&&

Examine this part of the program:

// check that there is enough of both ingredients
if ( flour >= 4 && sugar >= 2 )
  System.out.println("Enough for cookies!" );
else
  System.out.println("sorry...." );

For you to have enough ingredients, both relational expressions must be true. This is the role of the && (and-operator) between the two relational expressions. The && requires that both

flour >= 4 

and

sugar >= 2 

are true for the entire expression to be true. The entire question must be true in order for the true branch to execute.

The and operator && is a logical operator. A logical operator examines two true/false values and outputs a single true/false value.