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Data Types in Java Data is the information that a program has to work with. Data is of different types. The type of a piece of data tells Java what can.

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Presentation on theme: "Data Types in Java Data is the information that a program has to work with. Data is of different types. The type of a piece of data tells Java what can."— Presentation transcript:

1 Data Types in Java Data is the information that a program has to work with. Data is of different types. The type of a piece of data tells Java what can be done with it, and how much memory needs to be put aside for it. When we create a variable in Java, we need to specify: the type of the value we want to put in there, and the name we will use for that variable. We have seen that when we enter the name of a Person object, we need to put it in quotes, but when we enter the age, we don't. This is because they are of different types. Draw a picture of a box representing a variable. Remind students that a variable is really a memory location that is put aside to hold a certain value. We give a name to that location and we never need to know where it actually is. We can put values in there and find out what values are in there whenever we like, by using the name of the variable. For an object, a variable is a place in memory where the state of the object is stored.

2 Variables A variable is a name for a location in memory
A variable must be declared, specifying the variable's name and the type of information that will be held in it data type variable name int total; int count, temp, result; Multiple variables can be created in one declaration int count=1, temp=0, result; Variables can also be given initial values

3 final int MIN_HEIGHT = 69;
Constants A constant is an identifier that is similar to a variable except that it holds one value for its entire existence The compiler will issue an error if you try to change a constant In Java, we use the final modifier to declare a constant final int MIN_HEIGHT = 69; Constants: give names to otherwise unclear literal values facilitate changes to the code prevent inadvertent errors

4 Numeric Primitive Data
The difference between the various numeric primitive types is their size, and therefore the values they can store: Type byte short int long float double Storage 8 bits 16 bits 32 bits 64 bits Min Value -128 -32,768 -2,147,483,648 < -9 x 1018 +/- 3.4 x 1038 with 7 significant digits +/- 1.7 x with 15 significant digits Max Value 127 32,767 2,147,483,647 > 9 x 1018

5 Type int (byte, short, & long)
An int is a whole number (e.g. 21). You can do arithmetic with an int. int age = 21; addition + subtraction - multiplication * division / modulus % 21 age int age = 15; age + 3 2 * age - 4 age / 2 age % 10 Explain the last 3 lines here. The first one creates a variable of type int and calls it age. The next two are expressions that use whatever value is stored in that variable. Talk about the symbols used for arithmetic, and what modulus means. An int is not an object. It doesn't have attributes and behaviour. It just has a single value. It can be one of the attributes of an object. On the overhead, show that you cannot do arithmetic with a String that looks like it might be an int, e.g. "2" Don't go into too much detail about variables and types, as we will do more on that in week 3.

6 Type double (float) The type called double is used to store a real number , i.e. a number with a decimal point. It is stored in a different format from an int. You can do arithmetic on a double. double price = 3.95; 3.95 price * 0.1 might be working out a 10% discount, or the 10% GST. The final price of an item might be the last expression, i.e. the original price plus 10% of that. price addition + subtraction - multiplication * division / price * 0.10 price / 2 price + price * 0.1

7 Type boolean A boolean value represents a true or false condition
A boolean can also be used to represent any two states, such as a light bulb being on or off The reserved words true and false are the only valid values for a boolean type boolean done = false;

8 Type char A char is a character, i.e. a bit pattern you can produce by pressing a key (or a combination of keys) on a keyboard. Examples are 'a' 'A' '3' '?' '!' char response = 'Y'; You cannot do arithmetic on a char. A String is a collection of chars. 'Y' response Explain the difference between "2", '2' and 2 briefly, and what you can do with each of these.

9 Type String A String is a collection of characters (e.g. "Sally").
String name = "Sally"; A String value is always written in double quotes. You can have an empty String, shown as "". A String has many methods including: change itself to upper/lower case tell you how long it is (how many characters) give you the character at a specified position the string concatenation operator + Sally name Talk about what a character is. This was not mentioned in week 1's lecture but should be covered in CSE1200. Point out the line that creates a String with an initial value of "Sally". String is actually a class in Java. A String object has attributes including the collection of characters in it, and certain behaviours in its public interface. There are more than what are listed here. Open the editor window in BlueJ and show the part of the code for the Person class that defines the attributes. Don't worry about the rest of it.

10 result = total + count / max - offset;
Operator Precedence Operators can be combined into complex expressions result = total + count / max - offset; Operators have a well-defined precedence which determines the order in which they are evaluated Multiplication, division, and remainder are evaluated prior to addition, subtraction, and string concatenation Arithmetic operators with the same precedence are evaluated from left to right Parentheses can always be used to force the evaluation order

11 Operator Precedence What is the order of evaluation in the following expressions? a + b + c + d + e a + b * c - d / e 1 2 3 4 3 1 4 2 a / (b + c) - d % e 2 1 4 3 a / (b * (c + (d - e))) 4 3 2 1

12 Assignment Revisited The assignment operator has a lower precedence than the arithmetic operators First the expression on the right hand side of the = operator is evaluated answer = sum / 4 + MAX * lowest; 4 1 3 2 Then the result is stored in the variable on the left hand side

13 Assignment Revisited The right and left hand sides of an assignment statement can contain the same variable First, one is added to the original value of count count = count + 1; Then the result is stored back into count (overwriting the original value)

14 Data Conversions Sometimes it is convenient to convert data from one type to another For example, we may want to treat an integer as a floating point value during a computation Conversions must be handled carefully to avoid losing information Widening conversions are safest because they tend to go from a small data type to a larger one (such as a short to an int) Narrowing conversions can lose information because they tend to go from a large data type to a smaller one (such as an int to a short)

15 Data Conversions In Java, data conversions can occur in three ways:
assignment conversion arithmetic promotion casting Assignment conversion occurs when a value of one type is assigned to a variable of another Only widening conversions can happen via assignment Arithmetic promotion happens automatically when operators in expressions convert their operands

16 result = (float) total / count;
Data Conversions Casting is the most powerful, and dangerous, technique for conversion Both widening and narrowing conversions can be accomplished by explicitly casting a value To cast, the type is put in parentheses in front of the value being converted For example, if total and count are integers, but we want a floating point result when dividing them, we can cast total: result = (float) total / count;

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