activity_main.xmlMainActivity.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_horizontal"
    android:orientation="vertical"
    android:padding="16dp"
    tools:context="com.codinginflow.textwatcherexample.MainActivity">

    <EditText
        android:id="@+id/edit_text_username"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Username"
        android:inputType="text" />

    <EditText
        android:id="@+id/edit_text_password"
        android:layout_width="match_parent"
        android:layout_height="wrap_content"
        android:hint="Password"
        android:inputType="textPassword" />

    <Button
        android:id="@+id/button_confirm"
        android:layout_width="wrap_content"
        android:layout_height="wrap_content"
        android:enabled="false"
        android:text="Confirm" />

</LinearLayout>
package com.codinginflow.textwatcherexample;

import android.support.v7.app.AppCompatActivity;
import android.os.Bundle;
import android.text.Editable;
import android.text.TextWatcher;
import android.widget.Button;
import android.widget.EditText;

public class MainActivity extends AppCompatActivity {
    private EditText editTextUsername;
    private EditText editTextPassword;
    private Button buttonConfirm;

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

        editTextUsername = findViewById(R.id.edit_text_username);
        editTextPassword = findViewById(R.id.edit_text_password);
        buttonConfirm = findViewById(R.id.button_confirm);

        editTextUsername.addTextChangedListener(loginTextWatcher);
        editTextPassword.addTextChangedListener(loginTextWatcher);
    }

    private TextWatcher loginTextWatcher = new TextWatcher() {
        @Override
        public void beforeTextChanged(CharSequence s, int start, int count, int after) {

        }

        @Override
        public void onTextChanged(CharSequence s, int start, int before, int count) {
            String usernameInput = editTextUsername.getText().toString().trim();
            String passwordInput = editTextPassword.getText().toString().trim();

            buttonConfirm.setEnabled(!usernameInput.isEmpty() && !passwordInput.isEmpty());
        }

        @Override
        public void afterTextChanged(Editable s) {

        }
    };
}