我正在尝试制作基本的应用程序，将一个图像交叉淡化为另一个图像，然后再返回。但是，当我单击第一张图片时，该应用程序不断崩溃。
交叉淡入淡出方法fade独立工作。自从我添加了第二种将图像转换回来的方法以来，它刚刚开始崩溃。
我看到了InvocationTargetException，NullPointerException但我不明白这是如何/为什么会发生的。还有onClick设置为正确的方法。
package com.example.richardcurteis.layoutdemo;
import android.os.Bundle;
import android.support.design.widget.FloatingActionButton;
import android.support.design.widget.Snackbar;
import android.support.v7.app.AppCompatActivity;
import android.support.v7.widget.Toolbar;
import android.view.View;
import android.view.Menu;
import android.view.MenuItem;
import android.widget.ImageView;
public class MainActivity extends AppCompatActivity {
ImageView obama;
ImageView will;
public void fade(View view) {
obama = (ImageView) findViewById(R.id.obama);
will = (ImageView) findViewById(R.id.will);
obama.animate().alpha(0f).setDuration(2000);
will.animate().alpha(1f).setDuration(2000);
}
public void fadeBack(View view) {
will.animate().alpha(1f).setDuration(2000);
obama.animate().alpha(0f).setDuration(2000);
}
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
Toolbar toolbar = (Toolbar) findViewById(R.id.toolbar);
setSupportActionBar(toolbar);
FloatingActionButton fab = (FloatingActionButton) findViewById(R.id.fab);
fab.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View view) {
Snackbar.make(view, "Replace with your own action", Snackbar.LENGTH_LONG)
.setAction("Action", null).show();
}
});
}
@Override
public boolean onCreateOptionsMenu(Menu menu) {
// Inflate the menu; this adds items to the action bar if it is present.
getMenuInflater().inflate(R.menu.menu_main, menu);
return true;
}
@Override
public boolean onOptionsItemSelected(MenuItem item) {
// Handle action bar item clicks here. The action bar will
// automatically handle clicks on the Home/Up button, so long
// as you specify a parent activity in AndroidManifest.xml.
int id = item.getItemId();
//noinspection SimplifiableIfStatement
if (id == R.id.action_settings) {
return true;
}
return super.onOptionsItemSelected(item);
}
}
活动主要 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: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"
app:layout_behavior="@string/appbar_scrolling_view_behavior"
tools:context="com.example.richardcurteis.layoutdemo.MainActivity"
tools:showIn="@layout/activity_main"
android:clickable="false">
<ImageView
android:layout_width="match_parent"
android:layout_height="match_parent"
android:id="@+id/obama"
android:layout_alignParentTop="true"
android:layout_alignParentStart="true"
android:src="@drawable/obama"
android:onClick="fade" />
<ImageView
android:layout_width="fill_parent"
android:layout_height="fill_parent"
android:id="@+id/will"
android:layout_alignTop="@+id/obama"
android:layout_alignParentStart="true"
android:src="@drawable/will"
android:alpha="0"
android:onClick="fadeBack" />
</RelativeLayout>
错误：
12-29 17:17:32.003 3368-3368/com.example.richardcurteis.layoutdemo E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.richardcurteis.layoutdemo, PID: 3368
java.lang.IllegalStateException: Could not execute method for android:onClick
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:275)
at android.view.View.performClick(View.java:5198)
at android.view.View$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:270)
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 'android.view.ViewPropertyAnimator android.widget.ImageView.animate()' on a null object reference
at com.example.richardcurteis.layoutdemo.MainActivity.fadeBack(MainActivity.java:29)
at java.lang.reflect.Method.invoke(Native Method)
at android.support.v7.app.AppCompatViewInflater$DeclaredOnClickListener.onClick(AppCompatViewInflater.java:270)
at android.view.View.performClick(View.java:5198)
at android.view.View$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)