在主页上创建多个按钮。对此很陌生，知道的不多。"注册"按钮会导致"application stopping"

Question

package com.example.james.assignment1_18094969;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View.OnClickListener;
import android.view.View;
import android.content.Intent;
import android.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，这会导致"应用程序已停止工作"，即使它是完全相同的......困惑。在此处输入代码

        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 android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2665)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726)
    at android.app.ActivityThread.-wrap12(ActivityThread.java)
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477)
    at android.os.Handler.dispatchMessage(Handler.java:102)
    at android.os.Looper.loop(Looper.java:154)
    at android.app.ActivityThread.main(ActivityThread.java:6119)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference
    at com.example.james.assignment1_18094969.SignUp.onCreate(SignUp.java:22)
    at android.app.Activity.performCreate(Activity.java:6679)
    at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1118)
    at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2618)
    at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2726) 
    at android.app.ActivityThread.-wrap12(ActivityThread.java) 
    at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1477) 
    at android.os.Handler.dispatchMessage(Handler.java:102) 
    at android.os.Looper.loop(Looper.java:154) 
    at android.app.ActivityThread.main(ActivityThread.java:6119) 
    at java.lang.reflect.Method.invoke(Native Method) 
    at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886) 
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776) 

.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:id="@+id/activity_home"
    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"
    android:background="@color/black"
    tools:context="com.example.james.assignment1_18094969.Home">
    <ImageView
        android:layout_width="200dp"
        android:layout_height="150dp"
        app:srcCompat="@drawable/speedy"
        android:layout_marginTop="26dp"
        android:id="@+id/imageView"
        android:layout_below="@+id/textView"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:layout_marginLeft="64dp"
        android:layout_marginStart="64dp" />
    <Button
        android:text="Delivery"
        android:layout_width="200dp"
        android:layout_height="50dp"
        android:layout_below="@+id/imageView"
        android:layout_centerHorizontal="true"
        android:id="@+id/button_delivery"
        android:background="@color/red"
        android:textColor="@color/white" />
    <Button
        android:text="Pick Up"
        android:layout_width="200dp"
        android:layout_height="50dp"
        android:layout_below="@+id/button_delivery"
        android:layout_alignLeft="@+id/button_delivery"
        android:layout_alignStart="@+id/button_delivery"
        android:layout_marginTop="24dp"
        android:background="@color/red"
        android:id="@+id/button_pickUp"
        android:textColor="@color/white"/>
    <Button
        android:text="Quick Order"
        android:layout_width="200dp"
        android:layout_height="50dp"
        android:layout_below="@+id/button_pickUp"
        android:layout_alignRight="@+id/button_pickUp"
        android:layout_alignEnd="@+id/button_pickUp"
        android:layout_marginTop="21dp"
        android:background="@color/red"
        android:id="@+id/button_quickOrd"
        android:textColor="@color/white"/>
    <Button
        android:text="Login"
        android:layout_width="100dp"
        android:layout_height="wrap_content"
        android:id="@+id/button_login"
        android:layout_marginBottom="16dp"
        android:layout_alignParentBottom="true"
        android:layout_alignLeft="@+id/imageView"
        android:layout_alignStart="@+id/imageView"
        android:background="@color/white"
        android:textColor="@color/red"/>
    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Welcome to Speedy Pizza!"
        android:id="@+id/textView"
        android:layout_alignParentTop="true"
        android:layout_alignParentLeft="true"
        android:layout_alignParentStart="true"
        android:textColor="@color/red"/>
    <Button
        android:text="Sign Up"
        android:layout_width="100dp"
        android:layout_height="wrap_content"
        android:id="@+id/button_signUp"
        android:layout_marginLeft="37dp"
        android:layout_marginStart="37dp"
        android:background="@color/white"
        android:textColor="@color/red"
        android:layout_alignBaseline="@+id/button_login"
        android:layout_alignBottom="@+id/button_login"
        android:layout_toRightOf="@+id/textView"
        android:layout_toEndOf="@+id/textView" />
</RelativeLayout>

Answer 1

查看日志，应用找不到button_signUp视图，并因此引发异常。最好检查button_signUp ID 是否与 XML 中使用的 ID 匹配。

注意：如果未找到视图，则 findViewById 返回空

值有关 NullPointerException

的详细信息，请参阅什么是 NullPointerException，以及如何修复它？