Java: Elements of Statements

Literals, identifiers, symbols, values, types

A value is a number, a character, a string of characters, or a boolean, etc.

Literals make the names of values. A literal could be a digit, a character, or a “symbol”. For example, 100 contains the literals “1”, “0”, “0” and represents the value of one hundred.

A type is a set of values. Java is a statically-typed language which means every value has a designated type, or in other words, there’s no values that don’t have a type. Java types can be divided into two categories, primitive types and reference types (or object types).

Identifiers

An identifier is an unlimited-length sequence of Unicode letters and digits used as the names of variables, methods and classes. An indentifier can start with a letter, an underscore _, or a dollar sign $, but it’s good practice to always begin with a letter. It cannot start with any other kind of characters including digits, whitespace, hyphen, asterisk, equal sign, etc, and it cannot contain whitespaces. In Java identifiers are case sensitive, for example, foo and Foo are different identifiers.

Identifiers cannot be reserved words such as class, public, static, if, else, while, etc.

It’s a good practice to always use meaningful names for variables instead of abbreviations. If the name consists of only one word, spell it in all lowercase letters, and if it consists of more than one word, capitalize the first letter of each subsequent word, such as firstTime. This is called camel casing. If the variable contains a constant value, capitalize every letter and separate subsequent words with underscores, such as DEFAULT_VALUE.

By convention class names should begin with upper case letters, and variable and method names should begin with lower case letters.

Periods are used to indicate different levels of containment in compound names (also called qualified names), e.g. System.out.println().

Symbols

• { } Used to group things together. If there are no curly braces in above structures only the first line of the statements will be performed.
• // In-line comment
• /*…*/ Block comment
• /**…*/ Javadoc comment Spaces They do not affect the program, but are useful for organizing code.

Types of types

Java is a strictly-typed language. Every value and every variable has their own type, and each variable can only store a value of the same type. There are two types of Java types: primitive types and reference types.

1. Primitive types

   Primitive types of values are represented using only a certain finite number of bits (a bit is a 0 or 1 and 8 bits is one byte).

   Names of primitive types begin with lower-case letters.

   When you declare a primitive variable you get a storage space for a primitive value.

   You can not create new primitive types in Java.

   There are 8 primitive types in Java: byte, short, int, long, float, double, char, boolean.

2. Reference types

   Reference types (also called object types) usually represent more complicated data.

   Names of object types should begin with upper-case letters.

   When you declare an object variable you get a storage space for a reference to the object. To get space for the object itself you should use new.

   You can create new object types.

   Java is an object-oriented programming language and most Java types are object types, such as String, Point, Rectangle, etc. Arrays are also object types. null is a special value that means “no objects” and can be assign to any object type variable.

Numbers

• Numerical literals can include underscores as separators of digit groups, e.g. 1₀₀₀₀₀₀, 2₀₀₀.000₀₀₁
• Numbers can be represented using scientific notation, e.g. 1.3e10, 3.5E200, 10e10.

1. Integers

   Whole numbers. Integers are represented bitwise using Two’s Complement. See integer clock.

   A byte type integer takes up a single byte (a sequence of 8 bits) of memory, which makes this type of numbers range from -128 (-2⁷) to 127 (2⁷-1), inclusive. See Why is the range of bytes 128 to 127 in Java.

   A short type integer takes up 2 bytes (16 bits) and ranges from -2¹⁵ to 2¹⁵-1.

   An int type integer takes up 4 bytes (32 bits) and ranges from -2³¹ and 2³¹-1 [>] (approx. four billion numbers). It’s the default type for integers in Java. See StackOverflow: The range of int.

   A long type integer takes up 8 bytes (64 bits) and ranges from -2⁶³ to 2⁶³-1. Integers of this type are represented by appending an “l” or “L’ at the end of the number, e.g. 500L.

2. Floating-point numbers

   Fractional numbers, real numbers.

   A float type number takes up 4 bytes and ranges up to 10³⁸ with 7 significant digits at most. Numbers of this type end with an “f” or “F”, e.g. 1.0F, 2f.

   A double type number takes up 8 bytes and ranges up to 1.7x10³⁰⁸ with 15 significant digits at most. It’s the default type in Java for floating-point numbers.

3. Radices

   Numbers can be in different radices:

   • Binary: beginning with 0b or 0B, e.g. 0b101
   • Octal: beginning with with 0, e.g. 077
   • Hexadecimal: beginning with 0x or 0X, e.g. 0x9a

Characters

Characters are of type char and a character takes up 2 bytes of memory. A character is specified in the Unicode character set, which may be a letter, a number, or a symbol, and should be enclosed in single quotes when represented, such as ‘a’, ‘1’, etc.

A character is stored as a 16-bits (short) integer which represents its position in the Unicode set, called Unicode number. In Java the character and its Unicode number is equivalent and can be used interchangeably. For example, you can assign a number to a char type variable or convert a number to a character with typecasting:

char character = 233;
int number = 'é';
System.out.println((char) 233); // output: é

A character can be represented with \u followed by four hexadecimal digits. e.g. '\u00E9' -> 'é'. You can check a character’s hex number in the character map (Windows).

Not all characters can be represented with one letter or symbol either because they are invisible functional characters such as a tab or a linefeed, or the letters and symbols conflict with reserved literals. These special characters could be represented as escaped characters by using a backslash \. Some common escaped characters are:

• \t: a tab
• \r: a carriage return
• \n: a linefeed
• \b: a backspace
• \f: a formfeed
• \': a single quote
• \": a double quote
• \\: a back slash

Boolean values

The results of logical propositions. A boolean value is either true or false and has type boolean.

Strings

A sequence of one or more characters enclosed in double quotes.

Operators

• For these operators the order of operations is based on the rules of precedence.
• They only work with primitive types.

Arithmetic

• +: Add (numbers, characters), concatenating (strings)
• -: Subtract
• *: Multiply
• /: Divide
• %: Modulus operation, works on integers and yields the remainder of the first operand divided by the second. It also works on real numbers and yields the residue of the first operand subtracted as many copies as possible of the second.
• i++: Increment the value of i by 1 for its next use. i can be of type int or char.
• i--: Decrement the value of i by 1 for its next use. i can be of type int or char.
• ++i: Increment the value of i by 1 immediately. i can be of type int or char.
• --i: Decrement the value of i by 1 immediately. i can be of type int or char.

NOTE:

• For +,-,*,/ if the operands are all integers, the result is also an integer or otherwise a floating-point number. For the division of two integers the result always rounds down to the nearest integer.
• A char is converted into its Unicode code number when it is used with an arithmetic operator.

Assignment

• = Assign
• += Add a value to the current value of an int or double, e.g. x += y ⇔ x = x + y
• -= Subtract a value from the current value of an int or double, e.g. x -= y ⇔ x = x - y
• *= x *= y ⇔ x = x*y
• /= x /= y ⇔ x = x/y
• %= x %= y ⇔ x = x % y
• &&= p &&= q ⇔ p = p && q

Comparison/Relational

• == Equal to
• != Not equal to
• > Greater than
• < Less than
• >= Greater than or equal to
• <= Less than or equal to

NOTE:

• The two sides of of a condition operator have to be the same type.

• They can be used to compare values of type char according to their numeric Unicode values.

• instanceof: Compares an object to a specified type. You can use it to test if an object is an instance of a class, an instance of a subclass, or an instance of a class that implements a particular interface. null is not an instance of anything.

Logical/Conditional

• && Short-circuit and
• || Short-circuit or
• ! Not
• ? : The ternary operator. var = <exp1> ? <exp2> : <exp3>; If exp1 is true, returns exp2, or else returns
  • exp3

Precedence of logical operators:

• ! High
• && Medium
• || Low

Bitwise

• & Bitwise AND
• | Bitwise inclusive OR
• ^ Bitwise exclusive OR, XOR, e.g. 0⁰=0，1⁰=1，0¹=1，1¹=0
• ~ Unary bitwise complement, which inverts every bit of a value, e.g. if x = 1 is a byte, then ~x == -2, because ~00000001 == 11111110. A negative integer is represented by inverting its positive counterpart plus 1, e.g. -1 == ~1 + 1, which is called Two’s Complement. If a is an integer, then ~a=−a−1. The first bit of negative integers is always 1.
• << Signed left shift
• >> Signed right shift
• >>> Unsigned right shift

Variables

Type conversion

Variables have types. A type is a set of values. A variable is either a primitive types or a reference type. A variable can only hold a value of same type, or type which is a subset of the variable type. For example, an int variable can hold a byte or short value, but not a long value, neither a float or double value. But you can convert these values to int.

1. Automatic conversion

   In some cases values are converted automatically.

   When an integer is assigned to a double variable it is automatically converted to double. When the right side of the assingment is an expression with all operands being integers, Java evaluates the expression before the conversion. Thus, the result of the statement double q = 1/2; is 0.0, not 0.5. To gain 0.5 just write either or both of the operands to float-point, i.e., 1.0/2.0.

   When assigning char values to int variables, or int values between 0 and 65545 to char variables, the values are automatically converted. This is because char is in fact equivalent to int.

   When evaluating an arithmetic expression if all operands are integers then the result is an integer, and if one of the operands is a float-point number the result is a float-point number.

   When use + to concatenate strings the result is a string. For example, 1 + "foo" produces "1foo".

2. Typecast

   Typecast is to convert the type of a value to another type by using the typecasting operator (<type>) before the value or variable to convert. Not all types can be typecasted to one another. Typecast can happen between numeric types, between a numeric type and a char type, and between objects.

   Type conversion from a numeric type to a higher-bit numeric type is automatic, e.g. from short to int, but converting a numeric type to a lower-bit numeric type is forced by using the operator (<type>), e.g. short foo = (short) 100000. The result of this type of conversion is

   (n - max) % range - 1 + min
   

   • n: the integer to be converted
   • max: the max value of the the type to be converted into
   • range: the range of the values of the type from min to max
   • min: the min value of the type

   Type conversion from a floating-point to an integer always rounds down. For example, (int) 0.9 produces 0.

   Typecast takes precedence over arithmetic operations. For example, in

   double pi = 3.14; 
   int x = (int) pi * 20.0; 
   

   x gives 60 rather than 62.

   Between char and int: (char) 233 -> é, (int) 'é' -> 233

   Between object types that have inheritance relationship.

3. Using methods

   int to String:

   String s = String.valueOf(number); 
   String s = Integer.toString(int number); 
   String s = "" + number;
   

   String to int:

   int foo = Integer.parseInt("5");
   

   String to double:

   double dou = Double.parseDouble("3.14");
   

   int to Double:

   Double n = new Integer(i).doubleValue();
   Double n = Integer.valueOf(i).doubleValue();
   

Declaration

• It is the process of deciding the type of the variable before assigning the value.
• It is marked by putting the value type before the variable name, e.g. String bob.
• You can declare a list of variable names such as String bob, dylan, felix.

Assignment

• The left side of an assignment has to be a variable name for it indicates the storage location where the value will go.
• Expressions do not represent storage locations, only values.
• The type of the value and the variable in an assignment must be the same.

Initializtion

The process of declaring a variable and assigning a value to it at the same time. e.g. String name = "Bob";

You can initialize multiple variables with only one type declaration. E.g. String name1 = "Bob", name2 = "John", name3 = "Felix";

Default values: Variables that are not initialized have default values that depends on their types.

• int variable defaults to 0
• boolean variable defaults to false
• object variable defaults to null

Usages

• Intermediate/Temporary variables: Variables that are used in computation but never printed

• Flags: Variables that flag the presence or absence of some condition, e.g. evenFlag in

  boolean evenFlag = (n % 2 = 0); 
  if (evenFlag) { 
      System.out.println("n is even"); 
  }
  

• Local variables:

  • variables declared inside a method definition
  • only exist inside the method

• Member variables/Global variables:

  • Variables that are declared outside any method
  • Can be marked with modifiers such as static, public and private
    • static member variables
    • non-static member variables

• Parameters

• Class variables

• Instance variables

• Fields: refer to both class variables and instance variables. Variables refer to all of the above.

Mutable and immutable variables

• A variable is mutable if its orignal value can be modified by using some methods, and immutable if its original value can’t be modified through invoking methods.
• Primitive variables are immutable, while object (except string) variables are mutable.

Reserved words

• println short for “print line”, adds a special character called newline after each line, which moves the cursor to the next line of the display
• print displays the output from multiple print statements all on one line. The inside of a print statement can be an expression.
• return
• new
• import