Review of Relational Operators

In order to bake cookies two things must be true:

• You must have 4 or more cups of flour (or more).
• You must have 2 or more cups of sugar (or more).

If one of these requirements is false, then you do not have enough ingredients. Here is a program that follows this logic (I hope I don't need to remind you to copy it and to play with it):

// Cookie Ingredients Checker
//
import java.io.*;
class CookieChecker
{
  public static void main (String[] args) throws IOException
  { 
    BufferedReader stdin = 
        new BufferedReader ( new InputStreamReader( System.in ) );
 
    String inData;
    int    sugar, flour; 

    // get the number of cups of flour
    System.out.println("How much flour do you have?");
    inData   = stdin.readLine();
    flour    = Integer.parseInt( inData ); 

    // get the number of cups of sugar
    System.out.println("How much sugar do you have?");
    inData   = stdin.readLine();
    sugar    = Integer.parseInt( inData ); 

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

  }
}

The symbol in Java that means "and" is "&&" (ampersand ampersand). The if statement is asking a question with two parts:

if ( flour >= 4 && sugar >= 2 )
    ----------     ----------
    flour part      sugar part

Each one of these parts is a relational expression. A relational expression is a type of boolean expression that uses a relational operator to compute a true or false value. The entire expression between parentheses is also a boolean expression. It is correct (and common) for a boolean expression to be composed of smaller boolean expressions. This is similar to human language where "compound sentences" are made from smaller sentences.

               
flour >= 4  ________________

sugar >= 2  ________________