我的android studio代码构建成功，但输出应用程序崩溃

Question

下面是我的MainActivity的XML文件：

<?xml version="1.0" encoding="utf-8"?>
<androidx.constraintlayout.widget.ConstraintLayout 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:layout_width="match_parent"
    android:layout_height="match_parent"
    tools:context=".MainActivity">

    <TextView
        android:id="@+id/textView"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:text="Hello World!"
        app:layout_constraintBottom_toBottomOf="parent"
        app:layout_constraintHorizontal_bias="0.47"
        app:layout_constraintLeft_toLeftOf="parent"
        app:layout_constraintRight_toRightOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        app:layout_constraintVertical_bias="0.877" />

    <ImageView
        android:id="@+id/imageView"
        android:layout_width="404dp"
        android:layout_height="331dp"
        android:layout_marginStart="3dp"
        android:layout_marginTop="94dp"
        android:layout_marginEnd="4dp"
        android:layout_marginBottom="199dp"
        app:layout_constraintBottom_toTopOf="@+id/textView"
        app:layout_constraintEnd_toEndOf="parent"
        app:layout_constraintStart_toStartOf="parent"
        app:layout_constraintTop_toTopOf="parent"
        tools:ignore="ImageContrastCheck"
        tools:srcCompat="@tools:sample/avatars" />

</androidx.constraintlayout.widget.ConstraintLayout>

下面是Java文件：

package com.wavedevelopers.myapplication;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.widget.ImageView;

import java.util.Calendar;
import java.util.Date;

public class MainActivity extends AppCompatActivity {

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

        ImageView imageView;

        Calendar cal = Calendar.getInstance();

        Date now_date = null;
        cal.setTime(now_date);

        imageView = (ImageView)findViewById(R.id.content);

        if(cal.get(Calendar.DAY_OF_WEEK) == Calendar.MONDAY)
        {
            imageView.setImageResource(R.drawable.a1);
        }
        else if(cal.get(Calendar.DAY_OF_WEEK) == Calendar.TUESDAY)
        {
            imageView.setImageResource(R.drawable.a2);
        }

    }
}

示例：如果本地日历日期等于给定日期，则显示图像

Answer 1

您正在将空日期设置为CAL对象date now_date=null；将此行更改为date now_date=new date()；