1. Integer[] numbers = {1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12};
   
   void setup(){
     println(numbers);
   }
   
   void draw(){}
   
   void mouseReleased(){
     Collections.shuffle(Arrays.asList(numbers));
     println(numbers); 
   }

And a note to your code above: You use the variable mousePressed and the function mousePressed(). This is redundant, you only need one of them (i recommend the function).

1. void mousePressed{
2.       // pick the next card
   
3.       currentCard = (currentCard++) % images.length;
4.       // show it
   
5.       image(images[currentCard], 0, 0);
   
6. }
   

1. boolean begin = false; // my splash screen
   int currentCard;
   PImage backcard;
   PImage startup;
   String[] imageNames = { "Aclubs.png", "2clubs.png", "3clubs.png","4clubs.png", "5clubs.png", "6clubs.png", "7clubs.png", "8clubs.png", "9clubs.png", "Jclubs.png", "Qclubs.png", "Kclubs.png"};
   PImage[] images = new PImage[imageNames.length];
   
   void setup() {
   Collections.shuffle(Arrays.asList(images));
   currentCard = 0;  
   
   startup = loadImage("Party.jpg");
   
   backcard = loadImage("backcard.jpg");
   size (1000, 1000);
   
     for (int i=0; i < imageNames.length; i++){
       String imageName = imageNames[i];
       images[i] = loadImage(imageName);
    }
    
   }
   
   
   void mousePressed(){
      begin = true;
      background(0);
   
     println(images);
   
   
    currentCard =(currentCard++) % images.length;
    image(images[currentCard],0, 0);
   }
   
   void draw() {
     if (begin == false) {
       image(startup, 0, 0, 1000, 1000);}
        
    
     }
    
       
    
   
      
     
   
   
   
   

1. void mousePressed() {
     begin = true;
     background(0);
     image(images[currentCard], 0, 0);
     currentCard =(++currentCard) % images.length;
   }
   

1. boolean begin; // my splash screen
2. int currentCard;
3. PImage backcard;
4. PImage startup;
5. String[] imageNames = {
6.   "Aclubs.png", "2clubs.png", "3clubs.png", "4clubs.png", "5clubs.png", "6clubs.png", "7clubs.png", "8clubs.png", "9clubs.png", "Jclubs.png", "Qclubs.png", "Kclubs.png"
7. };
8. PImage[] images = new PImage[imageNames.length];
9. void setup() {
10.   Collections.shuffle(Arrays.asList(images));
11.   currentCard = 0;  
12.   //startup = loadImage("Party.jpg");
13.   //backcard = loadImage("backcard.jpg");
14.   size (1000, 1000); 
15.   for (int i=0; i < imageNames.length; i++) {   
16.     String imageName = imageNames[i];   
17.     //images[i] = loadImage(imageName);
18.   }
19. }
20. void mousePressed() {  
21.   begin = true;  
22.   background(0); 
23.   //println(images);
24.   currentCard = (currentCard +1)% images.length; 
25.   //println(currentCard);
26. }
27. void draw() { 
28.   if (begin == false) {   
29.     //image(startup, 0, 0, 1000, 1000);
30.   }
31.   else {
32.     //image(images[currentCard], 0, 0);}
33.   }
34. }

It should be "++currentCard" instead of "currentCard++"

1. PImage img;
2. int cardNb = 7;
3.  
4. void setup()
5. {
6.   size(800, 800);
7.   smooth();
8.   img = loadImage("Card.png");
9.   imageMode(CENTER);
10. }
11.  
12. void draw()
13. {
14.   background(255);
15.  
16.   for (float a = 0; a < TWO_PI; a += TWO_PI / cardNb)
17.   {
18.     pushMatrix();
19.     translate(width / 2, height / 2);
20.     rotate(a);
21.     translate(width / 3, 0);
22.     rotate(HALF_PI);
23.     image(img, 0, 0);
24.     popMatrix();
25.   }
26. }