unit 3.1

The operators == and != (not equal) can be used to compare values. They return true or false boolean values.

One = sign changes the value of a variable. Two == equal signs are used to test if a variable holds a certain value, without changing its value!

We can also use == or != to test if two reference values, like Turtle and String objects, refer to the same object. In the figure below, we are creating two separate Turtle objects called juan and mia. They do not refer to same object or turtle. Then, we create a reference variable called friend that is set to mia. The turtle mia will have two ways (references or aliases) to name her – she’s both mia and friend, and these variables refer to the same object (same Turtle) in memory. If two reference variables refer to the same object like the turtle on the right in the image below, the test with == will return true which you can see in the code below.

The Relational Operators below in Java are used to compare numeric values or arithmetic expressions. Although some programming languages allow using relational operators like < to compare strings, Java only uses these operators for numbers, and uses the methods compareTo and equals for comparing String values.

• < Less Than

• > Greater Than

• <= Less than or equal to

• >= Greater than or equal to

• == Equals

• != Does not equal

/Test if a number is even by seeing if the remainder is 0 when divided by 2(number % 2 0) //Test if a number is odd by seeing if there is a remainder when divided by 2(number % 2 > 0) //Test if a number is a multiple of x (or divisible by x with no remainder)(number % x 0)

The remainder operator has been used quite a bit on the AP CSA exam, so you should be familiar with it.

Use it to check for odd or even numbers. If num % 2 != 0 is true, num is odd and if num % 2 == 0 is true then num is even.

You can also use remainder to check if any number is evenly divisible by any other: If num1 % num2 == 0 is true then num1 is evenly divisible by num2.

Use it to get the last digit from an integer number: num % 10 gives us the rightmost digit of num.

Use it to get the number of minutes left when you convert a total number of minutes to hours and minutes:

int totalMinutes = 345; int hours = totalMinutes / 60; // Number of whole hours, i.e. 5int minutes = totalMinutes % 60; // Number of minutes left over, i.e. 45

Use it whenever you have limit in the value, and you need to wrap around to zero if the value goes over the limit: the value of num % limit will always be in the range from 0 (inclusive) to limit (exclusive) as long as num and limit are both positive.

A warning: because Java’s % is a remainder operator and not a true mathematical modulo operator (as we discussed briefly in section 1.4) you can’t check if a number is odd with the expression num % 2 == 1.

That expression will be true if num is positive and odd and false when num is even, both of which are correct. But if num is negative and odd, its remainder when divided by 2 is -1, not 1 and this expression will evaluate to false. Thus you should always use num % 2 != 0 to check if num is odd.

Primitive values and reference values can be compared using relational operators (i.e., == and !=) in Java.

Arithmetic expression values can be compared using relational operators (i.e., <, >, <=, >=) in Java.

An expression involving relational operators evaluates to a boolean value of true or false.