Useful Java Terms
Some definitions taken from:
http://java.about.com/od/beginningjava/a/javaglossary.htm
http://java.sun.com/docs/glossary.html
Assign to set the data of a variable (involves the equal
sign).
Example 1:
assign the value 2 to the integer var.
int
var = 2;
Example 2:
// myObject1 is assigned a new
instance of MyObject().
MyClass myObject1 = new
MyObject();
// myObject2 is assigned the same
instance as myObject1
MyClass myObject2 =
myObject1;
Class - A class describes a particular kind of object. It can contain related
methods and data members (variables). A class must have the same name as the
file it is contained in.
Example:
public class myClass(){
//data members
//constructors
//methods
}
Constructor - A special type of instance method that creates a
new object. In Java, constructors have the same name as their class and have no
return value in their declaration.
Example:
public
class
myClass{
// a constructor that takes no
parameters
public myClass(){
}
// a constructor that
takes one parameter
public myClass(int var){
}
}
Declaration - A statement that creates a variable, method, or
class identifier and its associated attributes but doesn't necessarily allocate
storage for variables or define an implementation for methods. Classes are
always defined when they are declared, i.e., a class cannot be declared and
then have its body defined elsewhere.
Example 1:
the variable var is declared
int var;
Example 2:
the object myObject is declared as an instance of the MyClass class.
MyClass
myObject;
Example 3:
abstract methods, found in abstract
classes, can be declared but not defined.
public abstract class myAbstractClass(){
// abstract
methods are declared.
public abstract myMethod()
}
Example 4:
methods found in interfaces can be
declared but not defined.
public interface myInterface(){
// methods in an interface
are declared
public void myMethod();
}
Definition Similar to a declaration except that it also
reserves storage (for variables) or provides implementations (for methods).
Example 1:
variables created from primitive data
types are defined when declared.
// this declaration also allocates
storage for var.
int
var;
Example 2:
a method is defined if it has brackets,
a space where code goes that says what the method does.
public void myMethod(){
/* these brackets can contain code that defines what this method
does */
}
Garbage Collection Programs require memory to run. Memory for objects
is allocated by the keyword new. When objects are no longer used or your
program terminates, Java automatically frees the used memory for other uses.
Initialize
an assignment that sets the starting value of a variable.
Example 1:
an integer var is
declared, defined, and initialized to the value 2.
int
var = 2;
Example 2:
an integer var is declared and defined. Then it is initialized to 2.
int
var;
var
= 2;
Example 3:
an object is declared via MyClass myObject,
instantiated via new, and
initialized by calling its constructor, MyClass().
MyClass
myObject = new
MyClass(/*parameters*/);
Instantiate
To allocate storage for an object in memory (involves the keyword new).
Example: the keyword new
instantiates the object below. An instantiation is always followed by a
constructor call that initializes the object.
MyClass myObject = new MyClass(/*parameters*/);
Method a collection of code found within a class. If the
data members of a class are nouns, the methods are the verbs (the action).
Example 1: a method, myMethod is defined within class myClass that
does NOT return a value.
public class myClass{
public void myMethod(){
}
}
Example 2: a method, myMethod is defined within class myClass that
returns a boolean.
public class myClass{
public boolean myMethod(){
return
true;
}
}
Object - The principal code building block of Java programs.
Each object in a program consists of both variables (data) and methods
(functionality).
Parameter
a variable or object passed into a method.
Example
1: a method, myMethod is defined which takes 2 parameters.
public class MyClass{
public void myMethod(int
var1, double
var2){
}
}
Example 2: myMethod is called.
int
myInteger = 2;
double
myDouble = 5.0;
myMethod(myInteger, myDouble);
Primitive A variable defined with a primitive data type: byte, short, int, long, float, double, char, or boolean.
Typecast (1) to demote a variable from a larger capacity
data type to a smaller one. (2) to re-establish the class of an object. The
cast associates itself with the expression to its immediate right.
Example
1: A double var is created and then typecast via (int) to a
variable of integer type ans.
double var = 3;
int ans = (int) var;
Example
2: All objects extend the Java Object class.
// define class MyClass in file
MyClass.java
public class MyClass{}
// in the main method of another class
// instantiate MyClass, but store it
in a variable of type Object.
Object o = new MyClass();
// typecast object o, back into an
instance of MyClass.
MyClass myObject = (MyClass) o;
Use/Read the use of a variable in the right hand side of an
assignment statement.
Example: an integer var
is declared and used in the
initialization of integer ans.
int
var = 2;
int
ans = Math.pow(var,2);