AlertaPersonalizada1.java

LinearLayout layout = new LinearLayout(MainActivity.this);
layout.setOrientation(LinearLayout.VERTICAL);
final EditText editTextNombre = new EditText(MainActivity.this);
editTextNombre.setHint("Nombre");
layout.addView(editTextNombre);
final EditText editTextPass = new EditText(MainActivity.this);
editTextPass.setInputType(EditorInfo.TYPE_TEXT_VARIATION_PASSWORD);
editTextPass.setHint("Contraseña");
layout.addView(editTextPass);
final AlertDialog.Builder builder = new AlertDialog.Builder(MainActivity.this);
builder
        .setTitle("El título")
        .setNegativeButton("Cancelar", null)
        .setPositiveButton("OK", new DialogInterface.OnClickListener() {
            @Override
            public void onClick(DialogInterface dialog, int which) {
                Toast.makeText(MainActivity.this, "Click en ok", Toast.LENGTH_LONG).show();
            }
        });
final AlertDialog alertDialogPersonalizado = builder.create();
alertDialogPersonalizado.setView(layout);
// después mostrarla:
alertDialogPersonalizado.show();