:49.112: E/AndroidRuntime(979):  at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:597)
09-23 19:06:49.112: E/AndroidRuntime(979):  at dalvik.system.NativeStart.main(Native Method)
09-23 19:06:49.112: E/AndroidRuntime(979): Caused by: java.lang.NullPointerException
09-23 19:06:49.112: E/AndroidRuntime(979):  at com.example.loginregisterwithsqlite.Home.onCreate(Home.java:22)
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1047)
09-23 19:06:49.112: E/AndroidRuntime(979):  at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:1611)
09-23 19:06:49.112: E/AndroidRuntime(979):  ... 11 more

Home.java：

package com.example.loginregisterwithsqlite;

import android.app.Activity;
import android.os.Bundle;
import android.view.View;
import android.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ImageView;

public class Home extends Activity{

    Button button1;
    ImageView imageView1;
    @Override
    protected void onCreate(Bundle savedInstanceState) {

        button1=(Button)findViewById(R.id.button1);
        imageView1=(ImageView)findViewById(R.id.imageView1);

        super.onCreate(savedInstanceState);
        setContentView(R.layout.home);

        button1.setOnClickListener(new OnClickListener(){ --->22nd line

            @Override
            public void onClick(View v) {


                   imageView1.setImageResource(R.drawable.android);

            }


        });

    }

}

home.xml：

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:background="#D8D8D8" >

<Button 
    android:id="@+id/button1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content" 
    android:text="Display an Image"/>  

<ImageView
    android:id="@+id/imageView1"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:background="@drawable/android"
    />



</RelativeLayout>

我需要在同一个屏幕上显示图像。任何人都可以帮助我。谢谢。

Answer 1

移动

    button1=(Button)findViewById(R.id.button1);
    imageView1=(ImageView)findViewById(R.id.imageView1);

后

    super.onCreate(savedInstanceState);
    setContentView(R.layout.home);

在

之前调用setContentView之后，视图将成为活动视图层次结构的一部分

Answer 2

只有在主视图准备就绪后才能生成子视图。使用setContentView（）函数设置主视图。只有在此之后，您才可以使用findViewById（）

创建子视图

所以在setContentView（）

之后移动button1和imageView1

像

super.onCreate(savedInstanceState);
setContentView(R.layout.home);

button1=(Button)findViewById(R.id.button1);
imageView1=(ImageView)findViewById(R.id.imageView1);

ImageView上的空指针异常

2 个答案: