Manning OCA Java SE 7 Reading Note: Chapter 2

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This is a post that I moved from my old blog site. This is the reading note for OCA Java SE 7 Certification Guide from Manning. Please note that all definations and examples in this post are taken from the book.

Chapter 2: Working with Java Data Types

This chapter main covers:

  • Primitive data types in Java

  • Literal values of primitive Java data types

  • Object reference variables in Java

  • Valid and invalid identifiers

  • Usage of Java operators

  • Modification of default operator precedence via parentheses

2.1 Primitivate Variables

A variable defined as one of the primitive data types is a primary variable.

Primitive data types is the simplest data types in a programming language.

Java has eight primitive data types:

  • char

  • byte

  • short

  • int

  • long

  • float

  • double

  • boolean

String is not primitive type in Java.

char and int are the only short-name primitive data types, with low case as begining, others are all full-name.

Integer or Character or Boolean is not primitive data type in Java.

Primitive Date Type can be devided to three categories:

  • Boolean: boolean

  • Character: char

  • Numeric:

    1. Integers: int, byte, short, long
    1. Floating-point/decimals: float, double

boolean

A literal is a fixed value that does not need further calculations in order for it to be assigned to any varible.

true and false are the only boolean literal variables.

Numeric

Range of byte: -128 to 127, inclusive. 8 bits.

The default type of a nondecimal number is int.

To designate an integer value as a long value, just add suffix L or l: 23333000000L.

Integer literal values comes in four categories: binary, decimal, octal, and hexoadecimal:

  • Binary: A base-2 system. Use prefix 0B or 0b to define it: int b = 0B1110011;

  • Decimal: A base-10 system. This is the default setting for System.out.println().

  • Octal: A base-8 system, which means in octal , 10 has decimal value 9. Use prefix 0 to define it: int o = 036620;

  • Hex: A base-16 system, A->F means 10->16. Use 0X or 0x to define it.Example: int h = 0X23232FA;

In Java7, you can use underscores. Underscore has no effect on values: long oc = 045_13;

Rules to Remember

  • You can not start or end a literal value with an underscore, such as _100_

  • You can not place an underscore right after the prefixes 0b for binary, 0 for octal and 0x for hex.

  • You can not place an underscre prior the an L suffix for long value

  • You can not use an underscore in positions where a string of digits is expected, such as int i = Integer.parseInt("45_98");. Note this will compile successfully but fail in run time.

In Java, you use float and double to store decimal numbers. float is smaller and takes less space. float has less precision.

The default type of a decial litercal is double in Java. You can place a suffix f/F to transfer to to float in compile time. a D/d suffix can also be used to transfer to double, but this can be skipped since double is default.

You can also assign scientific notation as a literal decimal value: double test = 1.4444e4;

In Java7, you can use underscore for floating-points. Rules:

  • All rules for numberic literals above

  • You can not place an underscore prior to a D/d or F/f suffix

  • You can not place an underscore adjacent(i.e. attched before or attched after) to a decimal point: 100_.21 or 89._45

char

A char can store a single 16-bit Unicode character.

Range of char: from \u0000or0to`ufffor65,535`, inclusive.

Use single quote for characters. Double quote will fail compile.

Since Java store characters internally as unsigned integer value (i.e. positivate integer), so it's fine to assign positive integer to a char. Negative number will fail to compile, but you can cast.

For example: char a = 122;, character is transferred to \u0122. The format is first a number based on10, then a number that is based on16`.

You must use quotes to assign Unicode values to char variables: char c = '\u0122';

You can cast only capatible data types.

If you cast a negative number for char, the sign will be recognize as a part of numberic value becuase Java does not store sign of an integer.

2.2 Identifiers

Indentifiers are names of packages, classes, interfaces, methods and variables.

Hyphens (-) are not allowed in identifier. Underscotres are allowed.

Valid Identifiers

Unlimited length

Starts with a letter ( a–z, upper- or lowercase), a currency sign, or an underscore Uses special characters: ! , @ , # , % , ^ , & , * , (, ) ,', : ,; , [ , / , \`` ,}`

Can use a digit (not at the starting position)

Can use an underscore (in any position)

Can use a currency sign (in any position): $ , £ , ¢ ,¥ and others

Invalid Indentifier

Same spelling as a Java reserved word or keyword

Starts with a Java digit ( 0 – 9 )

2.3 Object Reference Variables

The variables in Java can be categorized into two types: primitive variables and reference variables.

Reference variables are also known as object reference variables or object references.

Objects are instances of classes.

An object reference is a memory address that points to a memory area where an object’s data is located.

When an object is instantiated with the new operator, a heap-memory address value to that object is returned. That address is usually assigned to the reference variable.

Code Example:

Person person = new Person();

//Step 1: A new Person object is created
//Step 2: A variable named person is created in the stack with an empty ( null ) value.
//Step 3: The variable person is assigned the memory address value where the object is located.

The literal value of all types of object reference variables is null. You can also assign a null value to a reference variable explicitly.

The basic difference is that primitive variables store the actual values, whereas reference variables store the addresses of the objects they refer to.

2.4 Operators

Assignment Operators

The += , -= , *= , and /= operators are short forms of addition, subtraction, multiplication and division with assignment.

You cannot assign larger range of values to smaller range of values. For example: long num = 188883544333334L; int val = numl will give complie error. However, opposite and casting is allowed.

You cannot assisn boolean to other primitive data types.

You can also assign multiple values on the same line using the assignment operator and comma: int a=7, b=5, c=3;

Arithmetic Operators

Unary operators (++/--) can also be used in prefix and postfix notation.

In prefix notation, the unary calculation run first, then closest expression run.

In postfix notation, closest expression run first, then the unary calcuation run.

cloest expression means the one expression that this variable needs to attend now.

Code example for unaray operators:

int a  = 10;
a = a++ + a + a-- - a-- + ++a;
System.out.println(a)

For the code above:

  • a++ means a=10, after expression a=11. However, there is no expression before this step. so now totally we have a=10 as whole function, but the single a value on right for next step is 11;

  • Then + a now means + 11, so now a = 10 + 11;

  • Then a-- means first do +, then do --. So now a = 10 + 11 + 11, but single a on right side now is 10.

  • Then a-- means first do -, then do --. So now a = 10 + 11 + 11 - 10, but single a on right side now is 9.

  • Then ++a means first make single a = 10, then do +. So now a = 10 + 11 + 11 -10 + 10

  • Now we can hava a=32 for final printout.

During steps above, the variable assignment is the last step. It needs to waiting for all expressions on right finish.

Relational Operators

Relational operators are used to check one condition, with two categories:

  • Comparing greater ( > , >= ) and lesser values ( < , <= )

  • Comparing values for equality ( == ) and nonequality ( != )

The result of the relational operation is always a boolean value

The operators == (equal to) and !=(not equal to) can be used to compare all types of primitives: char , byte , short , int , long , float , double, and boolean.

Remember that you can’t apply these operators to incomparable types.

Logical Operators

These expressions should return a boolean value.

You can use the logical operators AND (&&) , OR (||) , and NOT (!) to check multiple conditions and proceed accordingly.

If there is any false, && always return false;

If there is any true, || always return true;

&& and || is called short-circuit operators, because as long as there is any expression that can decide the result of whole expression (Using the rule above), they will stop evaluating and give out result.

Operator Precedence

the operator on top has the highest precedence, and operators within the same group have the same precedence and are evaluated from left to right:

  • Exp++,Exp--

  • ++Exp, --Exp, +Exp, -Exp, !

  • *, /, %

  • +, -

  • <, >, <=, >=

  • ==, !=

  • &&

  • || Note OR has lower precedence than AND

  • =, +=, -=, *=, /=, %=

  • You can use () i.e. parentheses to override the default operator precedence.

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