android studio java.langNullPointerException null 对象引用

android studio java.langNullPointerException null object reference

提问by Caedon Bates

I am continually getting an error when trying to run an android app telling me that there is a nullobject reference. I am still a beginner with android and java and I would greatly appreciate any help I can get.

尝试运行 Android 应用程序时，我不断收到错误消息，告诉我存在null对象引用。我仍然是 android 和 java 的初学者，如果我能得到任何帮助，我将不胜感激。

Here is the logcat:

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)

and here is my MainActivity.java:

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);
    }

}

回答by natario

Your paintLayoutis null, and your problem is onCreate(). When overriding methods from a super class, you should always (most of the times) call the method from that class using super.method(), to allow normal processing.

你paintLayout是空的，你的问题是onCreate(). 当覆盖超类中的方法时，您应该始终（大多数情况下）使用super.method(),调用该类中的方法，以允许正常处理。

Also, you are missing a call to setContentView(R.layout.your_layout). So don't forget these two lines:

此外，您错过了对setContentView(R.layout.your_layout). 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);
}

回答by CommonsWare

Your onCreate()method has two flaws:

你的onCreate()方法有两个缺陷：

1. It does not chain to the superclass with super.onCreate(), which is typically the very first line in the onCreate()method.

2. It does not do anything to create a user interface, such as calling setContentView().

1. 它不使用 链接到超类super.onCreate()，这通常是onCreate()方法中的第一行。

2. 它不会为创建用户界面做任何事情，例如调用setContentView().

You are crashing because findViewById()is returning null, because you have no widgets yet.

您崩溃是因为findViewById()正在返回null，因为您还没有小部件。