/**Getting Firebase Database data Using HashMap Used addValueEventListener for Auto data display for*/ in Android Studio Java

Third_Activity.java

package com.example.learningfirebase;

import android.os.Bundle;
import android.view.View;
import android.widget.AdapterView;
import android.widget.ArrayAdapter;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ListView;
import android.widget.ProgressBar;
import android.widget.Toast;

import androidx.annotation.NonNull;
import androidx.annotation.Nullable;
import androidx.appcompat.app.AlertDialog;
import androidx.appcompat.app.AppCompatActivity;

import com.google.android.gms.tasks.OnFailureListener;
import com.google.android.gms.tasks.OnSuccessListener;
import com.google.firebase.database.DataSnapshot;
import com.google.firebase.database.DatabaseError;
import com.google.firebase.database.DatabaseReference;
import com.google.firebase.database.FirebaseDatabase;
import com.google.firebase.database.ValueEventListener;

import java.util.ArrayList;
import java.util.HashMap;
import java.util.Iterator;

public class Third_Activity extends AppCompatActivity {
    EditText editText,editText2,editText3,editText4;
    Button save,retrive;
    ProgressBar progressBar;
    ListView listView;
    ArrayList arrayList;
    ArrayAdapter arrayAdapter;

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

        editText = findViewById(R.id.editTextTextPersonName);
        editText2 = findViewById(R.id.editTextTextPersonName2);
        editText3 = findViewById(R.id.editTextTextPersonName3);
        editText4 = findViewById(R.id.editTextTextPersonName4);
        save = findViewById(R.id.button);
        retrive = findViewById(R.id.button2);
        progressBar = findViewById(R.id.progressBar2);
        progressBar.setVisibility(View.INVISIBLE);
        listView = findViewById(R.id.listview1);

        arrayList = new ArrayList();
        arrayAdapter = new ArrayAdapter(this,android.R.layout.simple_list_item_1,arrayList);
        listView.setAdapter(arrayAdapter);
        listView.setOnItemClickListener(new AdapterView.OnItemClickListener() {
            @Override
            public void onItemClick(AdapterView<?> parent, View view, int position, long id) {
                Toast.makeText(Third_Activity.this, "Im "+(position+1), Toast.LENGTH_SHORT).show();
            }
        });

        final DatabaseReference databaseReference = FirebaseDatabase.getInstance().getReference("Worker");

        /**Create Save method data using in to Firebase Database with another class*/
        save.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                progressBar.setVisibility(View.VISIBLE);
                try { // wenas data type eken asign unoth program eka off nowenna try ekak daala exception handle karanawa
                    String name = editText.getText().toString().trim();
                    String collage = editText3.getText().toString().trim();
                    int age = Integer.parseInt(editText2.getText().toString().trim());
                    double salary =  Double.parseDouble(editText4.getText().toString().trim());

                    Worker worker = new Worker(name,collage,age,salary); /**class ekak hadanawa data key walata*/

                    String key = databaseReference.push().getKey().trim(); /**awashya thna key eka hadawala eeka string ekak widihat gannawa*/

                    databaseReference.child(key).child(name + "'s Details! ").setValue(worker).addOnSuccessListener(new OnSuccessListener<Void>() {/**data assign worker OBJECT eka database ekata
                                                                                                                                                             set karanawa   */
                        @Override
                        public void onSuccess(Void aVoid) {
                            progressBar.setVisibility(View.INVISIBLE);
                            Toast.makeText(Third_Activity.this, "Successfully Save Data!", Toast.LENGTH_SHORT).show();
                        }
                    }).addOnFailureListener(new OnFailureListener() {
                        @Override
                        public void onFailure(@NonNull Exception e) {
                            progressBar.setVisibility(View.INVISIBLE);
                            Toast.makeText(Third_Activity.this, "Something error!", Toast.LENGTH_SHORT).show();
                        }
                    });
                    editText.setText(null);
                    editText3.setText(null);
                    editText2.setText(null);
                    editText4.setText(null);
                }catch (Exception e){
                    progressBar.setVisibility(View.INVISIBLE);
                    AlertDialog.Builder builder = new AlertDialog.Builder(Third_Activity.this);
                    builder.setIcon(R.drawable.common_google_signin_btn_icon_light_normal);
                    builder.setTitle("Invalid input!");
                    builder.setMessage("Name : Text  \n Age : Number only \n Collage = Text \n Salary = Number only");
                    builder.show();
                }
            }
        });
        /**End of Save method data using in to Firebase Database with another class*/


        /**Getting Firebase Database data Using HashMap Used addValueEventListener for Auto data display for*/
        progressBar.setVisibility(View.VISIBLE);
        databaseReference.addValueEventListener(new ValueEventListener() {
            @Override
            public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                arrayAdapter.clear();
                                                                        /**********MOST IMPORTANT!***********/
                HashMap<String,HashMap<String,HashMap<String,String>>> hashMap = (HashMap<String, HashMap<String, HashMap<String, String>>>) dataSnapshot.getValue();
                for (HashMap h : hashMap.values()){
                    HashMap<String,HashMap<String,String>> hashMap1 = new HashMap<>(h);
                    for (HashMap hh : hashMap1.values()){
                        arrayList.add("Name is "+hh.get("name")+". Age is "+hh.get("age")+". Collage is "+hh.get("collage")+". Salary is "+hh.get("salary")+".");
                    }
                }
                System.out.println("*  "+dataSnapshot.getKey());
                System.out.println("*  "+dataSnapshot.getValue());
                System.out.println("*  "+dataSnapshot.getChildren());
                System.out.println("\n");
                for (DataSnapshot ds : dataSnapshot.getChildren()){
                    System.out.println("#  "+ds.getKey());
                    System.out.println("#  "+ds.getValue());
                    System.out.println("#  "+ds.getChildren());
                    System.out.println("\n");
                    for (DataSnapshot dss : ds.getChildren()){
                        System.out.println("$  "+dss.getKey());
                        System.out.println("$  "+dss.getValue());
                        System.out.println("$  "+dss.getChildren());
                        System.out.println("\n");
                        for (DataSnapshot dsss : dss.getChildren()){
                            System.out.println("&  "+dsss.getKey());
                            System.out.println("&  "+dsss.getValue());
                            System.out.println("&  "+dsss.getChildren());
                            System.out.println("\n");
                            //arrayList.add(dsss.getKey() + "  " + dsss.getValue());
                        }
                    }
                   // System.out.println(ds);
                }
                System.out.println(arrayList);
                System.out.println(dataSnapshot.getValue());
                arrayAdapter.notifyDataSetChanged();
                System.out.println(arrayList);
                //System.out.println(datas);
                progressBar.setVisibility(View.INVISIBLE);
            }
            @Override
            public void onCancelled(@NonNull DatabaseError databaseError) {

            }
        });
        /**Closing Firebase Database data Using HashMap Used addValueEventListener for Auto data display for*/


        /**Getting Firebase Database data Using HashMap Used addListenerForSingleValueEvent in to click on the button and retrive method*/
        /*retrive.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                arrayAdapter.clear();
                progressBar.setVisibility(View.VISIBLE);
                databaseReference.addListenerForSingleValueEvent(new ValueEventListener() {
                    String datas;
                    @Override
                    public void onDataChange(@NonNull DataSnapshot dataSnapshot) {
                        datas = dataSnapshot.getValue().toString();

                        HashMap<String,HashMap<String,HashMap<String,String>>> hashMap = (HashMap<String, HashMap<String, HashMap<String, String>>>) dataSnapshot.getValue();
                        for (HashMap h : hashMap.values()){
                            HashMap<String,HashMap<String,String>> hashMap1 = new HashMap<>(h);
                            for (HashMap hh : hashMap1.values()){
                                arrayList.add("Name is "+hh.get("name")+". Age is "+hh.get("age")+". Collage is "+hh.get("collage")+". Salary is "+hh.get("salary")+".");
                            }
                        }
                        arrayAdapter.notifyDataSetChanged();
                        System.out.println(arrayList);
                        //System.out.println(datas);
                        progressBar.setVisibility(View.INVISIBLE);
                    }


                    @Override
                    public void onCancelled(@NonNull DatabaseError databaseError) {

                    }
                });
            }
        });*/
        /**Ending Firebase Database data Using HashMap Used addListenerForSingleValueEvent in to click on the button and retrive method*/
    }
}







class Worker{
    String name;
    String collage;
    int age;
    double salary;

    public Worker(String name, String collage, int age, double salary) {
        this.name = name;
        this.collage = collage;
        this.age = age;
        this.salary = salary;
    }

    public String getName() {
        return name;
    }

    public void setName(String name) {
        this.name = name;
    }

    public String getCollage() {
        return collage;
    }

    public void setCollage(String collage) {
        this.collage = collage;
    }

    public int getAge() {
        return age;
    }

    public void setAge(int age) {
        this.age = age;
    }

    public double getSalary() {
        return salary;
    }

    public void setSalary(double salary) {
        this.salary = salary;
    }
}

Thanks for lot.very important to me this code.

Thanks for lot.very important to me this code.

dont think

Thanks