It would be much easier to help if you included all the code - particularly where you instantiate your RectButton objects, and the exact nature of the error message you receive.  Not sure I follow the question about a two dimensional array of RectButtons.  Is this what you want to do:

Code:

int btnLength = 5;
// This needs to be declared outside of setup to ensure it has global scope.
RectButton[] rectBt = new RectButton[btnLength];
boolean locked;

void setup() {
  size(200,200);
  color col1 = #ff0000;
  color col2 = #ffff00;
  for(int i=0; i<btnLength; i++) {
      // Using a multiple of i means the buttons aren't all on top of each other
      rectBt[i] = new RectButton(40 * i, 20, 30, col1, col2);  
  }   
}

void draw() {
    for(int i=0; i<btnLength; i++) {
        rectBt[i].display();
        rectBt[i].update();
    }
}



class Button {
 int x, y;
 int size;
 color basecolor, highlightcolor;
 color currentcolor;
 boolean over = false;
 boolean pressed = false;  

 void update()  {
   if(over()) {
     currentcolor = highlightcolor;
   }
   else {
     currentcolor = basecolor;
   }
 }

 boolean pressed()  {
   if(over) {
     locked = true;
     return true;
   }
   else {
     locked = false;
     return false;
   }    
 }

 boolean over()  {
   return true;
 }

 boolean overRect(int x, int y, int width, int height)  {
   if (mouseX >= x && mouseX <= x+width &&
     mouseY >= y && mouseY <= y+height) {
     return true;
   }
   else {
     return false;
   }
 }
}

class RectButton extends Button {
 RectButton(int ix, int iy, int isize, color icolor, color ihighlight)  {
   x = ix;
   y = iy;
   size = isize;
   basecolor = icolor;
   highlightcolor = ihighlight;
   currentcolor = basecolor;
 }

 boolean over()  {
   if( overRect(x, y, size, size) ) {
     over = true;
     return true;
   }
   else {
     over = false;
     return false;
   }
 }

 void display()  {
   stroke(255);
   fill(currentcolor);
   rect(x, y, size, size);
 }
} 

Hello !

Thank you both for your reply!
You helped me a lot.

I post two complete codes based on yours.
Both have Mouse-Over.
The first code with an one-dimensional array. The buttons are now
clickable, Button-Number is printed in the Output-Window when clicked.

The second code with an two-dimensional array. The buttons are
clickable as well, Button-Number (x,y) is printed in the Output-Window when clicked.

A few Buttons are offline, meaning they don't exist. In fact, they exist, but are invisible and without function.

Thanks again!

Greetings, Chrisir

First-----------------------------------------------------------

int btnLength = 5;
// This needs to be declared outside of setup to ensure it has global scope.
RectButton[] rectBt = new RectButton[btnLength];
boolean locked;

void setup() {
 size(400,200);
color col1 = #ff0000;
color col2 = #ffff00;
 for(int i=0; i<btnLength; i++) {
   // Using a multiple of i means the buttons aren't all on top of each other
   rectBt[i] = new RectButton(40 * i + 11, 20, 30, col1, col2);  
 }  
}

void draw() {
 for(int i=0; i<btnLength; i++) {
   rectBt[i].display();
   rectBt[i].update();
 }
}

void mouseReleased()
{
 if  (mouseButton == LEFT) {
   for(int i=0; i<btnLength; i++) {
     if (rectBt[i].pressed()) {  
       println(i);  
     };

   }
 }
}

class Button {
 int x, y;
 int size;
 color basecolor, highlightcolor;
 color currentcolor;
 boolean over = false;
 boolean pressed = false;  

 void update()  {
   if(over()) {
     currentcolor = highlightcolor;
   }
   else {
     currentcolor = basecolor;
   }
 }

 boolean pressed()  {
   if(over) {
     locked = true;
     return true;
   }
   else {
     locked = false;
     return false;
   }    
 }

 boolean over()  {
   return true;
 }

 boolean overRect(int x, int y, int width, int height)  {
   if (mouseX >= x && mouseX <= x+width &&
     mouseY >= y && mouseY <= y+height) {
     return true;
   }
   else {
     return false;
   }
 }
}

class RectButton extends Button {
 RectButton(int ix, int iy, int isize, color icolor, color ihighlight)  {
   x = ix;
   y = iy;
   size = isize;
   basecolor = icolor;
   highlightcolor = ihighlight;
   currentcolor = basecolor;
 }

 boolean over()  {
   if( overRect(x, y, size, size) ) {
     over = true;
     return true;
   }
   else {
     over = false;
     return false;
   }
 }

 void display()  {
   stroke(255);
   fill(currentcolor);
   rect(x, y, size, size);
 }
}



Second -------------------------------------------------------

int btnLength = 5;
int btnLength2 = 15;
// This needs to be declared outside of setup to ensure it has global scope.
RectButton[][] rectBt = new RectButton[btnLength][btnLength2];
boolean locked;

void setup() {
 size(800,800);
color col1 = #ff0000;
color col2 = #ffff00;
 for(int i=0; i<btnLength; i++) {
   for(int j=0; j<btnLength2; j++) {
     // Using a multiple of i and j means the buttons aren't all on top of each other
     if (i != j) {
       rectBt[i][j] = new RectButton(40 * i + 19, 25 * j + 19, 20, col1, col2, true);  
     }  
     else {        
       rectBt[i][j] = new RectButton(40 * i + 19, 25 * j + 19, 20, col1, col2, false);  
     }
   }
 }
 rectBt[2][11].Exists = false;
}

void draw() {
 for(int i=0; i<btnLength; i++) {
   for(int j=0; j<btnLength2; j++) {
     if (rectBt[i][j].Exists) {
       rectBt[i][j].display();
       rectBt[i][j].update();
     }
   }
 }
}

void mouseReleased()
{
 if  (mouseButton == LEFT) {
   for(int i=0; i<btnLength; i++) {
     for(int j=0; j<btnLength2; j++) {
       if (rectBt[i][j].pressed()) {  
         print(i  );          
         print(' ');          
         println(j );  
       }
     }
   }
 }
}

class Button {
 int x, y;
 int size;
 color basecolor, highlightcolor;
 color currentcolor;
 boolean over = false;
 boolean pressed = false;  
 boolean Exists = false;  

 void update()  {
   if(over()) {
     currentcolor = highlightcolor;
   }
   else {
     currentcolor = basecolor;
   }
 }

 boolean pressed()  {
   if(over) {
     locked = true;
     return true;
   }
   else {
     locked = false;
     return false;
   }    
 }

 boolean over()  {
   return true;
 }

 boolean overRect(int x, int y, int width, int height)  {
   if (mouseX >= x && mouseX <= x+width &&
     mouseY >= y && mouseY <= y+height) {
     return true;
   }
   else {
     return false;
   }
 }
}

class RectButton extends Button {
 RectButton(int ix, int iy, int isize, color icolor, color ihighlight, boolean iExist)  {
   x = ix;
   y = iy;
   size = isize;
   basecolor = icolor;
   highlightcolor = ihighlight;
   currentcolor = basecolor;
   Exists = iExist;
 }

 boolean over()  {
   if( overRect(x, y, size, size) ) {
     over = true;
     return true;
   }
   else {
     over = false;
     return false;
   }
 }

 void display()  {
   stroke(255);
   fill(currentcolor);
   rect(x, y, size, size);
 }
}