What's the difference between " and '? [duplicate]

This question already has an answer here:

• Is there a difference between single and double quotes in Java? 2 answers

Why does my code work, when I use " in the following code:

for(int i=0;i<7;i++){
        if(grid[row][i]!=0){
            if(player == "yellow"){
                grid[row][i-1] = 'y';
            }
            else if(player == "red"){
                grid[row][i-1] = 'r';
            }
        }
    }

But don't work, when I use ' in the following code:

for(int i=0;i<7;i++){
        if(grid[row][i]!=0){
            if(player == 'yellow'){
                grid[row][i-1] = 'y';
            }
            else if(player == 'red'){
                grid[row][i-1] = 'r';
            }
        }
    }

It is always saying, "Invalid character constant".
- grid is a 2 dimensional char variable
- row is an interger
- player is the super class (window) protected and in the constructor of the super class occupied with the value yellow:

protected String player;

public window() {
    player = "yellow";
}