我在android studio中创建了clicker应用。但是,运行时,应用程序在调用“ incrementCount()”后崩溃。为什么?

迦勒·布朗(Caleb Brown):

打开应用程序后,它会运行平稳,直到我单击“增加值”按钮或“重置”按钮为止。由于某种原因,它在调用递增值()方法时遇到了麻烦,并且使应用程序崩溃。有人可以解释代码中的错误吗?

MainActivity.java

package com.example.fixedclicker;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class MainActivity extends AppCompatActivity {
    private TextView text;
    private Counter count;
//private int count;
    @Override
    protected void onCreate(Bundle savedInstanceState) {

        text = (TextView) findViewById(R.id.textView3);
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
    }


    public void incrementCount(View view) {
        count.increaseCount();
        text.setText((CharSequence) count.getCount().toString());
    }


    public void resetButton(View view) {
        count.reset();
        text.setText((CharSequence) count.getCount().toString());
    }


}

Counter.java

package com.example.fixedclicker;

public class Counter {
    private int value;
    public Counter()
    {
        value = 0;
    }
    public void increaseCount(){
        value++;
    }
    public void reset()
    {
        value = 0;
    }
    public Integer getCount()
    {
        return value;
    }
}

致命异常/主要

E/AndroidRuntime: FATAL EXCEPTION: main
    Process: com.example.fixedclicker, PID: 12147
    java.lang.IllegalStateException: Could not execute method for android:onClick
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:402)
        at android.view.View.performClick(View.java:7448)
        at android.view.View.performClickInternal(View.java:7425)
        at android.view.View.access$3600(View.java:810)
        at android.view.View$PerformClick.run(View.java:28305)
        at android.os.Handler.handleCallback(Handler.java:938)
        at android.os.Handler.dispatchMessage(Handler.java:99)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.reflect.InvocationTargetException
        at java.lang.reflect.Method.invoke(Native Method)
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:397)
        at android.view.View.performClick(View.java:7448) 
        at android.view.View.performClickInternal(View.java:7425) 
        at android.view.View.access$3600(View.java:810) 
        at android.view.View$PerformClick.run(View.java:28305) 
        at android.os.Handler.handleCallback(Handler.java:938) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.Integer com.example.fixedclicker.Counter.getCount()' on a null object reference
        at com.example.fixedclicker.MainActivity.incrementCount(MainActivity.java:25)
        at java.lang.reflect.Method.invoke(Native Method) 
        at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:397) 
        at android.view.View.performClick(View.java:7448) 
        at android.view.View.performClickInternal(View.java:7425) 
        at android.view.View.access$3600(View.java:810) 
        at android.view.View$PerformClick.run(View.java:28305) 
        at android.os.Handler.handleCallback(Handler.java:938) 
        at android.os.Handler.dispatchMessage(Handler.java:99) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947) 

清单代码(XML)

<manifest xmlns:android="http://schemas.android.com/apk/res/android"
    package="com.example.fixedclicker">

    <application
        android:allowBackup="true"
        android:icon="@mipmap/ic_launcher"
        android:label="@string/app_name"
        android:roundIcon="@mipmap/ic_launcher_round"
        android:supportsRtl="true"
        android:theme="@style/AppTheme">
        <activity android:name=".MainActivity">
            <intent-filter>
                <action android:name="android.intent.action.MAIN" />

                <category android:name="android.intent.category.LAUNCHER" />
            </intent-filter>
        </activity>
    </application>

</manifest>

多亏有谁能帮忙!

雅各布·卡杜拉:

尝试在setContentView()调用之后设置TextView的引用

    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    text = (TextView) findViewById(R.id.textView3);

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