Highlight Button onClick in if clause

I am recently struggling with creating a java based code on Android Studio which I am working on since 2days now. Unfortunately, I cannot find any resolution for my problem on the stackoverflow or any other forum..

I try to highlight a button when clicked in red or green depending on if the buttons text is text1 or text2 of a string-array.

If I run the application it always shows me an error or the button simply doesn't change it's color when I press it. I guess that this has to do with the if-clause, but I don't really know how to solve this problem in any other way.

I would really appreciate any help on this matter!

XML code for green button

<item
    android:state_pressed="true"
    android:drawable="#81c784"/>
<item
    android:state_selected="true"
    android:drawable="@color/#4caf50"/>

XML code for red button

    <item
    android:state_pressed="true"
    android:drawable="#FF4081"/>
    <item
    android:state_selected="true"
    android:drawable="#d50000"/>

Java code:

Button button1;
Button button2;

String[] txtString;            
@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.first_layout);

    txtString = getResources().getStringArray(R.array.optiontxt);
    String random = answerString[new random().nextInt(txtString.length)];


    button1 = (Button)findViewById(R.id.btn1);
    button2 = (Button)findViewById(R.id.btn2);


    if (random == txtString[0])
    {
        button1.setText(txtString[0]);
        button1.setBackgroundResource(R.drawable.button_green);
        button2.setText(txtString[1]);
        button2.setBackgroundResource(R.drawable.button_red);

    }
    else
    {
        button1.setText(txtString[1]);
        button1.setBackgroundResource(R.drawable.button_red);
        button2.setText(txtString[0]);
        button2.setBackgroundResource(R.drawable.button_green);
    }       button1.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {}}; button2.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {}};

I also tried varies other codes which didn't work aswell, like

button1.setBackground(Drawable.createFromPath("/drawable/button_green"));

or

button1.getBackground().setState(new int[]{R.drawable.button_green});

or

button1.setBackgroundResource(R.drawable.button_green);

If anybody could help me with my problem I would be very thankful. Thank you for any help in advance!!