- The if-else-statement
is a 2-way selection statement
- The if-else-statement
can be used to write
a N-way selection statement
in Java...
.... but the if-else-statement is not intrinsically an N-way selection statement
- We will now study an intrinsically N-way selection statement: the switch statement
- Syntax of the switch-statement:
switch ( INT-EXPRESSION ) { case INT-VALUE1: STATEMENT11; STATEMENT12; ... break; // Marks the end of case INT-VALUE1 case INT-VALUE2: STATEMENT21; STATEMENT22; ... break; // Marks the end of case INT-VALUE2 ... (more cases if desired) ... [default: STATEMENTd1; // Optional clause STATEMENTd2; ... break; ] }Meaning of the switch statement:
- The INT-EXPRESSION in the
switch statement is
first evaluated
It INT-EXPRESSION must be an integer valued expression
- The result of the
INT-EXPRESSION is
compared to the
integer values
given in the
case clauses
- If the result of the
INT-EXPRESSION is equal to
some INT-VALUE in a case clause,
the statements
following the case clause
upto the break statement
are
executed
- If the result of the
INT-EXPRESSION is
not equal to
any INT-VALUE
in the case clauses, then
the statements
in the default case
are executed if it is specified
If the default case is not specified, then the switch statement will terminate without executing any statement.
- The INT-EXPRESSION in the
switch statement is
first evaluated
- Note: the execution of the
switch statement is
terminated by:
- A
break; statement,
or
- When execution reaches the end of the switch statement
- A
break; statement,
or
- Programming example:
translate a numeric month
to a string name
import java.util.Scanner; public class Switch01 { public static void main(String[] args) { int a; String name = ""; Scanner in = new Scanner(System.in); // Construct Scanner object System.out.print("Enter a numeric month (1-12): "); a = in.nextInt(); // Read in next number into a switch ( a ) { case 1: name = "January"; break; // For brevity, I put case 2: name = "February"; break; // the "break" statement case 3: name = "March"; break; // on the same line. case 4: name = "April"; break; case 5: name = "May"; break; case 6: name = "June"; break; case 7: name = "July"; break; case 8: name = "August"; break; case 9: name = "September"; break; case 10: name = "October"; break; case 11: name = "November"; break; case 12: name = "December"; break; default: name = "Invalid month"; break; // <-- This break statement // is redundant } System.out.println("Name of month = " + name); } }Explanation:
- The INT-EXPRESSION
in the switch statement is
a
Therefore, based on the value of a, one of the cases will be executed
- When the variable a
is equal to 1, the
statements:
name = "January"; break;
are executed.
- When the variable a
is equal to 2, the
statements:
name = "February"; break;
are executed.
And so on.
- If the value of the variable a
is not one of the 12 values (1, 2, ..., 12)
listed in the cases, then
the statements of the default case:
name = "Invalid month"; break;
are executed.
- The INT-EXPRESSION
in the switch statement is
a
- Previously discussed:
switch ( INT-EXPRESSION ) <--- must be an integer valued expression { ... }
- The following types are
automatically converted
into an
integer typed value
in expressions:
- byte
- short
- char
Therefore, we can also use them in the switch statement !!
Example:
public class Switch02 { public static void main(String[] args) { char x; String name = ""; x = 'C'; switch ( x ) // char is converted to int automatically ! { case 'A': name = "January"; break; case 'B': name = "February"; break; case 'C': name = "March"; break; case 'D': name = "April"; break; case 'E': name = "May"; break; case 'F': name = "June"; break; case 'G': name = "July"; break; case 'H': name = "August"; break; case 'I': name = "September"; break; case 'J': name = "October"; break; case 'K': name = "November"; break; case 'L': name = "December"; break; default: name = "Invalid month"; break; } System.out.println("Name of month = " + name); } }This program will compile correctly and run (prints "March").
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile: javac Switch02.java
- To run: java Switch02
- A long typed integer value
is not automatically converted
to int
- Therefore, you
cannot use a
long typed integer value
in a
switch statement
without a
casting operation.
- Example:
public class Switch03 { public static void main(String[] args) { long a; String name = ""; a = 10; switch ( a ) // Illegal ! { case 1: name = "January"; break; case 2: name = "February"; break; case 3: name = "March"; break; case 4: name = "April"; break; case 5: name = "May"; break; case 6: name = "June"; break; case 7: name = "July"; break; case 8: name = "August"; break; case 9: name = "September"; break; case 10: name = "October"; break; case 11: name = "November"; break; case 12: name = "December"; break; default: name = "Invalid month"; break; } System.out.println("Name of month = " + name); } }You will get the following compile error:
Switch03.java:11: possible loss of precision found : long required: int switch ( a ) ^
- According the the
Java development website:
click here
- The switch statement in
Java version SE 7 and later
will
also allow
String typed
expressions
(But Java SE 7 has not been released yet)
(This is not the case in Java 6 and other languages such as C/C++)
- The switch statement in
Java version SE 7 and later
will
also allow
String typed
expressions
- Example: the following program
converts a
month name (string) into
an integer
using a
switch statement
import java.util.Scanner; public class Switch04 { public static void main(String[] args) { String name = ""; int a; Scanner in = new Scanner(System.in); // Construct Scanner object System.out.print("Enter the name of a month (lower case): "); name = in.next(); // Read in name switch ( name ) { case "january": a = 1; break; case "february": a = 2; break; case "march": a = 3; break; case "april": a = 4; break; case "may": a = 5; break; case "june": a = 6; break; case "july": a = 7; break; case "august": a = 8; break; case "september": a = 9; break; case "october": a = 10; break; case "november": a = 11; break; case "december": a = 12; break; default: a = 0; break; } System.out.println("Index of month = " + a); } }We cannot compile and run this program because Java SE7 is not yet available !!!
- We can use a
switch statement to
assign a letter grade
to a number grade (read from input)
- Algorithm:
- We must use integer
numeric grade
(floating point numbers
are not allowed
in a switch statement !)
- Assign letter grade
A for
numeric grades 90, 91, ..., 100
- Assign letter grade
B for
numeric grades 80, 81, ..., 89
- Assign letter grade
C for
numeric grades 70, 71, ..., 79
- Assign letter grade
D for
numeric grades 60, 61, ..., 69
- Default: assign letter grade F otherwise.
- We must use integer
numeric grade
(floating point numbers
are not allowed
in a switch statement !)
- Java program:
public class Switch05 { public static void main(String[] args) { int ng; String lg = ""; Scanner in = new Scanner(System.in); // Construct Scanner object ng = in.nextInt(); // Read in next number switch ( ng ) { case 100: .... // I left out a lot of "cases" to save space ! case 90: lg = "A"; break; case 89: ... case 80: lg = "B"; break; case 79: ... case 70: lg = "C"; break; case 69: ... case 60: lg = "D"; break; default: lg = "F"; break; } System.out.println("Letter grade = " + lg); } }
- Example Program:
(Demo above code)
- Prog file: click here
How to run the program:
- Right click on link and
save in a scratch directory
- To compile: javac Switch05.java
- To run: java Switch05