Control Statements

Selection Statements

if statment

use the if statement to selectively execute instructions based on whether a condition (test) is true
format

if(boolean condition)
statement or block
else
statement or block

condition must evaluate to true or false
you may have a single statement following the if or a block of statements in {}
the else part is optional
conditions for comparing numbers

> for greater than
< for less than
!= for not equal
== for equal (note double equal signs)
>= for greater than or equal to
<= for less than or equal to
&& for AND
|| for OR

example:

int x = 0, y = 1;

if(x == y)
System.out.println("x equals y");
else
System.out.println("x is not equal to y");

if(y >= x)
{
y = x;
x = 0;
}
else
x = 1;

if(x > 0 || y > 0)
System.out.println("One or both X or Y is greater than zero);

if(y > 0 && x > 0)
System.out.println("Both X and Y are greater than zero");

conditions for comparing strings use String funtions equals, equalsIgnoreCase, compareTo, compareToIgnoreCase. In each you are asking one string to compare itself to another string. equals and equalsIgnoreCase return trrue or false, compareTo and compareToIgnoreCase return a negative number if the string being asked comes before, 0 if the strings are equal, and positive if the string being asked comes after the other string.
example:

String s1 = "Cat", s2 = "Dog", s3 = "dog";
if(s2.equals(s3))
System.out.println("s2 equals s3");
else
System.out.println("s2 is not equal to s3"); // this will print

if(s2.equalsIgnoreCase(s3))
System.out.println("s2 equals s3"); // this will print
else
System.out.println("s2 is not equal to s3");

if(s1.compareTo(s2) > 0)
System.out.println("s1 comes after s2");
else
if(s1.compareTo(s2) < 0)
System.out.println("s1 comes before s2"); // this will print
else
System.out.println("s1 equals s2");

conditional expression

this is a short cut for simple if else statements and has the format:

(boolean expression) ? expression 1 : expression 2

if the boolean expression is true, then expression 1 is evaluated, else expression 2 is evaluated. Note, be sure to use the ? and : in the right spots
Example

if(x > 0)
y = 1;
else
y = -1;

could be done as:

y = (x > 0) ? 1 : -1;

switch statement

use a switch statement to selectively execute one of a number of groups of statements based on the result of an expression or value of a variable
replaces multiple ifs and nested ifs when examining the same expression for different values
format

switch (expression)
{
case value1:
statements
break;
case value2:
statements
break;
case value3:
statements
break;
default:
statements;
}

the expression can evaluate to any number,or single character ( not a String)
execution will start after the colon following the correct value and continue until a break statement is encountered.
the default case is optional
values must be constants or literals
example:

int x = 1, y = 2;
switch (x + y)
{
case 1:
System.out.println("x+y = 1");
break;
case 2:
System.out.println("x+y = 2");
break;
case 3:
System.out.println("x+y = 3");
break;
default:
System.out.println("x+y is not 1, 2 or 3");
}

using if statements would yield

         if(x+y == 1)
            System.out.println("x+y = 1");
         else
            if(x+y == 2)
               System.out.println("x+y = 2");
            else
               if(x+y == 3)
                System.out.println("x+y = 3");
               else
                 System.out.println("x+y is not 1, 2 or 3");