因此，基本上，当我启动程序时，它可以正常工作，但是当我按“后退”按钮返回到上一个活动时，它几乎立即崩溃。我已经使用调试工具进行了检查，它可以完成我的onActivity Result方法。
错误日志
2018-11-13 00:37:48.874 8349-8349/com.example.caesp.todolist E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.caesp.todolist, PID: 8349
java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.Object.toString()' on a null object reference
at android.widget.ArrayAdapter.createViewFromResource(ArrayAdapter.java:445)
at android.widget.ArrayAdapter.getView(ArrayAdapter.java:407)
at android.widget.AbsListView.obtainView(AbsListView.java:3189)
at android.widget.ListView.makeAndAddView(ListView.java:2197)
at android.widget.ListView.fillDown(ListView.java:824)
at android.widget.ListView.fillSpecific(ListView.java:1534)
at android.widget.ListView.layoutChildren(ListView.java:1914)
at android.widget.AbsListView.onLayout(AbsListView.java:2961)
at android.view.View.layout(View.java:20836)
at android.view.ViewGroup.layout(ViewGroup.java:6401)
at android.support.constraint.ConstraintLayout.onLayout(ConstraintLayout.java:1915)
at android.view.View.layout(View.java:20836)
at android.view.ViewGroup.layout(ViewGroup.java:6401)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at android.view.View.layout(View.java:20836)
at android.view.ViewGroup.layout(ViewGroup.java:6401)
at android.support.v7.widget.ActionBarOverlayLayout.onLayout(ActionBarOverlayLayout.java:444)
at android.view.View.layout(View.java:20836)
at android.view.ViewGroup.layout(ViewGroup.java:6401)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at android.view.View.layout(View.java:20836)
at android.view.ViewGroup.layout(ViewGroup.java:6401)
at android.widget.LinearLayout.setChildFrame(LinearLayout.java:1791)
at android.widget.LinearLayout.layoutVertical(LinearLayout.java:1635)
at android.widget.LinearLayout.onLayout(LinearLayout.java:1544)
at android.view.View.layout(View.java:20836)
at android.view.ViewGroup.layout(ViewGroup.java:6401)
at android.widget.FrameLayout.layoutChildren(FrameLayout.java:323)
at android.widget.FrameLayout.onLayout(FrameLayout.java:261)
at com.android.internal.policy.DecorView.onLayout(DecorView.java:944)
at android.view.View.layout(View.java:20836)
at android.view.ViewGroup.layout(ViewGroup.java:6401)
at android.view.ViewRootImpl.performLayout(ViewRootImpl.java:2948)
at android.view.ViewRootImpl.performTraversals(ViewRootImpl.java:2635)
at android.view.ViewRootImpl.doTraversal(ViewRootImpl.java:1779)
at android.view.ViewRootImpl$TraversalRunnable.run(ViewRootImpl.java:7810)
at android.view.Choreographer$CallbackRecord.run(Choreographer.java:911)
at android.view.Choreographer.doCallbacks(Choreographer.java:723)
at android.view.Choreographer.doFrame(Choreographer.java:658)
at android.view.Choreographer$FrameDisplayEventReceiver.run(Choreographer.java:897)
at android.os.Handler.handleCallback(Handler.java:789)
at android.os.Handler.dispatchMessage(Handler.java:98)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6938)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.Zygote$MethodAndArgsCaller.run(Zygote.java:327)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1374)
现在，我的目标是仅解决崩溃问题。
对不起，我忘记添加实际的代码。
package com.example.caesp.todolist;
import android.content.Intent;
import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.ListView;
import java.util.ArrayList;
public class MainActivity extends AppCompatActivity {
ListView ListofList;
ArrayList<String> Lists;
ArrayAdapter<String> ListAdapter;
Button button;
ArrayList<String> Subject = new ArrayList<>();
ArrayList<String> tit = new ArrayList<>();
int Size;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
ListofList = findViewById(R.id.LofL);
Lists = new ArrayList<String>();
ListAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, Lists);
ListofList.setAdapter(ListAdapter);
button = findViewById(R.id.AddList);
Lists.add("Test");
ListofList.setOnItemClickListener(new AdapterView.OnItemClickListener() {
@Override
public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
Intent i = new Intent(getBaseContext(), ItemList.class);
String s = (String) ListofList.getItemAtPosition(position);
i.putExtra("Title", s);
i.putExtra("size", Size);
int a = 1;
for (String t : Subject) {
i.putExtra("Item" + a, t);
a++;
}
a = 1;
for (String u : tit) {
i.putExtra("title" + a, u);
a++;
}
startActivityForResult(i, 2);
}
});
button.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
Intent i = new Intent(getBaseContext(), CreateItem.class);
String message = "Please enter the name of the list below.";
i.putExtra("mess", message);
startActivityForResult(i, 1);
}
});
}
@Override
protected void onActivityResult (int requestCode, int resultCode, Intent data) {
Bundle extras = getIntent().getExtras();
if (data == null) {
}
if (data != null) {
String item = (data.getStringExtra("item"));
Lists.add(item);
ListAdapter.notifyDataSetChanged();
}
if (requestCode == 2) {
ArrayList<String> subject = new ArrayList<String>();
ArrayList<String> title = new ArrayList<String>();
int i = 1;
assert data != null;
int size = (data.getIntExtra("ListSize", 0));
Size = size;
while (i <= size) {
String item = (data.getStringExtra("item" + i));
subject.add(item);
i++;
}
i = 1;
while (i <= size) {
String item = (data.getStringExtra("title" + i));
title.add(item);
i++;
}
Subject = subject;
tit = title;
}
}
}
答案 0 :(得分：0)
我发现我的代码有问题。从单独的活动返回后，onActivityResult在以下位置创建了空值
if (data != null) {
String item = (data.getStringExtra("item"));
Lists.add(item);
ListAdapter.notifyDataSetChanged();
}
由于没有Extra称为“ item”，因此它创建了一个空值并将其添加到ListAdapter。抱歉，如果您这样做，那会浪费您的时间。