Operators and conditionals

Operators

• An operator is one of a small set of expressions built into Java. An operator takes one, two, or three arguments (called operands) and performs some operation on them. Binary operators, those taking two operands, appear between the operands, unlike method identifiers, which appear before their arguments.
• One set of important operands implement arithmetic functions. These include +, -, /, *, %. Each returns a number, the specific type of which depends on the types of the operands.
• The relational operators test for the relationship between their operands and return a boolean (see the next section for examples). These include ==, !=, >, <, >=, <=.
• The conditional operators take boolean operands and return booleans. These include && (and), || (or), ! (not).
• The assignment operator =, which returns nothing but changes the value of the variable which appears on its left to the value which appears on its right.

Conditionals

• We often want a method to do different things depending on the state of some object or the state of the world. So we need to able to ask questions in Java. The answers to these questions will always be true or false, that is, booleans.
• We ask questions about equality using the == operator. For example, if there is an integer variable called speed in a particular class, the following expression asks whether the value of that variable is currently zero:
  speed == 0
  We ask questions about inequality using the following operators (the first means "not equal to"): !=, >, <, >=, <=. For example, if getHeading() returns the heading of a Critter (an integer), the following expression would ask whether the heading of the Critter is greater than 3 (that is, whether the Critter is headed to the left):
  getHeading() > 3
• When we want to do something under a certain condition, we use if:
  if (condition)
    statement;
  The condition is an expression which returns a boolean, and the statement is an expression that gets executed if the condition returns true. It must be enclosed in parentheses.
• When we want to do one thing under a certain condition, and something else otherwise, we use if together with else:
  if (condition)
    statement;
  else
    statement;
  
• We can nest if (else) expressions by making the statements themselves if (else) expressions. In particular when there are more than two conditions with corresponding actions, we can do the following:
  if (condition)
    statement;
  else if (condition)
    statement;
  else if (condition)
    statement;
  ...
  else
    statement;
  
• Whenever a statement is called for, for example, in an if expression, we can replace it with a block statement, which includes multiple statements which are to be executed in sequence:
  {
    statement;
    statement;
    statement;
    ...
  }
  
  Note that a block statement is not followed by a semicolon.

© 2004. Indiana University and Michael Gasser. www.indiana.edu/~gasser/Q530/Notes/conditionals.html 5 Sep 2004