How do i make an image solid?

How can I make an image solid? For example, I am making a game in Java. My basic character movement is working fine. However, i need to add a block (draw an image using paintComponent (Graphics g ) method ). I dont want my character moved character to go through it. How can I set a perimeters so that it wont go through the block? Can someone please provide me with some guidance? Thank you so much