activity_main.xmlUser.javaMainActivity.java
<?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"
    android:gravity="center"
    tools:context="com.codinginflow.spinnercustomobjectexample.MainActivity">

    <Spinner
        android:id="@+id/spinner"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content" />

    <Button
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:onClick="getSelectedUser"
        android:text="Get selected User" />

</LinearLayout>
package com.codinginflow.spinnercustomobjectexample;

public class User {
    private String name;
    private int age;
    private String mail;

    public User(String name, int age, String mail) {
        this.name = name;
        this.age = age;
        this.mail = mail;
    }

    public String getName() {
        return name;
    }

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

    public int getAge() {
        return age;
    }

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

    public String getMail() {
        return mail;
    }

    public void setMail(String mail) {
        this.mail = mail;
    }

    @Override
    public String toString() {
        return name;
    }
}
package com.codinginflow.spinnercustomobjectexample;

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.Spinner;
import android.widget.Toast;

import java.util.ArrayList;
import java.util.List;

public class MainActivity extends AppCompatActivity {
    private Spinner spinner;

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

        spinner = findViewById(R.id.spinner);

        List<User> userList = new ArrayList<>();
        User user1 = new User("Jim", 20, "jim@gmail.com");
        userList.add(user1);
        User user2 = new User("John", 23, "john@gmail.com");
        userList.add(user2);
        User user3 = new User("Jenny", 25, "jenny@gmail.com");
        userList.add(user3);

        ArrayAdapter<User> adapter = new ArrayAdapter<User>(this,
                android.R.layout.simple_spinner_item, userList);
        adapter.setDropDownViewResource(android.R.layout.simple_spinner_dropdown_item);

        spinner.setAdapter(adapter);

        spinner.setOnItemSelectedListener(new AdapterView.OnItemSelectedListener() {
            @Override
            public void onItemSelected(AdapterView<?> parent, View view, int position, long id) {
                User user = (User) parent.getSelectedItem();
                displayUserData(user);
            }

            @Override
            public void onNothingSelected(AdapterView<?> parent) {

            }
        });
    }

    public void getSelectedUser(View v) {
        User user = (User) spinner.getSelectedItem();
        displayUserData(user);
    }

    private void displayUserData(User user) {
        String name = user.getName();
        int age = user.getAge();
        String mail = user.getMail();

        String userData = "Name: " + name + "\nAge: " + age + "\nMail: " + mail;

        Toast.makeText(this, userData, Toast.LENGTH_LONG).show();
    }
}