android工作室错误E/AndroidRuntime:致命异常:main

Question

This a error
E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.loginappaplication, PID: 20994
java.lang.RuntimeException: Unable to instantiate activity ComponentInfo{com.example.loginappaplication/com.example.loginappaplication.MainActivity}: java.lang.InstantiationException: java.lang.Class<com.example.loginappaplication.MainActivity> has no zero argument constructor
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3703)
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83)
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2216)
at android.os.Handler.dispatchMessage(Handler.java:107)
at android.os.Looper.loop(Looper.java:237)
at android.app.ActivityThread.main(ActivityThread.java:7948)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075)
Caused by: java.lang.InstantiationException: java.lang.Class<com.example.loginappaplication.MainActivity> has no zero argument constructor
at java.lang.Class.newInstance(Native Method)
at android.app.AppComponentFactory.instantiateActivity(AppComponentFactory.java:95)
at androidx.core.app.CoreComponentFactory.instantiateActivity(CoreComponentFactory.java:45)
at android.app.Instrumentation.newActivity(Instrumentation.java:1251)
at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3437)
at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3703) 
at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:83) 
at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2216) 
at android.os.Handler.dispatchMessage(Handler.java:107) 
at android.os.Looper.loop(Looper.java:237) 
at android.app.ActivityThread.main(ActivityThread.java:7948) 
at java.lang.reflect.Method.invoke(Native Method) 
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493) 
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1075) 

公共类MainActivity扩展了AppCompatActivity｛

private EditText eName;
private EditText ePassword;
private Button eLogin;
private TextView eAttemptsInfo;
private String Username = "Azahracaca";
private String Password = "Caca12345";
boolean isvalid = false;
private int counter = 3;
public MainActivity(Button eLogin) {
this.eLogin = eLogin;
}

@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
eName = findViewById(R.id.IdName);
ePassword = findViewById(R.id.etPassword);
eAttemptsInfo = findViewById(R.id.tvAttemptsInfo);
eLogin.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
String inputName = eName.getText().toString();
String inputPassword = ePassword.getText().toString();
if (inputName.isEmpty() || inputPassword.isEmpty())
{
Toast.makeText(MainActivity.this, "Pastikan Semuanya Benar !", Toast.LENGTH_SHORT).show();
}else{
isvalid = valited(inputName, inputPassword);
if(!isvalid){
counter--;
Toast.makeText(MainActivity.this, "Salah Memasukkan Data !", Toast.LENGTH_SHORT).show();
eAttemptsInfo.setText("Jumlah Login Tersisa : 3 Kali" + counter);
if(counter == 0){
eLogin.setEnabled(false);
}
}else {
Toast.makeText(MainActivity.this, "Login Sukses !", Toast.LENGTH_SHORT).show(); //ke menu selanjutnya
Intent intent = new Intent(MainActivity.this, HomePage.class);
startActivity(intent);
}
}
}
});
}
private boolean valited(String name, String password){
if(name.equals(Username) && password.equals(Password)){
return true;
}
return  false;
}

}

Answer 1

public MainActivity(Button eLogin) {
this.eLogin = eLogin;
}

使该构造函数为空，并使用findViewById初始化eLogin按钮，就像对eName所做的那样。您应该在onCreate()中/之后初始化所有视图。无法初始化类构造函数中的视图。

Answer 2

为什么构造函数Activity传递Param？

编辑：

public class MainActivity extends AppcompatActivity

和

Button button = findViewById(R.id....)

Answer 3

结构应该像

public class MainActivity extends AppCompatActivity {
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);
}
}