我在我的android studio应用程序中创建了一个新活动，我有一个按钮，可以在单击时执行操作。我不确定是什么导致了错误，因为当我实际运行活动时会发生这种情况。如果我删除onClick代码，则活动将运行。谢谢你的帮助。我收到了这个错误：
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.masesk.kalkulator, PID: 4102
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.masesk.kalkulator/com.masesk.kalkulator.Main2Activity}: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2416)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
at com.masesk.kalkulator.Main2Activity.onCreate(Main2Activity.java:29)
at android.app.Activity.performCreate(Activity.java:6237)
at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1107)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2369)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2476)
at android.app.ActivityThread.-wrap11(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1344)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:148)
at android.app.ActivityThread.main(ActivityThread.java:5417)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:726)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
这是我的Main2Activity.java代码：
package com.masesk.kalkulator;
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.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.ToggleButton;
public class Main2Activity extends AppCompatActivity {
private ToggleButton textToBinary;
private Button trace;
private EditText text;
private EditText binary;
private String textInput;
private String binaryInput;
StringBuilder binaryText = new StringBuilder();
@Override
protected void onCreate(Bundle savedInstanceState) {
this.textToBinary = (ToggleButton)findViewById(R.id.textToBinary);
this.trace = (Button)findViewById(R.id.trace);
this.text = (EditText)findViewById(R.id.text);
this.binary = (EditText)findViewById(R.id.binary);
trace.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
if(textToBinary.isActivated() == false){
textInput = text.getText().toString();
byte[] bytes = textInput.getBytes();
StringBuilder binary = new StringBuilder();
for (byte b : bytes)
{
int val = b;
for (int i = 0; i < 8; i++)
{
binaryText.append((val & 128) == 0 ? 0 : 1);
val <<= 1;
}
binaryText.append(' ');
}
}
}
});
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
}
@Override
public void onResume(){
this.binary = (EditText)findViewById(R.id.binary);
binary.setText(binaryText.toString(), TextView.BufferType.EDITABLE);
}
}
这是我的XML代码
<?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:id="@+id/content_main2"
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.masesk.kalkulator.Main2Activity"
tools:showIn="@layout/activity_main2">
<ToggleButton
android:text="ToggleButton"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_alignParentTop="true"
android:layout_alignParentStart="true"
android:id="@+id/textToBinary"
android:layout_alignParentEnd="true"
android:textOff="Binary To Text"
android:textOn="Text to Binary" />
<TextView
android:text="Text:"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/textToBinary"
android:layout_alignParentStart="true"
android:id="@+id/textView2"
android:layout_alignParentEnd="true" />
<EditText
android:background="@drawable/back"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inputType="textMultiLine"
android:ems="10"
android:layout_below="@+id/textView2"
android:layout_alignParentStart="true"
android:id="@+id/text"
android:layout_alignParentEnd="true" />
<TextView
android:text="Binary Code:"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/text"
android:layout_alignParentStart="true"
android:id="@+id/textView3" />
<EditText
android:background="@drawable/back"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:inputType="textMultiLine"
android:ems="10"
android:layout_below="@+id/textView3"
android:layout_alignParentStart="true"
android:id="@+id/binary"
android:layout_alignParentEnd="true" />
<Button
android:text="convert"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_below="@+id/binary"
android:layout_alignParentStart="true"
android:layout_marginTop="14dp"
android:id="@+id/trace"
android:layout_alignParentEnd="true" />
</RelativeLayout>
答案 0 :(得分：4)
在OnCreate方法中，您应始终首先放置
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main2);
这是因为在您使用setContentView设置视图之前，视图不会膨胀。这意味着任何findViewById之前的调用都会返回null。
此外，您还必须在super.onResume(...)的最开始处设置onResume()来电，正如Android文档所述。这也适用于其他生命周期方法。这也应解决您的setText问题。