我在MainActivity中实现了一个接口，在回调方法中我想更新我的TextView但是我得到了nullpointer异常。
这是我的MainActivity类
public class MainActivity extends AppCompatActivity implements GenericCallback
{
Context mcontext;
TextView tv;
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mcontext = this;
Button btn = findViewById(R.id.btn);
tv = findViewById(R.id.tv);
btn.setOnClickListener(new View.OnClickListener()
{
@Override
public void onClick(View v)
{
startActivity(new Intent(MainActivity.this,SecondActivity.class));
}
});
}
@Override
public void doSomething(Context context, String... a)
{
Toast.makeText(context,"Calback"+a[0]+a[1],Toast.LENGTH_SHORT).show();
tv = findViewById(R.id.tv);//Line 43
tv.setText(a[0]+a[1]);
}
我的界面看起来像这样
public interface GenericCallback
{
void doSomething(Context context, String... a);
}
我的SecomdActivity
public class SecondActivity extends AppCompatActivity
{
Context context;
GenericCallback genericCallback = new MainActivity();
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
context = this;
String a="Secomd",b = "Activity";
genericCallback.doSomething(context,a,b);
finish();
}
}
栈跟踪
引起：java.lang.NullPointerException：尝试调用虚拟 方法'android.view.Window $ Callback 在null对象引用上的android.view.Window.getCallback（）' 在android.support.v7.app.AppCompatDelegateImplBase。（AppCompatDelegateImplBase.java:117） 在android.support.v7.app.AppCompatDelegateImplV9。（AppCompatDelegateImplV9.java:149） 在android.support.v7.app.AppCompatDelegateImplV14。（AppCompatDelegateImplV14.java:56） 在android.support.v7.app.AppCompatDelegateImplV23。（AppCompatDelegateImplV23.java:31） 在android.support.v7.app.AppCompatDelegate.create（AppCompatDelegate.java:200） 在android.support.v7.app.AppCompatDelegate.create（AppCompatDelegate.java:183） 在android.support.v7.app.AppCompatActivity.getDelegate（AppCompatActivity.java:519） 在android.support.v7.app.AppCompatActivity.findViewById（AppCompatActivity.java:190） 在android.com.callback.MainActivity.doSomething（MainActivity.java:43） 在android.com.callback.SecondActivity.onCreate（SecondActivity.java:21）
答案 0 :(得分：1)
您可以使用EventBus Library
安装
编译'org.greenrobot：eventbus：3.1.1'
首先，创建一个Java类
override func viewDidAppear(_ animated: Bool) {
super.viewDidAppear(animated)
let url = URL(string:"http://192.168.0.113/attendance/check_attend.php")
let parameters = ["emp_id": self.att_emp_id]
var request = URLRequest(url : url!)
request.httpMethod = "POST"
request.httpBody = try? JSONSerialization.data(withJSONObject:parameters, options: [])
request.addValue("application/json", forHTTPHeaderField: "Content-Type")
let session = URLSession.shared
session.dataTask(with: request, completionHandler: { (data, response, error) in
if let data = data {
do {
let json = try? JSONSerialization.jsonObject(with: data, options: []) as! Dictionary<String, Any>
if let json = json {
print("HERE SHOULD BE YOUR JSON \(json)")
//var datas = [String:AnyObject]()
if let datas = json["data"] as? [String:AnyObject] , let in_time_Str = datas["in_time"] as? String, let out_time_Str = datas["out_time"] as? String , let grace_Str = datas["grace"] as? String, let signal_Str = datas["signal"] as? String
{
print("here is attendance Data : \(datas)")
print("Here is attendance Signal : \(signal_Str)")
//datas = data
if in_time_Str != ""
{
self.server_in_time = in_time_Str
print("Here is attendance IN TIME : \(self.server_in_time)")
}
if out_time_Str != ""
{
self.server_out_time = out_time_Str
print("Here is attendance OUT TIME : \(self.server_out_time)")
}
if signal_Str == "10" {
self.grace_str_time = grace_Str
DispatchQueue.main.async {
self.out_time_button.isEnabled = false
}
} else if signal_Str == "01" {
self.server_in_time = in_time_Str
DispatchQueue.main.async {
self.inTimeTextField.text = self.server_in_time
self.in_time_button.isEnabled = false
self.out_time_button.isEnabled = true
}
} else if signal_Str == "00" {
self.server_out_time = out_time_Str
DispatchQueue.main.async {
self.inTimeTextField.text = self.server_in_time
self.outTimeTextField.text = self.server_out_time
self.in_time_button.isEnabled = false
self.out_time_button.isEnabled = false
}
} else {
print("Error : \(String(describing: error))")
}
}
}
}
} else {
print("Error \(String(describing: error?.localizedDescription))")
}
}).resume()
}
然后在SecondActivity中
public class MsgEvent {
String oo;
public String getOo() {
return oo;
}
public void setOo(String oo) {
this.oo = oo;
}
}
然后在MainActivity中
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
context = this;
String a="Secomd",b = "Activity";
MsgEvent msgEvent = new MsgEvnet();
msgEvent.setOo(a);
EventBus.getDefault().post(msgEvent);
finish();
}
答案 1 :(得分：1)
您可以使用 BroadcastReceiver
检查以下示例
<强> MainActivity
public class MainActivity extends AppCompatActivity {
TextView mTitle;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
mTitle = findViewById(R.id.toolbar_title);
startActivity(new Intent(this, SecondActivity.class));
}
private BroadcastReceiver myBroadcastReceiver = new BroadcastReceiver() {
@Override
public void onReceive(Context context, Intent intent) {
mTitle.setText(intent.getStringExtra("DATA"));
Toast.makeText(context, "recieved text : " + intent.getStringExtra("DATA"), Toast.LENGTH_SHORT).show();
}
};
@Override
protected void onResume() {
IntentFilter filter = new IntentFilter();
filter.addAction("MY_ACTION");
registerReceiver(myBroadcastReceiver, filter);
super.onResume();
}
@Override
protected void onPause() {
super.onPause();
}
}
XML表格MainActivity
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
xmlns:app="http://schemas.android.com/apk/res-auto"
android:id="@+id/toolbarIcon"
android:layout_width="match_parent"
android:layout_height="wrap_content"
android:gravity="center"
android:orientation="vertical">
<TextView
android:id="@+id/toolbar_title"
android:layout_width="wrap_content"
android:layout_height="wrap_content"
android:layout_marginTop="50dp"
android:text="Nilesh"
android:textStyle="bold" />
</LinearLayout>
<强> Second_activity
public class SecondActivity extends AppCompatActivity {
TextView tvText;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
getSupportActionBar().setTitle("SECOND Activity");
tvText = findViewById(R.id.tvText);
tvText.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
sendBroadcast();
}
});
}
public void sendBroadcast() {
Intent broadcast = new Intent();
broadcast.putExtra("DATA", "MY NAME IS NILESH");
broadcast.setAction("MY_ACTION");
sendBroadcast(broadcast);
}
}
第二项活动的布局
<?xml version="1.0" encoding="utf-8"?>
<LinearLayout 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=".SecondActivity">
<TextView
android:layout_width="match_parent"
android:id="@+id/tvText"
android:text="Click to Send Broadcas"
android:layout_height="wrap_content" />
</LinearLayout>
答案 2 :(得分：0)
您获得该null异常的原因是您的MainActivity视图不会膨胀，因为它没有被框架实例化。
您可以从第一个活动创建实例getter方法。
private static GenericCallback instance;
public static GenericCallback getHandler() {
return instance;
}
@Override
protected void onCreate(Bundle savedInstanceState)
{
...
instance = this;
...
}
然后在第二个活动中，获取该实例而不是实例化活动。
@Override
protected void onCreate(Bundle savedInstanceState)
{
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_second);
context = this;
String a="Secomd",b = "Activity";
GenericCallback genericCallback = MainActivity.getHandler();
genericCallback.doSomething(context,a,b);
finish();
}
答案 3 :(得分：0)
将GenericCallback genericCallback = new MainActivity();更改为
SecondActivity中的GenericCallback genericCallback;
在onCreate中声明genericCallback对象中的值，如genericCallback = this;