<?xml version="1.0" encoding="utf-8"?>
<!--In linearlayout, screen is rendered and hence all the below
    components come line by line and provide neat layout -->
<LinearLayout 
    xmlns:android="http://schemas.android.com/apk/res/android"
    xmlns:tools="http://schemas.android.com/tools"
    android:layout_width="match_parent"
    android:layout_height="match_parent"
    android:layout_gravity="center"
    android:layout_margin="16dp"
    android:orientation="vertical"
    tools:context=".MainActivity">

    <!-- To indicate the user that first MultiAutoCompleteTextView 
         is supported with comma seperated this textview is introduced.
         It just displays Text Separated by Commas at the top -->
    <TextView
        android:id="@+id/textView"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="Text Separated by Commas"
        android:textSize="18sp" />

    <!-- 1st MultiAutoCompleteTextView instance identified with 
         multiAutoCompleteTextViewDefault and here when user starts
         to type, it will show relevant substrings and after user chooses it,
         the text is ended with "," as comma is the default tokenizer -->
    <MultiAutoCompleteTextView
        android:id="@+id/multiAutoCompleteTextViewDefault"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="20dp"
        android:ems="10"
        android:hint="Enter Search Terms here" />

    <TextView
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:gravity="center"
        android:text="Text Separated by Custom Token"
        android:textSize="18sp" />

    <!-- 2nd MultiAutoCompleteTextView instance identified with 
         multiAutoCompleteTextViewCustom and here when user starts
         to type, it will show relevant substrings and after user 
         chooses it, the text is ended with " " as code is done to 
         have space as separator -->
    <MultiAutoCompleteTextView
        android:id="@+id/multiAutoCompleteTextViewCustom"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:layout_margin="20dp"
        android:ems="10"
        android:hint="Add your necessary tags here" />

</LinearLayout>

import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.MultiAutoCompleteTextView;
import androidx.appcompat.app.AppCompatActivity;

public class MainActivity extends AppCompatActivity {

    // Defining two MultiAutoCompleteTextView
    // This is to recognize comma separated.
    MultiAutoCompleteTextView multiAutoCompleteTextViewDefault;

    // This is the second one and required for custom features
    MultiAutoCompleteTextView multiAutoCompleteTextViewCustom;

    // As a sample, few text are given below which can be populated in dropdown, when user starts typing
    // For example, when user types "a", text whichever starting with "a" will be displayed in dropdown
    // As we are using two MultiAutoCompleteTextView components, using two string array separately
    String[] fewRandomSuggestedText = {"a", "ant", "apple", "asp", "android", "animation", "adobe",
            "chrome", "chromium", "firefox", "freeware", "fedora"};
    String[] fewTags = {"Java", "JavaScript", "Spring", "Java EE", "Java 8", "Java 9", "Java 10",
            "MongoDB", "MarshMallow", "NoSQL", "NativeApp", "SQL", "SQLite"};

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

        multiAutoCompleteTextViewDefault = findViewById(R.id.multiAutoCompleteTextViewDefault);
        multiAutoCompleteTextViewCustom = findViewById(R.id.multiAutoCompleteTextViewCustom);

        // In order to show the substring options in a dropdown, we need ArrayAdapter 
        // and here it is using simple_list_item_1
        ArrayAdapter<String> randomArrayAdapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, fewRandomSuggestedText);
        multiAutoCompleteTextViewDefault.setAdapter(randomArrayAdapter);

        // setThreshold() is used to specify the number of characters after which 
        // the dropdown with the autocomplete suggestions list would be displayed.
        // For multiAutoCompleteTextViewDefault, after 1 character, the dropdown shows substring
        multiAutoCompleteTextViewDefault.setThreshold(1);

        // Default CommaTokenizer is used here
        multiAutoCompleteTextViewDefault.setTokenizer(new MultiAutoCompleteTextView.CommaTokenizer());
        ArrayAdapter<String> tagArrayAdapter = new ArrayAdapter<>(this, android.R.layout.simple_list_item_1, fewTags);
        multiAutoCompleteTextViewCustom.setAdapter(tagArrayAdapter);

        // For multiAutoCompleteTextViewCustom, after 2 characters, the dropdown shows substring
        multiAutoCompleteTextViewCustom.setThreshold(2);

        // As multiAutoCompleteTextViewCustom is customized , we are using SpaceTokenizer
        // which is written as a separate java class to handle space
        // SpaceTokenizer can be customized as per our needs, here for this example, 
        // after user types 2 character
        // the substring of the text is shown in the dropdown and once selected, 
        // a space is appended at the
        // end of the substring. So for customized MultiAutoCompleteTextView, 
        // we need to write code like SpaceTokenizer
        // It has 3 methods namely findTokenStart,findTokenEnd and terminateToken
        multiAutoCompleteTextViewCustom.setTokenizer(new SpaceTokenizer());
    }
}

import android.text.SpannableString;
import android.text.Spanned;
import android.text.TextUtils;
import android.widget.MultiAutoCompleteTextView;

// As this java class implements MultiAutoCompleteTextView.Tokenizer,
// we should write 3 methods i.e. findTokenStart,findTokenEnd and terminateToken
public class SpaceTokenizer implements MultiAutoCompleteTextView.Tokenizer {
    private int i;

    // Returns the start of the token that ends at offset cursor within text.
    public int findTokenStart(CharSequence inputText, int cursor) {
        int idx = cursor;

        while (idx > 0 && inputText.charAt(idx - 1) != ' ') {
            idx--;
        }
        while (idx < cursor && inputText.charAt(idx) == ' ') {
            idx++;
        }
        return idx;
    }

    // Returns the end of the token (minus trailing punctuation) that 
    // begins at offset cursor within text.
    public int findTokenEnd(CharSequence inputText, int cursor) {
        int idx = cursor;
        int length = inputText.length();

        while (idx < length) {
            if (inputText.charAt(i) == ' ') {
                return idx;
            } else {
                idx++;
            }
        }
        return length;
    }

    // Returns text, modified, if necessary, to ensure that it ends with a token terminator
    // (for example a space or comma).
    public CharSequence terminateToken(CharSequence inputText) {
        int idx = inputText.length();

        while (idx > 0 && inputText.charAt(idx - 1) == ' ') {
            idx--;
        }

        if (idx > 0 && inputText.charAt(idx - 1) == ' ') {
            return inputText;
        } else {
            if (inputText instanceof Spanned) {
                SpannableString sp = new SpannableString(inputText + " ");
                TextUtils.copySpansFrom((Spanned) inputText, 0, inputText.length(),
                        Object.class, sp, 0);
                return sp;
            } else {
                return inputText + " ";
            }
        }
    }
}