Hi i have a problem and i dont know what wrong whit the code

I'm trying to get the String in the MainActivity2 to create a "login" page and the MyApplication should be the password so i can change it with a set in another Activity

Error :

Process: com.example.test1, PID: 23014
    java.lang.RuntimeException: Unable to start activity ComponentInfo{com.example.test1/com.example.test1.MainActivity2}: java.lang.ClassCastException: android.app.Application cannot be cast to com.example.test1.MyApplication
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3449)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601)
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85)
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135)
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95)
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066)
        at android.os.Handler.dispatchMessage(Handler.java:106)
        at android.os.Looper.loop(Looper.java:223)
        at android.app.ActivityThread.main(ActivityThread.java:7656)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592)
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)
     Caused by: java.lang.ClassCastException: android.app.Application cannot be cast to com.example.test1.MyApplication
        at com.example.test1.MainActivity2.onCreate(MainActivity2.java:28)
        at android.app.Activity.performCreate(Activity.java:8000)
        at android.app.Activity.performCreate(Activity.java:7984)
        at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1309)
        at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:3422)
        at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:3601) 
        at android.app.servertransaction.LaunchActivityItem.execute(LaunchActivityItem.java:85) 
        at android.app.servertransaction.TransactionExecutor.executeCallbacks(TransactionExecutor.java:135) 
        at android.app.servertransaction.TransactionExecutor.execute(TransactionExecutor.java:95) 
        at android.app.ActivityThread$H.handleMessage(ActivityThread.java:2066) 
        at android.os.Handler.dispatchMessage(Handler.java:106) 
        at android.os.Looper.loop(Looper.java:223) 
        at android.app.ActivityThread.main(ActivityThread.java:7656) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:592) 
        at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:947)

MyApplication Activity:

package com.example.test1;

import android.app.Application;


public class MyApplication extends Application {

    private String vpassword = "1234";


    public String getSomeVariable() {
        return vpassword;
    }

    public void setSomeVariable(String vpassword) {
        this.vpassword = vpassword;
    }
}

MainActivity2

package com.example.test1;

import androidx.appcompat.app.AppCompatActivity;
import android.content.Intent;
import android.os.Bundle;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import com.google.android.material.button.MaterialButton;

public class MainActivity2 extends AppCompatActivity {

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main2);

        EditText username=(EditText) findViewById(R.id.et_email1);
        EditText password=(EditText) findViewById(R.id.et_password1);


        Button button2 = (Button) findViewById(R.id.button2);
        TextView tv_cl2 = (TextView) findViewById(R.id.tv_cl2);

        String s = ((MyApplication) this.getApplication()).getSomeVariable();

        button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                if(username.getText().toString().equals("user") && password.getText().toString().equals(s)){
                    //correct
                    Intent intent = new Intent(MainActivity2.this, MainActivity4.class);
                    startActivity(intent);

                    Toast.makeText(MainActivity2.this, "Login SUCCESSFUL", Toast.LENGTH_SHORT).show();
                }else{
                    //incorrect

                    Toast.makeText(MainActivity2.this, "Login FAILED!!", Toast.LENGTH_SHORT).show();
                }

            }
        });
        tv_cl2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                Intent intent = new Intent(MainActivity2.this, MainActivity3.class);
                startActivity(intent);
            }
        });
    }
}

CodePudding user response：

Add android:name="com.example.test1.MyApplication" to your <application> element in your manifest. Without that, Android will not know to use MyApplication.

For more, see https://developer.android.com/guide/topics/manifest/application-element#nm and https://developer.android.com/reference/android/app/Application.