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

我在 Android Studio 1.5.1 中使用最小 API 15 创建了新的空白活动项目

在生成代码后，我收到此警告/错误:

MainActivity.java 中的代码:

package 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 AppCompatActivity {@覆盖protected 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(工具栏);FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);fab.setOnClickListener(new View.OnClickListener() {@覆盖公共无效onClick(查看视图){Snackbar.make(view, "替换为你自己的操作", Snackbar.LENGTH_LONG).setAction("Action", null).show();}});}@覆盖公共布尔 onCreateOptionsMenu(菜单菜单){//给菜单充气；如果它存在，这会将项目添加到操作栏.getMenuInflater().inflate(R.menu.my_menu, menu);返回真；}@覆盖公共布尔 onOptionsItemSelected(MenuItem item) {//在此处处理操作栏项目的点击.操作栏将//自动处理 Home/Up 按钮上的点击，这么长时间//当您在 AndroidManifest.xml 中指定父活动时.int id = item.getItemId();//不检查SimplifiableIfStatement如果(id == R.id.action_settings){返回真；}返回 super.onOptionsItemSelected(item);}}

content_my.xml 中的代码:

<文本视图android:layout_width="wrap_content"android:layout_height="wrap_content"android:text="你好世界！"/></RelativeLayout>

my_layout.xml 中的代码:

<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/工具栏"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="底部|结束"android:layout_margin="@dimen/fab_margin"android:src="@android:drawable/ic_dialog_email"/></android.support.design.widget.CoordinatorLayout>

请告诉我如何解决这个问题以及它为什么会发生.我是 Android 和 Android Studio 的新手.以前我使用 Apache Cordova.

解决方案

只检查/断言 fab 是否为空:

if (fab != null)

或:

assert fab != null;

像这样:

 FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);如果(工厂！= null)fab.setOnClickListener(new View.OnClickListener() {@覆盖公共无效onClick(查看视图){Snackbar.make(view, "替换为你自己的操作", 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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