Useful Java Terms


Some definitions taken from:


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.



public class myClass(){

     //data members






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.



     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

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);