Question

主要活动 这些是代码

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.TextView;

public class FunfactMainActivity extends AppCompatActivity {
    // Declare our view variables
    private TextView mFactTextView;
    private Button mShowFactButton;


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

        //Assign the views from the layout file to the corresponding variables

        mFactTextView= (TextView) findViewById(R.id.factTextView);
        mShowFactButton= (Button) findViewById(R.id.showFactButton);

        View.OnClickListener listener = new View.OnClickListener() {
            @Override
            public void onClick(View view) {
                // The button was clicked, so update the fact TextView with a new fact

                String fact = "Ostriches can run faster than horses";
                mFactTextView.setText(fact);
            }
        };
        mShowFactButton.setOnClickListener(listener);
    }
}

main.xml中 这些是代码

<?xml version="1.0" encoding="utf-8"?>
<RelativeLayout xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    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"
    tools:context="com.example.oluwatomisinekpek.funfacts.FunfactMainActivity"
    android:background="@android:color/holo_green_dark">

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Did you know?"
        android:id="@+id/textView2"
        android:textSize="20sp"
        android:textColor="#80ffffff" />

    <TextView
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:textAppearance="?android:attr/textAppearanceMedium"
        android:text="Ants stretch when they wake up in the morning."
        android:id="@+id/textView"
        android:layout_below="@+id/textView2"
        android:layout_centerHorizontal="true"
        android:layout_marginTop="150dp"
        android:textSize="24sp"
        android:textColor="@android:color/white" />

    <Button
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:text="Next Fun Fact"
        android:id="@+id/Funfactbutton"
        android:layout_alignParentBottom="true"
        android:layout_centerHorizontal="true"
        android:background="@android:color/white" />

</RelativeLayout>

logcat的

这些是错误报告

java.lang.NullPointerException: Attempt to invoke virtual method 'void android.widget.Button.setOnClickListener(android.view.View$OnClickListener)' on a null object reference

我是android开发的新手。到目前为止，我了解基础知识并学习如何调试应用程序。我发现了车祸，但不知道怎么回事。

Answer 1

您没有ID showFactButton的任何按钮。将mShowFactButton代码更改为：

mShowFactButton= (Button) findViewById(R.id.Funfactbutton);

`

在null对象引用上调用虚方法的解决方案

1 个答案: