java - 尝试将按钮值与进度条值进行比较

Question

我的应用程序中有一个按钮，称为“启动引擎”。如果我按下按钮，我希望它检查燃料是否为空，然后将按钮内的文本更改为“引擎正在运行..”，但它不断崩溃，我不明白错误消息

新错误:

    05-04 19:06:52.435 18652-18652/com.example.schwarzerritter.remotecv02 E/AndroidRuntime: FATAL EXCEPTION: main
                                                                                        Process: com.example.schwarzerritter.remotecv02, PID: 18652
                                                                                        java.lang.IllegalStateException: Could not execute method for android:onClick
                                                                                            at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:293)
                                                                                            at android.view.View.performClick(View.java:5198)
                                                                                            at android.view.View$PerformClick.run(View.java:21147)
                                                                                            at android.os.Handler.handleCallback(Handler.java:739)
                                                                                            at android.os.Handler.dispatchMessage(Handler.java:95)
                                                                                            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$MethodAndArgsCaller.run(ZygoteInit.java:726)
                                                                                            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:616)
                                                                                         Caused by: java.lang.reflect.InvocationTargetException
                                                                                            at java.lang.reflect.Method.invoke(Native Method)
                                                                                            at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288)
                                                                                            at android.view.View.performClick(View.java:5198) 
                                                                                            at android.view.View$PerformClick.run(View.java:21147) 
                                                                                            at android.os.Handler.handleCallback(Handler.java:739) 
                                                                                            at android.os.Handler.dispatchMessage(Handler.java:95) 
                                                                                            at android.os.Looper.loop(Looper.java:148) 
                                                                                            at android.app.ActivityThread.main(ActivityThread.java:5417) 
                                                                                            at java.lang.reflect.Method.invoke(Native Method) 
                                                                                            at com.android.internal.os.ZygoteInit$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.setText(java.lang.CharSequence)' on a null object reference
                                                                                            at com.example.schwarzerritter.remotecv02.MainActivity.onClick(MainActivity.java:30)
                                                                                            at java.lang.reflect.Method.invoke(Native Method) 
                                                                                            at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:288) 
                                                                                            at android.view.View.performClick(View.java:5198) 
                                                                                            at android.view.View$PerformClick.run(View.java:21147) 
                                                                                            at android.os.Handler.handleCallback(Handler.java:739) 
                                                                                            at android.os.Handler.dispatchMessage(Handler.java:95) 
                                                                                            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) 
05-04 19:06:53.787 18652-18652/? I/Process: Sending signal. PID: 18652 SIG: 9

JAVA 代码主要 Activity//

> package com.example.schwarzerritter.remotecv02;

import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;

public class MainActivity extends AppCompatActivity {

    public Button lockB;
    public Button engB;

    public void onClick(View v){
        ProgressBar fuelProgressBar = (ProgressBar)findViewById(R.id.fuelBar);
        //if(fuelProgressBar.getProgress() > 0){
        engB.setText("ENGINE RUNNING *BRUMBRUM*");
        //}
    }

    public void lockPage(){
        lockB = (Button)findViewById(R.id.lockB);
        lockB.setOnClickListener(new View.OnClickListener(){
            @Override
            public void onClick(View v){
                Intent lockPage = new Intent(MainActivity.this, lockDoor.class);
                startActivity(lockPage);
            }
        });
    }

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        lockPage();
    }

    }

ACTIVITYXML//

android:layout_width="match_parent"
android:layout_height="match_parent"
tools:context="com.example.schwarzerritter.remotecv02.MainActivity">

<TextView
    android:id="@+id/textView"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="RemoteC"
    app:layout_constraintTop_toTopOf="parent"
    android:layout_marginTop="8dp"
    android:layout_marginLeft="8dp"
    app:layout_constraintLeft_toLeftOf="parent"
    android:layout_marginRight="8dp"
    app:layout_constraintRight_toRightOf="parent" />

<Button
    android:id="@+id/lockB"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginBottom="8dp"
    android:layout_marginLeft="64dp"
    android:layout_marginRight="8dp"
    android:layout_marginTop="39dp"
    android:fontFamily="monospace"
    android:text="LOCK"
    app:layout_constraintBottom_toTopOf="@+id/engB"
    app:layout_constraintHorizontal_bias="0.26"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toLeftOf="@+id/locationB"
    app:layout_constraintTop_toBottomOf="@+id/textView"
    app:layout_constraintVertical_bias="0.0" />

<Button
    android:id="@+id/locationB"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginRight="86dp"
    android:layout_marginTop="39dp"
    android:fontFamily="monospace"
    android:text="LOCATION"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/textView" />

<Button
    android:id="@+id/engB"
    android:layout_width="221dp"
    android:layout_height="41dp"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="8dp"
    android:layout_marginTop="51dp"
    android:fontFamily="monospace"
    android:text="START ENGINE"
    android:onClick="onClick"
    app:layout_constraintHorizontal_bias="0.503"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/locationB" />

<ProgressBar
    android:id="@+id/fuelBar"
    style="?android:attr/progressBarStyleHorizontal"
    android:layout_width="220dp"
    android:layout_height="18dp"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="8dp"
    android:max="100"
    android:progress="75"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    android:layout_marginTop="8dp"
    app:layout_constraintTop_toBottomOf="@+id/fuelT" />

<TextView
    android:id="@+id/fuelT"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="8dp"
    android:fontFamily="monospace"
    android:text="FUEL BAR"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintRight_toRightOf="parent"
    android:layout_marginTop="52dp"
    app:layout_constraintTop_toBottomOf="@+id/engB" />

<TextView
    android:id="@+id/acT"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="82dp"
    android:layout_marginTop="27dp"
    android:fontFamily="monospace"
    android:text="AC:"
    android:textSize="20sp"
    app:layout_constraintLeft_toLeftOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/fuelBar" />

<TextView
    android:id="@+id/textView4"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:layout_marginLeft="8dp"
    android:layout_marginRight="104dp"
    android:layout_marginTop="27dp"
    android:text="20"
    android:textSize="20sp"
    app:layout_constraintHorizontal_bias="0.416"
    app:layout_constraintLeft_toRightOf="@+id/acT"
    app:layout_constraintRight_toRightOf="parent"
    app:layout_constraintTop_toBottomOf="@+id/fuelBar" />

<Button
    android:id="@+id/button6"
    android:layout_width="38dp"
    android:layout_height="46dp"
    android:text="+"
    android:layout_marginTop="19dp"
    app:layout_constraintTop_toBottomOf="@+id/fuelBar"
    app:layout_constraintRight_toLeftOf="@+id/button8"
    android:layout_marginRight="9dp"
    app:layout_constraintLeft_toRightOf="@+id/textView4"
    android:layout_marginLeft="8dp" />

<Button
    android:id="@+id/button8"
    android:layout_width="41dp"
    android:layout_height="46dp"
    android:text="-"
    android:layout_marginTop="19dp"
    app:layout_constraintTop_toBottomOf="@+id/fuelBar"
    android:layout_marginRight="63dp"
    app:layout_constraintRight_toRightOf="parent" />

Answer 1

将此行:ProgressBarfuelProgressBar = (ProgressBar)findViewById(R.id.fuelBar);移动到您的lockPage方法:

public void lockPage(){    
        fuelProgressBar = (ProgressBar)findViewById(R.id.fuelBar); 
        //you have to find your Button before you use it.
        engB = (Button) findViewById(R.id.engB);                        
        lockB = (Button)findViewById(R.id.lockB);                                 
        lockB.setOnClickListener(new View.OnClickListener(){                      
            @Override                                                             
            public void onClick(View v){                                          
                Intent lockPage = new Intent(MainActivity.this, lockDoor.class);  
                startActivity(lockPage);                                          
            }                                                                     
        });                                                                       
    }          

当您的Activity被创建时，您无权访问Views，因为它们还不可用。

此外，您的方法 onClick 永远不会被调用，您必须将其添加到 XML 文件中，如下所示:

<Button
        android:id="@+id/engB"
        android:layout_width="221dp"
        android:layout_height="41dp"
        android:layout_marginLeft="8dp"
        android:layout_marginRight="8dp"
        android:layout_marginTop="51dp"
        android:fontFamily="monospace"
        android:text="START ENGINE"
        android:onClick="onClick"
        app:layout_constraintHorizontal_bias="0.503"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toBottomOf="@+id/locationB" />