java在主页上创建多个按钮。对这件事很陌生，知道的不多。注册按钮导致“应用程序停止”
package com.example.james.assignment1_18094969;
import 安卓.support.v7.app.AppCompatActivity;
import 安卓.os.Bundle;
import 安卓.view.View.OnClickListener;
import 安卓.view.View;
import 安卓.content.Intent;
import 安卓.widget.Button;
public class Home extends AppCompatActivity {
//login button
Button loginButton;
//sign up button
Button signupButton;
enter code here
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_home);
登录按钮将用户带到登录屏幕，此按钮工作正常
loginButton = (Button) findViewById(R.id.button_login);
loginButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick (View view) {
Intent intent = new Intent(Home.this, Login.class);
startActivity(intent);
}
}
);
“注册”按钮，将用户带到“注册表单”活动。这将导致“应用程序已停止工作”，即使它完全相同。。。困惑的在这里输入代码
signupButton = (Button) findViewById(R.id.button_signUp);
signupButton.setOnClickListener(new OnClickListener() {
@Override
public void onClick(View view) {
Intent intent = new Intent(Home.this, SignUp.class);
startActivity(intent);
}
});
}
}
日志：
01-06 13:27:42.934 2417-2417/com.example.james.assignment1_18094969 E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.james.assignment1_18094969, PID: 2417
java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.james.assignment1_18094969/com.example.james.assignment1_18094969.SignUp}: java.lang.NullPointerException: Attempt to invoke virtual method 'void 安卓.widget.Button.setOnClickListener(安卓.view.View$OnClickListener)' on a null object reference
at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at 安卓.app.ActivityThread.-wrap12(ActivityThread.java)
at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at 安卓.os.Handler.dispatchMessage(Handler.java:102)
at 安卓.os.Looper.loop(Looper.java:154)
at 安卓.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void 安卓.widget.Button.setOnClickListener(安卓.view.View$OnClickListener)' on a null object reference
at com.example.james.assignment1_18094969.SignUp.onCreate(SignUp.java:22)
at 安卓.app.Activity.performCreate(Activity.java:6679)
at 安卓.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
at 安卓.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
at 安卓.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
at 安卓.app.ActivityThread.-wrap12(ActivityThread.java)
at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
at 安卓.os.Handler.dispatchMessage(Handler.java:102)
at 安卓.os.Looper.loop(Looper.java:154)
at 安卓.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.安卓.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:776)
XML
<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:安卓="http://schemas.安卓.com/apk/res/安卓"
xmlns:app="http://schemas.安卓.com/apk/res-auto"
xmlns:tools="http://schemas.安卓.com/tools"
安卓:id="@+id/activity_home"
安卓:layout_width="match_parent"
安卓:layout_height="match_parent"
安卓:paddingBottom="@dimen/activity_vertical_margin"
安卓:paddingLeft="@dimen/activity_horizontal_margin"
安卓:paddingRight="@dimen/activity_horizontal_margin"
安卓:paddingTop="@dimen/activity_vertical_margin"
安卓:background="@color/black"
tools:context="com.example.james.assignment1_18094969.Home">
<ImageView
安卓:layout_width="200dp"
安卓:layout_height="150dp"
app:srcCompat="@drawable/speedy"
安卓:layout_marginTop="26dp"
安卓:id="@+id/imageView"
安卓:layout_below="@+id/textView"
安卓:layout_alignParentLeft="true"
安卓:layout_alignParentStart="true"
安卓:layout_marginLeft="64dp"
安卓:layout_marginStart="64dp" />
<Button
安卓:text="Delivery"
安卓:layout_width="200dp"
安卓:layout_height="50dp"
安卓:layout_below="@+id/imageView"
安卓:layout_centerHorizontal="true"
安卓:id="@+id/button_delivery"
安卓:background="@color/red"
安卓:textColor="@color/white" />
<Button
安卓:text="Pick Up"
安卓:layout_width="200dp"
安卓:layout_height="50dp"
安卓:layout_below="@+id/button_delivery"
安卓:layout_alignLeft="@+id/button_delivery"
安卓:layout_alignStart="@+id/button_delivery"
安卓:layout_marginTop="24dp"
安卓:background="@color/red"
安卓:id="@+id/button_pickUp"
安卓:textColor="@color/white"/>
<Button
安卓:text="Quick Order"
安卓:layout_width="200dp"
安卓:layout_height="50dp"
安卓:layout_below="@+id/button_pickUp"
安卓:layout_alignRight="@+id/button_pickUp"
安卓:layout_alignEnd="@+id/button_pickUp"
安卓:layout_marginTop="21dp"
安卓:background="@color/red"
安卓:id="@+id/button_quickOrd"
安卓:textColor="@color/white"/>
<Button
安卓:text="Login"
安卓:layout_width="100dp"
安卓:layout_height="wrap_content"
安卓:id="@+id/button_login"
安卓:layout_marginBottom="16dp"
安卓:layout_alignParentBottom="true"
安卓:layout_alignLeft="@+id/imageView"
安卓:layout_alignStart="@+id/imageView"
安卓:background="@color/white"
安卓:textColor="@color/red"/>
<TextView
安卓:layout_width="wrap_content"
安卓:layout_height="wrap_content"
安卓:text="Welcome to Speedy Pizza!"
安卓:id="@+id/textView"
安卓:layout_alignParentTop="true"
安卓:layout_alignParentLeft="true"
安卓:layout_alignParentStart="true"
安卓:textColor="@color/red"/>
<Button
安卓:text="Sign Up"
安卓:layout_width="100dp"
安卓:layout_height="wrap_content"
安卓:id="@+id/button_signUp"
安卓:layout_marginLeft="37dp"
安卓:layout_marginStart="37dp"
安卓:background="@color/white"
安卓:textColor="@color/red"
安卓:layout_alignBaseline="@+id/button_login"
安卓:layout_alignBottom="@+id/button_login"
安卓:layout_toRightOf="@+id/textView"
安卓:layout_toEndOf="@+id/textView" />
</RelativeLayout>
# 1 楼答案
查看您的日志，应用程序找不到
button_signUp视图，因此引发异常。最好检查button_signUpid是否与XML中使用的id匹配注意：findViewById如果找不到视图，则返回null
有关NullPointerException的更多信息，请参见What is a NullPointerException, and how do I fix it?