1. The Java switch Statement
A multi-path decision statement
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2. Copyright © 2004-2017 - Curt Hill
Flow of Control in Java
Recall the flow statements in Java:
Decisions If
Switch Case
Loops
for
while do
Break
Here we consider the switch
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Why?
There are times when we would use the following type of nested if if(a == 2) x = 5; else if(a == 4) { x = 12 * y; … } else if(a == 8) {…} else if(a == 12) {…} else System.out.println(“Error”);
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General Form of This
Each if has a similar condition
One variable compared with a constant
Comparison is always equality
The Then statement does something
The Else statement does another if
There is only one variable and one comparison in all the ifs
Generally such a construction needs a switch-case not an if
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If and Switch
An if is a decision of one path out of two
A switch is a decision of one path out of many
An if is based on a boolean
May only have two values
A switch is based on an int, char, enumeration
May have many values
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General Form of Switch
switch (integral expression) { case con1: many statements break;
case con2: many statements break;
case con3: many statements break; … }
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Notes
switch, case and break are reserved words
The parenthesized expression is evaluated to obtain a value
This value chooses the path
The type of the value must be an integer, character, byte or enumeration
May not be floating point or any object
The case indicates a path
Must be followed by a constant
The constant value following the case must match the type of the expression
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Example
switch (credit_hours / 32) {
case 0:
standing = “freshman”;
break;
case 1:
standing = “sophomore”;
case 2:
standing = “junior”;
case 3:
standing = “senior”; }
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More Notes
There is only one set of braces which surrounds all the cases
The path starts with a case and ends with a break
Multiple cases allow for multiple ways to identify a path
The cases do not have to be in any particular order
Duplicate cases are not allowed
Compiler actually checks for this
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Multiple case example
switch (grade) {
case ‘a’:
case ‘A’:
points = 4 * credit_hours;
break;
case ‘b’:
case ‘B’:
points = 3 * credit_hours; … }
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Missing Path
What happens if the switch expression does not match any case label?
Equivalent to the correct case followed by a break
It does nothing
The default label allows a catch all case
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Example Revisited
switch (credit_hours / 32) {
case 0:
standing = “freshman”;
break; …
case 3:
standing = “senior”;
default:
standing = “unknown”; System.out.println(“Error”); }
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13. Copyright © 2004-2017 - Curt Hill
default
default is a reserved word
The default label acts like a case label with out case or specifying a value
It catches any value not explicitly mentioned in a case
It is customarily last, but does not have to be
Leaving it out is the same as doing nothing for all the unmentioned cases
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The Break Statement
The break has the effect of leaving a switch or loop
The last path does not need a break
A return will also work since it leaves the entire method not just the switch
It is not required, but leaving it out makes for interesting flow
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Leaving out a break
If a break is left out, execution will drop into the next part
Leaving out the last break is not a problem since there is no next part
Consider the following example
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Omitted break example
switch (val) {
case 5:
x += 5;
case 12:
x += 8;
y = 2;
break;
case 6:
y = 4; }
If val is 5, x will have 13 added to it and y will be set to 2.
All the effect of val == 12 is also executed for x == 5.
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Omitted break notes
This feature, inherited from C, is both good and bad
It is somewhat good in that it allows an economy of code
Very few switches actually use this feature
It is usually bad because we seldom want it but the compiler never gives an error when a break is left out
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18. Equivalence of if and switch
We have already seen an if doing what a switch can do
The reverse is also true: switch(b>c){ case true: … break; case false: … break; }
Always use what is more natural
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Common mistakes
We cannot use objects for the case expression or labels
We cannot use floating point values (float or double) for the expression or case labels
There are problems with floating point equality
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Java 7
Starting in Java 7 a string may be used
Internally it converts into a sequence of nested if statements
It is case sensitive
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Example
int dayofweek;
String day = scan.next();
switch (day) {
case “Sunday”:
dayofweek = 0;
break; …
case “Saturday”:
dayofweek = 6;
default:
System.out.println(“Error”); }
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Nope!
switch(yertle){
...
switch(real){
case 2.1: }
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Uses
switch is much less used than an if
Just not as much need
Often used to decode an integer or enumeration into a string
Month number into a name
Used to select different actions for these as well
Often used for a menu in console programs
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Ranges
Similar statements may be able to handle a range of values, but not this one
If the values between 1 and 5 inclusive are to use the same path then five case constructs are needed
The only easy range is the default construct
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Finally
The if chooses one of two paths, but the switch chooses one of many paths
The new reserved words are:
switch
case
default
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