android 工作室 java.langNullPointerException 空对象引用

Question

我在尝试运行一个 android 应用程序时不断收到错误消息，告诉我有一个 null 对象引用。我仍然是 android 和 java 的初学者，如果能得到任何帮助，我将不胜感激。

这是 logcat：

01-17 17:11:49.156  27778-27778/com.example.owner.sketchy D/AndroidRuntime﹕ Shutting down VM
    --------- beginning of crash
01-17 17:11:49.157  27778-27778/com.example.owner.sketchy E/AndroidRuntime﹕ FATAL EXCEPTION: main
    Process: com.example.owner.sketchy, PID: 27778
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.owner.sketchy/com.example.owner.sketchy.MainActivity}: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.LinearLayout.getChildAt(int)' on a null object reference
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2298)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access0(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.view.View android.widget.LinearLayout.getChildAt(int)' on a null object reference
            at com.example.owner.sketchy.MainActivity.onCreate(MainActivity.java:33)
            at android.app.Activity.performCreate(Activity.java:5933)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1105)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2251)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2360)
            at android.app.ActivityThread.access0(ActivityThread.java:144)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1278)
            at android.os.Handler.dispatchMessage(Handler.java:102)
            at android.os.Looper.loop(Looper.java:135)
            at android.app.ActivityThread.main(ActivityThread.java:5221)
            at java.lang.reflect.Method.invoke(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:372)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:899)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:694)

这是我的 MainActivity.java:

    package com.example.owner.sketchy;

import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.view.Menu;
import android.view.MenuItem;
import android.view.View;
import android.widget.ImageButton;
import android.widget.LinearLayout;

public class MainActivity extends ActionBarActivity {


    private ImageButton currPaint;
    private DrawingView drawView;

    public void paintClicked(View view) {
        if (view != currPaint) {

            ImageButton imgView = (ImageButton) view;
            String color = view.getTag().toString();
            imgView.setImageDrawable(getResources().getDrawable(R.drawable.paint_pressed));
            currPaint.setImageDrawable(getResources().getDrawable(R.drawable.paint));
            currPaint = (ImageButton) view;
            drawView.setColor(color);
        }
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        drawView = (DrawingView)findViewById(R.id.drawing);
        LinearLayout paintLayout = (LinearLayout)findViewById(R.id.paint_colors);
        currPaint = (ImageButton)paintLayout.getChildAt(0);
    }
    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

}

Answer 1

您的 onCreate() 方法有两个缺陷：

它不会链接到具有 super.onCreate() 的超类，这通常是 onCreate() 方法中的第一行。
它不做任何创建用户界面的事情，比如调用setContentView()。

您正在崩溃，因为 findViewById() 正在返回 null，因为您还没有小部件。

Answer 2

你的paintLayout是空的，你的问题是onCreate()。从超级 class 覆盖方法时，您应该始终（大多数时候）使用 super.method() 从 class 调用方法以允许正常处理。

此外，您错过了对 setContentView(R.layout.your_layout) 的调用。所以不要忘记这两行：

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.your_layout);

    drawView = (DrawingView)findViewById(R.id.drawing);
    LinearLayout paintLayout = (LinearLayout)findViewById(R.id.paint_colors);
    currPaint = (ImageButton)paintLayout.getChildAt(0);
}

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