Section 2.5. Concepts Summary

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2.5. Concepts Summary

This chapter introduced many basic concepts: printing the result of a statement (expression), math operators, relational operators, types, casting, and variables.

2.5.1. Statements

Java programs are made of statements. Java statements can end in semicolons ';' just like sentences can end in periods "." in English. When you type statements in the definitions pane (when you define methods) they must have some sort of punctuation to show the end of the statement. One way to do this is to use a semicolon ';'.

If you leave off the semicolon in the interactions pane, it will print the result of the statement. If you do end a statement with a semicolon in the interactions pane, and you want to print the result, use System.out.println(expression); to print the result of the expression.

> int numPeople = 3; > double bill = 52.49; > double amountPerPerson = bill / numPeople; > System.out.println("Each person should pay: " + amountPerPerson); Each person should pay: 17.496666666666666

Math Operators

+	Addition	Used to add numbers together (3 + 4) = 7
-	Subtraction	Used to subtract one number from another (5 - 2) = 3
*	Multiplication	Used to multiply two numbers together (2 * 3) = 6
/	Division	Used to divide one number by another (18/2) = 9
%	Modulus (Remainder)	Used to find the remainder of one number divided by another (19 % 2) = 1

2.5.2. Relational Operators

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<	Less Than	Used to check if one value is less than another (2 < 3) is true
>	Greater Than	Used to check if one value is greater than another (3 > 2) is true
==	Equals	Used to check if two values are the same (2 == 2) is true
!=	Not Equals	Used to check if two values aren't equal (2 != 3) is true
<=	Less Than or Equal	Used to check if one value is less than or equal to another (2 <= 3) is true
>=	Greater Than or Equal	Used to check if one value is greater than or equal to another (3 >= 2) is true

2.5.3. Types

A type is a description of the "kind of" thing something is. It affects how much space is reserved for a variable and how the bits in that space are interpreted. In this chapter, we talked about several kinds of types (encodings) of data.

Floating point numbers	Java primitive types double or float, e.g., 5.2, -3.01, 928.3092	Numbers with a decimal point in them.
Integers	Java primitive types int, byte, short, long, e.g., -3, 5,239, 0	Numbers without a decimal pointthey can't represent fractions.
Characters	Java primitive type char, e.g., 'a', 'b', '?'	A character is delimited by a pair of single quotes.
Strings	Java String object, e.g., "Hello!"	A sequence of characters (including spaces, punctuation, etc.) delimited on either end with a double quote character.
Booleans	Java primitive type boolean with only two possible values	The value of a boolean can be the reserved word true or the reserved word false.

2.5.4. Casting

Java compilers recognize integer (-3) and floating point values (32.43). The result of a mathematical expression depends on the types involved in the expression. Expressions that involve integer values will have integer results. Expressions that have floating point (decimal) values will have floating point results.

This can lead to unexpected results.

> 1 / 2 0

There are two ways to fix this problem. One is to make one of the numbers a floating point number by adding '.0' (it doesn't matter which one) and the other is to use casting to change the type of one of the numbers to a floating point number (the primitive type float or double).

> 1.0 / 2 0.5 > (double) 1 / 2 0.5

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2.5.5. Variables

Variables are used to store and access values. You create variables by declaring them: type name; or type name = expression;. Declaring a variable reserves space for the variable and allows the computer to map the variable name to the address of that reserved space.

We introduced two types of variables: primitive and object. Primitive variables are any of the types: int, byte, short, long, float, double, char, or boolean. Object variables refer to an object of a class. Use the class name as the type when declaring object variables ClassName name; or ClassName name = expression;.

Primitive variables store a value in the reserved space for that variable. You can change the value using variableName = value;. You can access the value using variableName.

Object variables store a reference to an object in the reserved space for that variable. Object variables do not just store the address of the object. They store a reference to an object which allows the address of the object to be determined.

If the object variable doesn't refer to any object yet it has the value null. You can change what object a variable references using variableName = object-Reference;. You can access the referenced object using variableName.