Selection in Java

Explains how to use selection (if and switch statements) in a Java program.

Prerequisites

• Understand Java primitive types and variables
• Understand Java arithmetic operators

Objectives

• Know the relational and Boolean logic operators
• Understand Boolean expressions and data types
• Use basic if statements
• Use if … else statements
• Use nested if statements
• Use switch statements
• Use ternary operators
• Know the precendence of operators

Relational Operators

The relational operators are used in a program to compare two values. There are six relational operators in Java.

• < (less than)
• > (greater than)
• <= (less than or equal to)
• >= (greater than or equal to)
• == (equals)
• != (not equal)

Boolean Operators

The Boolean logic operators are used to combine multiple conditions into a condition that evaluates to a single Boolean result (true or false). There are four Boolean operators available in Java.

• ! (not)
• ^ (exclusive or)
• && (and)
• || (or)

Boolean Expressions

• A boolean variable can hold either true or false
• A Boolean expression also evaluates to true or false
• Examples:

  double bobHeight = 1.92;
  double sueHeight = 1.65;
  double tall = 1.8;
  boolean bobIsTall = (bobHeight >= tall);  // bobIsTall will store true
  boolean sueIsTall = (sueHeight >= tall);  // sueIsTall will store false

• Boolean expressions can be simple like the above examples, or complex
• Example:

  boolean b = ((x < y && z != v) || (x + 4 == a));

if Statements

• An if statement has two parts: the keyword if and a boolean expression
• Syntax:

      if (boolean_expression) {
          statements;
      }

• If the Boolean expression is true, the statements in the body of the if statement are executed
• If the Boolean expression is false, the statements in the body of the if statement are skipped
• Examples:

      if (sueHeight >= tall) {
          System.out.println("Sue is tall!");
      }
  
      if (x == y) {
          System.out.println("x and y are the same");
      }

if … else Statements

• In the above examples, if an if statement is false, the statements in the body are skipped
• In an if … else statement, if the condition in if statement is false, then the statements in the body of the else statement are executed
• Syntax:

      if (boolean_expression) {
          statements;
      }
      else {
          statements;
      }

• Example:

      if (sueHeight >= tall) {
          System.out.println("Sue is tall!");
      }
      else {
          System.out.println("Sue is short.");
      }

if … else if … else Statements

• There can be many conditions in an if … else if … else statement.
• Syntax:

      if (expression) {
          statements;
      }
      else if (expression) {
          statements;
      }
      else {
          statements;
      }

• If the first if statement is false, the next else if statement is checked
• If no else if statement is evaluated true, the statements in the else statement are executed.
• Example:

      if (sueHeight >= tall) {
          System.out.println("Sue is tall!");
      }
      else if (sueHeight == 5.9) {
          System.out.println("Sue is of medium stature.");
      }
      else {
          System.out.println("Sue is short.");
      }

Nested if Statements

• if statements can be nested within other if statements
• Example:

      if (x > y) {
          if (z > y) {
              System.out.println("x and z are greater than y");
          }
      }
      else {
          System.out.println("y is not greater than x and z");
      }

• What does the computer do when executing the above code when x = 5, y = 3, and z = 1?

switch Statements

• if statements make selection based on a condition being true or false, while switch statements can have many conditions
• Syntax:

      switch (variable) {
          case 0:
              statements;
              break;
          case 1:
              statements;
              break;
          default:
              statements;
              break;
      }

• If the value is found in one of the cases, the expression is evaluated
• The keyword break ends further evaluation of the other cases
• If the value is not found in any of the cases, the default case is evaluated
• Example:

      char grade = 'C';
      double percent;
  
      switch (grade){
          case 'A': percent = 90.0;
              break;
          case 'B': percent = 80.0;
              break;
          case 'C': percent = 70.0;
              break;
          case 'D': percent = 60.0;
              break;
          case 'F': percent = 50.0;
              break;
          default: percent = 0;
              break;
      }

Ternary Operator

• A ternary operator is a conditional statement like an if statement with a different syntax
• Syntax:

      boolean_expression ? expression_if_true : expression_if_false;

• If the boolean expression is true, expression_if_true is executed
• If the boolean expression is false, expression2_if_false is executed
• Example:

  int z = -54;
  String isPositive = (z > 0) ? "z is positive" : "z is zero or negative";

Operator Precedence

The precedence of operators is as follows, with the bottom of the list having the lowest precedence, and the top the highest:

• var++ var-- (postfix)
• + - (unary plus and minus), ++var --var (prefix)
• (int) (double) (type cast)
• ! (not)
• * / % (multiplication, division, modulus)
• + - (binary addition and subtraction)
• < <= > >= (comparison)
• == != (equality)
• ^ (exclusive or)
• && (and)
• || (or)
• = += -= *= /= %= (assignment operators)