Java Statements

For the <java statement> you can fill in any valid statement. Examples include the following:

• a valid input or output statement

• a valid java assignment statement

• another valid if statement

  Note that here we 1) use an if statement as part of the definition of a java statement, and 2) use a java statement in the definition of an if statement. This apparently circular reasoning is nevertheless quite precise.

• A block of statements:

  {
  	<java statement>
  	<java statement>
  	...
  	<java statement>
  }

  This says that a left curly brace, followed by a series of one or more valid statements, followed by a right curly brace is itself a valid statement. Note that here a valid java statement is used inside its own definition.

Let us see how the rules are applied:

Example 1:

int temperature = 
   inBox.getInteger("Enter your temperature.");
if (temperature <= 95)
   outBox.printLine("You have a low body temperature.");
else
   outBox.printLine("You do not have a low body temperature.");

Example 2: Method for asking a yes/no question:

public boolean ask(String question)
{
   int x = inBox.getInteger(question + " (1=yes, 0=no)");
   if (x==1)
      return true;
   else
      return false;
}

Example 3: Using rule 4, we can replace the temperature output statement with

{
   int pulse = inBox.getInteger("What is your heart rate?");
   outBox.printLine("Your heart rate is " + pulse);
}

to give:

int temperature = inBox.getInteger("Enter your temperature.");
if (temperature <= 95) // low body temperature
{
   int pulse = inBox.getInteger("What is your heart rate?");
   outBox.printLine("Your heart rate is " + pulse);
}
else
   outBox.printLine("You do not have a low body temperature.");

Nested If-Statements

By Rule 4, we could add more valid statements between the curly braces, including valid if statements:

int temperature = inBox.getInteger("Enter your temperature.");
if (temperature <= 95) // low body temperature
{
   int pulse = inBox.getInteger("What is your heart rate?");
   outBox.printLine("Your heart rate is " + pulse);
   if (pulse > 0)
      outBox.printLine("Draw a hot bath for yourself.");
   else
      outBox.printLine("Better call the morgue.");
}
else
   outBox.printLine("You do not have a low body temperature.");

Similarly, we could substitute another if statement for the last output statement in the else clause:

int temperature = inBox.getInteger("Enter your temperature.");
if (temperature <= 95) // low body temperature
{
   ....
}
else
{
   if (temperature > 101) // fever
      outBox.printLine("You have a fever!");
   else // normal temperature
      outBox.printLine("Your temperature is normal.");
}

Notes on Indentation

Note that the syntax rule for the if-statement implies an indentation scheme for laying out an if statement on the screen. When one if-statement appears inside another, some lines are indented two levels, because the inner if-statement is itself indented one level. If an if-statement is then substituted inside one of these if-statements, some statements will end up indented three levels deep, etc.

Though the compiler pays no attention to the indentation, you should always follow this indentation scheme in order to make your program comprehensible to yourself, instructors, and other programmers.

Multi-way If-Else Statements

There is one exception to this indentation scheme. Let us add a new syntax rule regarding indentation. The three places where there are comments about fever correspond to three mutually exclusive conditions (low, high, or normal body temperature).

In such a case, it is acceptable to indent the if and else clauses having to do with these conditions at the same level of indentation. For example:

int temperature = inBox.getInteger("Enter your temperature.");
if (temperature <= 95) // low body temperature
   outBox.printLine("You have low body temperature.");
else if (temperature > 101) // fever
   outBox.printLine("You have a fever!");
else // normal temperature
   outBox.printLine("Your temperature is normal.");

In general, any set of mutually exclusive conditions can be handled in this way.

Let us formalize this indentation scheme with an additional syntax rule for the if statement. The following is also a valid if statement:

if (<conditional expression>)
   <java statement>
else if (<conditional expression>)
   <java statement>
else if (<conditional expression>)
   <java statement>
...
else
   <java statement>

This tells the computer to find the first valid conditional expression in the series that evaluates to true, to execute the java statement immediately under it, and then to skip the remainder of the series.

Another Example:
Have the user enter a number from 1 to 12 corresponding to the index of the month. The program then prints out the name of the month:

if (num == 1)
   outBox.printLine("January");
else if (num == 2)
   outBox.printLine("February");
else if (num == 3)
   outBox.printLine("March");
...

else if (num == 12)
   outBox.printLine("December");
else
   outBox.printLine("Not a valid month.");

Order Matters

Consider the example to print out “big” if a number is bigger than 100, “medium” if a number is between 20 and 100, and small otherwise. The code would be

if (num > 100)
  outBox.printLine("Big");
else if (num > 20)
  outBox.printLine("Medium");
else
  outBox.printLine("Small");

Why does the second condition (num>20) only get executed for numbers between 20 and 100? If the order of the if statements had been rearranged:

if (num > 20)
   outBox.printLine("Medium");
else if (num > 100)
   outBox.printLine("Big");
else 
   outBox.printLine("Small");

would this work properly?

Boolean Operators: AND, OR, NOT

Boolean operators take boolean operands and return boolean values.

AND:

a	b	a && b
true	true	true
true	false	false
false	true	false
false	false	false

OR:

a	b	a || b
true	true	true
true	false	true
false	true	true
false	false	false

NOT:

a	!a
true	false
false	true

Using Java syntax, write a boolean expression that evaluates to true when:

• The temperature is above 90 or the humidity equals 90.
• The mathematical expression a > b > c.
• It is not true that a and b are both 0.
• The person’s gender is not female and the age is less than 21
• s is equal to the ascii letter q
• The character c is a letter of the alphabet.
• neither x nor y are bigger than 10
• x is larger than y but smaller than 2y

Boolean Algebra

• Identity: true && a = a

false || a = a

• Commutative: a && b = b && a

a || b = b || a

• Distributive: a && (b || c) = (a && b) || (a&&c)

a || (b && c) = (a || b) && (a||c)

• Inverse: a && (!a) = false

a || (!a) = true

• Idempotent: a && a = a

a || a = a

• DeMorgan’s Laws: ! (a && b) = !a || !b

! (a || b) = !a && !b

• Involution: !! a = a

---

Switch Statements

Switch statements are a special case of multi-way if-else statements. That is, all switch statements can be written as multi-way if-else statements but not vice versa.

switch (e) 
{
   case v1:
      <java statement>;
      <java statement>;
      ...
      break;
   case v2:
      <java statement>;
      <java statement>;
      ...
      break; 
   case v3:
      <java statement>;
      <java statement>;
      ...
      break; 
   ...

   default: // this is optional
      <java statement>;
      <java statement>;
      ...

}

Where:

• e must be an expression that evaluates to an integer or character
• v1, v2,... must be of the same type as e
• The case statement works as follows:
  e is compared to v1, v2, etc. When the first match is found, the java statements starting just under the match are executed until a break statement is encountered. When the break statement is encountered, execution skips to after the final bracket of the case statement.
  Note, if break statements are not used, the all statements after the match are executed including those in the subsequent cases.
• It may be that you don’t want to use the break statement (see example below)

Example 1:
Converts the day of the in numeric form (1, 2, ... 7) to the name of the day (Sunday, Monday,...)

String name="not a day";
int day = inBox.getInteger(
  "Enter a number for the day of the week " + 
  "(1=Sunday,2=Monday, ..): ");
switch(day)
{
   case 1:  
      name = "Sunday";
      break;
   case 2:
      name = "Monday"; 
      break; 
   case 3:
      name = "Tuesday";
      break;
   case 4: 
      name = "Wednesday";
      break;
   case 5:
      name = "Thursday";
      break;
   case 6:
      name = "Friday";
      break;
   case 7:
      name = "Saturday";
      break;
}
outBox.printLine("The day you entered is " + name);


Example 2:
switch(day)
{
   case 1:
   case 7:
      name = "Weekend";
      break;
   case 2:
   case 3:
   case 4:
   case 5:
   case 6:
      name = "Weekday";
}
outBox.printLine("It is a " + name);





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