Java NullPointerException setOnClickListner

我是android的初学者。 这是我从书上抄的代码。但当我下载并在智能手机上运行时，就会出现NullPointerException

例外情况的信息如下所示：

java.lang.NullPointerException:尝试在…处的空对象引用上调用虚拟方法'void android.widget.Button.setOnClickListner'

这是我代码的一部分：

public class MainActivity extends AppCompatActivity {

    Button btn1 , btn2;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);

       // requestWindowFeature(Window.FEATURE_PROGRESS);
        requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
        btn1 = (Button) findViewById(R.id.btn1);
        btn2 = (Button) findViewById(R.id.btn2);
        btn1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                setProgressBarIndeterminateVisibility(true);
                setProgressBarVisibility(true);
                setProgress(4500);
            }
        });
        btn2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                setProgressBarIndeterminateVisibility(false);
                setProgressBarVisibility(false);
            }
        });
        setContentView(R.layout.activity_main);

        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();
            }
        });
    }

下面是XML文件：

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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.neoncowboy.titleprogressbar.MainActivity"
    tools:showIn="@layout/activity_main"
    android:orientation="horizontal">

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="显示"
        android:id="@+id/btn1" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="隐藏"
        android:id="@+id/btn2" />

</LinearLayout>

---

您需要设置contentview（R.layout.activity\u main）添加前，单击列表

public class MainActivity extends AppCompatActivity {

Button btn1 , btn2;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

   // requestWindowFeature(Window.FEATURE_PROGRESS);
    requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
    setContentView(R.layout.activity_main);

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    btn1 = (Button) findViewById(R.id.btn1);
    btn2 = (Button) findViewById(R.id.btn2);
    btn1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            setProgressBarIndeterminateVisibility(true);
            setProgressBarVisibility(true);
            setProgress(4500);
        }
    });
    btn2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            setProgressBarIndeterminateVisibility(false);
            setProgressBarVisibility(false);
        }
    });


    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();
        }
    });
}

---

您需要

setContentView（R.layout.activity\u main）添加前，单击列表

public class MainActivity extends AppCompatActivity {

Button btn1 , btn2;
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

   // requestWindowFeature(Window.FEATURE_PROGRESS);
    requestWindowFeature(Window.FEATURE_INDETERMINATE_PROGRESS);
    setContentView(R.layout.activity_main);

    Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
    setSupportActionBar(toolbar);
    btn1 = (Button) findViewById(R.id.btn1);
    btn2 = (Button) findViewById(R.id.btn2);
    btn1.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            setProgressBarIndeterminateVisibility(true);
            setProgressBarVisibility(true);
            setProgress(4500);
        }
    });
    btn2.setOnClickListener(new View.OnClickListener() {
        @Override
        public void onClick(View v) {
            setProgressBarIndeterminateVisibility(false);
            setProgressBarVisibility(false);
        }
    });


    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();
        }
    });
}

setContentView（）
行必须在
findViewById（）
行之前
 第
setContentView（）
行必须在第
findViewById（）
行之前
 您应该使用
setContentView（R.layout.yourXmlFileName）将xml布局连接到活动类在这一行之后：
super.onCreate（savedInstanceState）
您应该使用
setContentView（R.layout.yourXmlFileName）将xml布局连接到活动类在这一行之后：
super.onCreate（savedInstanceState）
如果你逐字复制那本书，那本书就错了。如果你逐字复制那本书，那本书就错了。