drawing a board in which each tile has a 0.4 chance to be filled

I'm new to making java games and especially new to java graphics.
I'm trying to make a game in which each tile of the board has a 40% chance to be a filled barrier.
I have coded some parts,but it doesn't work the way I want it to.It fills up the entire board,but I want only some parts to be a barrier.
Here is my chance and drawing method code:

    /////////////////////////////////////////
	//Draws the graphics needed for the game.
	////////////////////////////////////////
	public void paint(Graphics g) { 
		
		g.setColor(Color.BLUE);
		
		
		for(int i=1;i<boardWidth/10;i++) {
			g.drawLine(i*100, 100, i*100, HEIGHT-100);
		}
		
		
		for(int j=1;j<boardHeight/10;j++)
			g.drawLine(100, j*100, WIDTH-100, j*100);
		
		
		for(int i=100;i<WIDTH-200;i++)
			for(int j=100;j<HEIGHT-200;j++)
				if(randomBarrier()) 
			    g.fillRect(i, j, 100, 100);
			   
		
	}
	
	
	
	
	///////////////////////////////////////////////////////////////////////////
	//finds a random place for the barrier objects in the beginning of the game.
	///////////////////////////////////////////////////////////////////////////

	public static boolean randomBarrier() {  //Should put all the parts to this method to see if the are barrier material or not.
		
		
		int row = WIDTH/100;
		int column = HEIGHT/100;
		
		int min = 0;
		int max = row*column;
		
		double random = Math.random();
		
		if(random<0.4)
			return true;
		
	    else if(random>=0.4)
		   return false;
		
		return true;
		
	}

Could someone please help me with this?

A painting method should NEVER change the state of the component (you can't control when Swing determines the component needs to be repainted).

Therefore you should NOT be using random code in the painting method.

Instead you can:

1. create a 2D array as an instance variable in your class containing boolean values
2. in the constructor of your class you iterate through all the items in the array and assign a true/false value based on your random logic
3. in the painting method, then you then iterate through the 2D array and paint each rectangle based on the value.

Also, custom painting is done by overriding the paintComponent() method not paint() and you need to invoke super.paintComponent().

Your painting code uses too many hard coded values. Instead you should be using instance variables in your class to avoid confusion. Something like:

int rows = 100;
int columns = 200;
int tileSize = 10;

Then your painting code becomes (untested) something like:

    super.paintComponent();

    int width = columns * tileSize;
    int height = rows * tileSize;

    for(int i = 0; i <= rows; i++) {
        int lineY = i * tileSize;
        g.drawLine(0, lineY, width, lineY);
    }
    
    
    for(int j = 1; j <= columns; j++) {
        int lineX = j * tileSize;
        g.drawLine(lineX, 0, lineX, height);
    }
    
    
    for(int i = 0;i < rows;i++)
        for(int j=0; j < columns;j++)
            if( randomBarrier[i][j]) 
                g.fillRect(i * tileSize, j * tileSize, tileSize, tileSize);

Now you can dynamically change the rows/columns/cell size without affecting the painting code. Avoid the usage of hard coded values in your programs. Make the values properties of you class.

You are potentially drawing a 100x100 tile for every pixel on the board because i and j increment by 1. With a 40% probability of drawing the tile, it's extremely likely you end drawing on all pixels on the board. You should increment i and j by the tile size 100:

    for(int i=100; i<WIDTH-200; i += 100)
        for(int j=100; j<HEIGHT-200; j += 100)
            if (randomBarrier()) g.fillRect(i, j, 100, 100);

You should also follow the advice in camickr's answer: With your current code, repainting the component in an animation would fill a different set of tiles and you would get a flickering effect. Or you will see all tiles get filled in because the component background does not get drawn.