Java Basics

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Variables
Operators
Expressions, Statments and Blocks
Control Flow Statments
What Does It Mean To Compile Code
Explain Like I’m Five
Questions

Variables

The Java programming language defines the following kinds of variables:

• Instance Variables (Non-Static Fields) Technically speaking, objects store their individual states in “non-static fields”, that is, fields declared without the static keyword. Non-static fields are also known as instance variables because their values are unique to each instance of a class (to each object, in other words); the currentSpeed of one bicycle is independent from the currentSpeed of another.

• Class Variables (Static Fields) A class variable is any field declared with the static modifier; this tells the compiler that there is exactly one copy of this variable in existence, regardless of how many times the class has been instantiated. A field defining the number of gears for a particular kind of bicycle could be marked as static since conceptually the same number of gears will apply to all instances. The code static int numGears = 6; would create such a static field. Additionally, the keyword final could be added to indicate that the number of gears will never change.

• Local Variables Similar to how an object stores its state in fields, a method will often store its temporary state in local variables. The syntax for declaring a local variable is similar to declaring a field (for example, int count = 0;). There is no special keyword designating a variable as local; that determination comes entirely from the location in which the variable is declared — which is between the opening and closing braces of a method. As such, local variables are only visible to the methods in which they are declared; they are not accessible from the rest of the class.

• Parameters Given public static void main(String[] args), the args variable is the parameter to this method. The important thing to remember is that parameters are always classified as “variables” not “fields”. This applies to other parameter-accepting constructs as well (such as constructors and exception handlers).

Operators

postfix	expr++ expr–
unary	++expr –expr +expr -expr ~ !
multiplicative	* / %
additive	+ -
shift	« » »>
relational	< > <= >= instanceof
equality	== !=
bitwise AND	&
bitwise exclusive OR	^
bitwise inclusive OR	|
logical AND	&&
logical OR	||
ternary	? :
assignment	= += -= *= /= %= &= ^= |= «= »= »>=

Expressions, Statements, and Blocks

Expressions

An expression is a construct made up of variables, operators, and method invocations, which are constructed according to the syntax of the language, that evaluates to a single value.

Examples:

• Taken int cadence = 0; cadence = 0 is the expression.
• Taken int result = 1 + 2; result = 1 + 2 is the expression.

The data type of the value returned by an expression depends on the elements used in the expression.

Statements

Statements are roughly equivalent to sentences in natural languages. A statement forms a complete unit of execution. The following types of expressions can be made into a statement by terminating the expression with a semicolon (;).

• Assignment expressions
• Any use of ++ or –
• Method invocations
• Object creation expressions

// assignment statement
aValue = 8933.234;
// increment statement
aValue++;
// method invocation statement
System.out.println("Hello World!");
// object creation statement
Bicycle myBike = new Bicycle();

Blocks

A block is a group of zero or more statements between balanced braces and can be used anywhere a single statement is allowed.

class BlockDemo {
     public static void main(String[] args) {
          boolean condition = true;
          if (condition) { // begin block 1
               System.out.println("Condition is true.");
          } // end block one
          else { // begin block 2
               System.out.println("Condition is false.");
          } // end block 2
     }
}

Control Flow Statements

The statements inside your source files are generally executed from top to bottom, in the order that they appear. Control flow statements, however, break up the flow of execution by employing decision making, looping, and branching, enabling your program to conditionally execute particular blocks of code. This section describes the decision-making statements (if-then, if-then-else, switch), the looping statements (for, while, do-while), and the branching statements (break, continue, return) supported by the Java programming language.

What Does It Mean To Compile Code

Explain Like I’m Five

Questions

1. What does “strong typed” mean?

   • Strong typed languages require data to be explicitly declared and will not allow implied but not declared data types.

2. Explain to a non-technical friend the difference in how compilation works in Java and JavaScript.

   • Java is a compiled programming language. Meaning that written code is handled by another program that translates it to machine language (0’s and 1’s). Think of Java acting as a way to better read and write code for humans and the compiler translates that to a non english speaking entity (a computer processor). Compiled languages are typically faster than interpred languages.

   • JavaScript is an interpreted language. A program called an interpreter reads line by line and executes the code it reads immediately, never handing it down to the processor in machine language.

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