Syntax

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Java derives much of its syntax from the C programming language: basic assignment statement syntax, expressions, control flow statements and blocks, etc. will be very familiar to C programmers.

Unicode 

Java source code are built by Unicode characters.

Tokens 

Java programs consist of a sequence of different kinds of tokens. For example, there are word tokens such as class and public which are keywords.

Keywords 

Those are special words with reserved meaning in Java. Those words can not be used by the programers to name identifiers.

Identifiers 

Other words (non keywords) are identifiers. Identifiers have many different uses in Java but primarily they are used as names, class names, method names, and variable names... .

literals 

Java also has tokens to represent numbers, such as 1 and 3; these are known as literals.

String literals, such as "http://en.wikibooks.org/Java_Programming", consist of zero or more characters embedded in double quotes.

Operators 

And operators such as + and = are used to express basic computation such as addition or String concatenation or assignment.

Blocks 

There are also left and right braces ({ and }) which enclose blocks. The body of a class is one such block.

Statements 

A Block contains one or more Java statement(s), separated by semicolons. A statement is the smallest building block of Java.

Separators 

Some tokens are punctuation, such as periods . and commas , and semicolons ;.

whitespace 

You use whitespace such as spaces, tabs, and newlines, to separate tokens. For example, whitespace is required between keywords and identifiers: publicstatic is a single identifier with twelve characters, not two Java keywords.

Comments 

Comments are not part of the executing code. Comments are used to document the code.

Unicode

Most Java program text consists of ASCII characters, but any Unicode character can be used as part of identifier names, in comments, and in character and string literals. Unicode escape sequences may also be used to express a Unicode character.

For example, π (which is the Greek Lowercase Letter pi) is a valid Java identifier.

double π = Math.PI;

and in a string literal

String pi = "π";

π may also be represented in Java as the Unicode escape sequence \u03C0. Thus, the following is a valid, but not very readable, declaration and assignment:

double \u03C0 = Math.PI;

The following demonstrates the use of Unicode escape sequences in other Java syntax:

// Declare Strings pi and quote which contain \u03C0 and \u0027 respectively: String pi = "\u03C0"; String quote = "\u0027";

Note that a Unicode escape sequence functions just like any other character in the source code. E.g., \u0022 (double quote, ") needs to be quoted in a string just like ".

// Declare Strings doubleQuote1 and doubleQuote2 which both contain " (double quote): String doubleQuote1 = "\""; String doubleQuote2 = "\\u0022"; // "\u0022" doesn't work since """ doesn't work.

See Unicode escape sequences for full details.

Literals

Java Literals are syntactic representations of boolean, character, numeric, or string data. Literals provide a means of expressing specific values in your program. For example, in the following statement, an integer variable named count is declared and assigned an integer value. The literal 0 represents, natually enough, the value zero.

int count = 0;

The following method call passes a String literal "int count = 0;" the boolean literal true and the special null value null to the method parse():

List items = parse("int count = 0;", true, null);

• Boolean Literals
• Numeric Literals
  • Character Literals
  • Integer Literals
  • Floating Point Literals
• String Literals
• null

Blocks

Java has a concept called block that is enclosed between the { and } characters, called curly braces. A block executed as a single statetement, and can be used where a single statetement is accepted.

After a block is executed all local variables defined inside the block is discarded, go out of scope.

{
...
 // -- This is a block ---
}

Blocks can be nested:

{
...
  {
     // -- This is a nested block ---
  }
}

Whitespaces

Whitespace in Java is used to separate the tokens in a Java source file. Whitespace is required in some places, such as between access modifiers, type names and Identifiers, and is used to improve readability elsewhere.

Wherever whitespace is required in Java, one or more whitespace characters may be used. Wherever whitespace is optional in Java, zero or more whitespace characters may be used.

Java whitespace consists of the

• space character ' ' (0x20),
• the tab character (hex 0x09),
• the form feed character (hex 0x0c),
• the line separators characters newline (hex 0x0a) or carriage return (hex 0x0d) characters.

Line separators are special whitespace characters in that they also terminate line comments, whereas normal whitespace does not.

Other Unicode space characters, including vertical tab, are not allowed as whitespace in Java.

Required Whitespace

Below is the declaration of an abstract method taken from a Java class

public abstract Distance distanceTo(Destination dest);

Whitespace is required between public and abstract, between abstract and Distance, between Distance and distanceTo, and between Destination and dest.

However, the following is not legal:

publicabstractDistance distanceTo(Destination dest);

because whitespace is required between keywords and identifiers. The following is lexically valid

publicabstractDistance distanceTo(Destination dest);

but means something completely different: it declares a method which has the return type publicabstractDistance It is unlikely that this type exists and the method is no longer abstract, so the above would result in a semantic error.

Indentation

Java ignores all whitespace in front of a statement. As this, these two code snippets are identical for the compiler:

public static void main(String[] args) {     printMessage(); } void printMessage() {     System.out.println("Hello World!"); }

public static void main(String[] args) { printMessage(); } void printMessage() { System.out.println("Hello World!"); }

However, the first one's style (with whitespace) is preferred, as the readability is higher. The method body is easier to distinguish from the head, even at a higher reading speed.