方法调用'fab.setOnClickListener(new View.OnClickListener(){@Override public void on ...')可能会产生'java.lang.NullPointerException' [英] Method invocation 'fab.setOnClickListener(new View.OnClickListener() { @Override public void on...' may produce 'java.lang.NullPointerException'

问题描述

我使用最低API 15在Android Studio 1.5.1中创建了新的Blank Activity项目

在生成代码后，我有以下警告/错误:

MainActivity.java 中的代码:

 包com.gamecodeschool.myapplication;导入android.os.Bundle;导入android.support.design.widget.FloatingActionButton;导入android.support.design.widget.Snackbar;导入android.support.v7.app.AppCompatActivity;导入android.support.v7.widget.Toolbar;导入android.view.View;导入android.view.Menu;导入android.view.MenuItem;公共类MyActivity扩展了AppCompatActivity {@Override受保护的void onCreate(Bundle savedInstanceState){super.onCreate(savedInstanceState);setContentView(R.layout.my_layout);工具栏工具栏=(工具栏)findViewById(R.id.toolbar);setSupportActionBar(toolbar);FloatingActionButton fab =(FloatingActionButton)findViewById(R.id.fab);fab.setOnClickListener(new View.OnClickListener(){@Overridepublic void onClick(View view){Snackbar.make(视图，用自己的动作替换"，Snackbar.LENGTH_LONG).setAction("Action"，null).show();}});}@Overridepublic boolean onCreateOptionsMenu(Menu menu){//膨胀菜单；这会将项目添加到操作栏(如果有).getMenuInflater().inflate(R.menu.my_menu，menu);返回true；}@Overridepublic boolean onOptionsItemSelected(MenuItem item){//单击操作栏项.动作栏将//自动处理主页"/向上"按钮上的点击，//当您在AndroidManifest.xml中指定父活动时.int id = item.getItemId();//noinspection SimplifiableIfStatement如果(id == R.id.action_settings){返回true；}返回super.onOptionsItemSelected(item);}} 

content_my.xml :

 <?xml version ="1.0" encoding ="utf-8"?>< RelativeLayout xmlns:android ="http://schemas.android.com/apk/res/android"xmlns:app ="http://schemas.android.com/apk/res-auto"xmlns:tools ="http://schemas.android.com/tools"android:layout_width ="match_parent"android:layout_height ="match_parent"android:paddingBottom ="@ dimen/activity_vertical_margin"android:paddingLeft ="@ dimen/activity_horizo​​ntal_margin"android:paddingRight ="@ dimen/activity_horizo​​ntal_margin"android:paddingTop ="@ dimen/activity_vertical_margin"app:layout_behavior ="@ string/appbar_scrolling_view_behavior"工具:context ="com.gamecodeschool.myapplication.MyActivity"工具:showIn ="@ layout/my_layout">< TextViewandroid:layout_width ="wrap_content"android:layout_height ="wrap_content"android:text ="Hello World！"/></RelativeLayout> 

my_layout.xml :

 <?xml version ="1.0" encoding ="utf-8"?>< android.support.design.widget.CoordinatorLayout xmlns:android ="http://schemas.android.com/apk/res/android"xmlns:app ="http://schemas.android.com/apk/res-auto"xmlns:tools ="http://schemas.android.com/tools"android:layout_width ="match_parent"android:layout_height ="match_parent"android:fitsSystemWindows ="true"工具:context ="com.gamecodeschool.myapplication.MyActivity">< android.support.design.widget.AppBarLayoutandroid:layout_width ="match_parent"android:layout_height ="wrap_content"android:theme ="@ style/AppTheme.AppBarOverlay">< android.support.v7.widget.Toolbarandroid:id ="@ + id/toolbar"android:layout_width ="match_parent"android:layout_height =?attr/actionBarSize"android:background =?attr/colorPrimary"app:popupTheme ="@ style/AppTheme.PopupOverlay"/></android.support.design.widget.AppBarLayout>< include layout ="@ layout/content_my"/>< android.support.design.widget.FloatingActionButtonandroid:id ="@ + id/fab"android:layout_width ="wrap_content"android:layout_height ="wrap_content"android:layout_gravity ="bottom | end"android:layout_margin ="@ dimen/fab_margin"android:src ="@ android:drawable/ic_dialog_email"/></android.support.design.widget.CoordinatorLayout> 

请告诉我如何解决此问题以及为什么会发生.我是Android和Android Studio的新手.以前我使用过Apache Cordova.

解决方案

只需检查/确认fab是否为空:

  if(fab！=空) 

或:

  assert fab！= null; 

喜欢这个:

  FloatingActionButton fab =(FloatingActionButton)findViewById(R.id.fab);如果(fab！= null)fab.setOnClickListener(new View.OnClickListener(){@Overridepublic void onClick(View view){Snackbar.make(视图，用自己的动作替换"，Snackbar.LENGTH_LONG).setAction("Action"，null).show();}}); 

I create new Blank Activity project in Android Studio 1.5.1 with min API 15

and after generated code i have this warning/error:

on pasteBin

Code in MainActivity.java:

package com.gamecodeschool.myapplication;

import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;

public class MyActivity extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.my_layout);
        Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
        setSupportActionBar(toolbar);

        FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
        fab.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                        .setAction("Action", null).show();
            }
        });
    }

    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.my_menu, 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);
    }
}

Code in content_my.xml:

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:paddingBottom="@dimen/activity_vertical_margin"
    android:paddingLeft="@dimen/activity_horizontal_margin"
    android:paddingRight="@dimen/activity_horizontal_margin"
    android:paddingTop="@dimen/activity_vertical_margin"
    app:layout_behavior="@string/appbar_scrolling_view_behavior"
    tools:context="com.gamecodeschool.myapplication.MyActivity"
    tools:showIn="@layout/my_layout">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!" />
</RelativeLayout>

Code in my_layout.xml:

<?xml version="1.0" encoding="utf-8"?>
<android.support.design.widget.CoordinatorLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:app="http://schemas.android.com/apk/res-auto"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:fitsSystemWindows="true"
    tools:context="com.gamecodeschool.myapplication.MyActivity">

    <android.support.design.widget.AppBarLayout
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:theme="@style/AppTheme.AppBarOverlay">

        <android.support.v7.widget.Toolbar
            android:id="@+id/toolbar"
            android:layout_width="match_parent"
            android:layout_height="?attr/actionBarSize"
            android:background="?attr/colorPrimary"
            app:popupTheme="@style/AppTheme.PopupOverlay" />

    </android.support.design.widget.AppBarLayout>

    <include layout="@layout/content_my" />

    <android.support.design.widget.FloatingActionButton
        android:id="@+id/fab"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:layout_gravity="bottom|end"
        android:layout_margin="@dimen/fab_margin"
        android:src="@android:drawable/ic_dialog_email" />

</android.support.design.widget.CoordinatorLayout>

Tell me please how to fix this problem and why it happens. I'm new in Android and Android Studio. Previously I used the Apache Cordova.

解决方案

Just check/assert whether fab is null:

if (fab != null)

or:

assert fab != null;

Like this:

    FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
    if (fab != null)
    fab.setOnClickListener(new View.OnClickListener() {
          @Override
          public void onClick(View view) {
                 Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
                 .setAction("Action", null).show();
           }
     });

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