1. Lesson A4 – Object Behavior
References from
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2. Objects
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3. Object Instantiation Chicken yeller = new Chicken();
yeller is a Chicken reference. new Chicken() creates a new Chicken Object out in memory.
yeller stores the location of that new Chicken Object.
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4. Object Instantiation Chicken yeller = new Chicken(); yeller
0x234
0x234
Chicken
yeller is a Chicken reference. new Chicken() creates a new Chicken Object out in memory.
yeller stores the location of that new Chicken Object.
yeller is a reference variable that refers
to a Chicken object.
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5. Methods
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6. What is a method? A method is a storage location
for related program statements.
When called, a method usually
performs a specific task.
System.out.println( )
Methods store commands / program statements. When called, the code inside the method is activated.
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7. What methods have we used?
dude.goHome()
keyboard.nextInt( )
System.out.println( )
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8. out.println("cluck-cluck"); }
methods
public void speak() {
out.println("cluck-cluck"); }
The speak method shown above contains a single println command.
The speak method would print out cluck-cluck on the console window.
OUTPUT
cluck-cluck
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9. methods public void speak( ) { System.out.println("cluck-cluck"); }
access
return type
name
params
code
public void speak( ) {
System.out.println("cluck-cluck"); }
A method has a signature. The signature provides information about the method. The name is most used and recognizable part of the signature. The method shown above is named print. The return type states what the method will return. Method print has a return type of void which means the method does not return a value. The access of method print is public. This states that the method print can be called from any location.
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10. All members with public access can be accessed or
What does public mean?
All members with public
access can be accessed or
modified inside and outside of the class where they are
defined.
Public access simply means the member can be used anywhere inside or outside of the class.
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11. chicken OUTPUT public class Chicken { public void speak()
out.println("cluck-cluck"); }
public static void main(String[] args)
Chicken red = new Chicken();
red.speak();
OUTPUT
cluck-cluck
In the Chicken example, method speak() prints out cluck-cluck each time it is called. Method speak() is called three times; thus, it prints out cluck-cluck three times.
OUTPUT
cluck-cluck
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12. Variables
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13. Instance Variables When you need many methods
to have access to the same
variable, make the variable an instance variable / instance field.
The scope of an instance variable
is the entire class where the
variable is defined.
An instance variable is a variable tied to an instance of a class. Each time an Object is instantiated, it is given its own set of instance variables.
Instance variables are also commonly called instance fields.
Monster x = new Monster();
x refers to a unique Monster that contains its own set of Monster instance variables.
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14. Instance Variables OUTPUT 11 public class InstanceVars {
private int one = 8, two = 3; //instance variables / fields
private int answer = 0; //exist throughout the class
public void add(){
answer = one + two; }
public void print(){
System.out.println(answer);
public static void main(String args[])
InstanceVars test = new InstanceVars();
test.add();
test.print();
OUTPUT 11
Class InstanceVars contains three instance variables : one, two, and total. Each time class InstanceVars is instantiated, a new set of instance variables is created inside of the new Object.
InstanceVars test = new InstanceVars();
test refers to an InstanceVars Object that contains one, two, and three.
InstanceVars diff = new InstanceVars();
diff refers to an InstanceVars Object that contains one, two, and three.
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15. InstanceVars one two answer 8 3 0
add
print
All methods in class InstanceVars can access instance vars one, two, and answer.
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16. All members with private access can be accessed
What does private mean?
All members with private
access can be accessed
or modified only inside the
class where they are
defined.
All members of a class with private access can be accessed or modified within the class where they are defined only. Private members cannot be accessed outside of the class.
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17. All data members should have private access. A set of public
Encapsulation
All data members should have
private access. A set of public
methods should be provided to
manipulate the private data.
Data should be declared with private access and public methods should be provided to manipulate the private data.
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18. defining
parameters
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19. defining parameters public void times( int num1, int num2 ) {
out.println(num1*num2); }
There will be times that we define parameters when we define a method. The parameters allow us to specify the type of data the method will receive.
Methods are often defined with a parameter list. Parameters are defined within the parenthesis following the method name.
public void method( parameter list )
When defining parameters, a data type and name must be provided for each parameter.
public void method(int one, double two)
public void go(String word, int num)
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20. passing parameters OUTPUT15 //code in main in another class
Fun test = new Fun();
test.times(3 , 5);
OUTPUT15
public class Fun {
public void times( int num1, int num2 )
out.println(num1*num2); }
When calling a method with parameters, the data types and number of parameters are very important.
public void method(int one, double two)
A call to method would have to have 2 parameters. A call to method would require passing in an integer and a double in that order.
method(6, 9.3);
method(562, );
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21. passing parameters OUTPUT15 //code in main in another class
public class Fun {
public static void times( int one, int two )
out.println(one*two); }
When calling a method with parameters, the data types and number of parameters are very important.
public static void times(int one, double two)
times is defined as static. A static method exists without the need for an instantiation of the class. Notice that there is no Fun x = new Fun() line in the main example.
Fun.times(6, 9);
Fun.times(11,22);
//code in main in another class
Fun.times(3 , 5);
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22. Variable
Scope
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23. Scope { int fun = 99; } Any variable defined inside of braces,
only exists within those braces.
That variable has a scope limited to
those braces.
When a variable is defined within a set of braces, that variable can only be accessed inside those braces.
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24. Instance Variables When you need many methods
to have access to the same
variable, you make that variable
an instance variable.
The scope of an instance variable
is the entire class where that
variable is defined.
An instance variable is a variable tied to an instance of a class. Each time an Object is instantiated, it is given its own set of instance variables.
Monster x = new Monster();
x would refer to a new Monster that contains its own set of Monster instance variables.
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25. Instance Variables OUTPUT 11 public class InstanceVars {
private int one = 8, two = 3; //instance variables
private int total = 0; //exist throughout the class
public void add(){
total = one + two; }
public void print(){
System.out.println(total);
public static void main(String args[])
InstanceVars test = new InstanceVars();
test.add();
test.print();
OUTPUT 11
Class InstanceVars contains three instance variables : one, two, and total. Each time class InstanceVars is instantiated, a new set of instance variables is created.
InstanceVars test = new InstanceVars();
test refers to an InstanceVars Object that contains its own set of one, two, and three.
InstanceVars diff = new InstanceVars();
diff refers to an InstanceVars Object that contains its own set of one, two, and three.
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26. Defining VS. Assigning int num; int num = 99; num = 56;
definition only
definition and assignment
When defining a variable, the type must be listed and the name. When defining and assigning a variable, the type, name, and value must be listed. When assigning a variable only, the name and value must be listed.
assignment only
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27. Local Variables When you need only one method
to have access to a variable,
you should make that variable
a local variable.
The scope of a local variable is
limited to the method where it
is defined.
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28. Local Vars OUTPUT public class LocalVars {
private int fun; //instance variable
public void change() {
int fun = 99; //local variable }
public void print() {
System.out.println(fun);
public static void main(String args[])
LocalVars test = new LocalVars();
test.change();
test.print();
OUTPUT
Class LocalVars has an one instance variable named fun.
Method change contains a local variable named fun. Local variables take precedence over instance variables. Local variables can only be used inside the method in which they are defined.
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29. fun = 99 LocalVars fun 0 change print
Class LocalVars has an one instance variable named fun.
Instance variable fun can be used anywhere in class LocalVars.
Method change contains a local variable named fun. Method change does not make any changes to the instance variable fun. Local variable fun can only be used in method change.
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30. Constructors
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31. very similar to methods have the same name as the class
Constructors
very similar to methods
have the same name as the class
have no return type – no void,int,etc.
initialize all instance variables
Constructors are used to initialize all of the data. Typically, all variables are assigned a value in the constructor.
A constructor is very similar to a method, but it is technically not a method.
Constructors have no return type and are named the same as the class. This is slightly different from a method.
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32. Constructors VS. Methods
access
name
params
code
Constructors and methods are more similar than different and sometimes constructors are referred to as constructor methods. Most of the differences are not visible. The only real visible differences exist in that the constructor has to be named the same name as the class and that the constructor has no return type. Methods and constructors both have a name, a list of parameters, and a block of code to execute. When you call a method or a constructor, the block of code associated with the name will be executed.
method
access
return type
name
params
code
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33. Triangle triangle = new Triangle();
class Triangle {
private int sideA, sideB, sideC;
public Triangle()
sideA=0;
sideB=0;
sideC=0; }
Default
Constructor
Default constructors have no parameter list. When a default constructor is called, all instance variables / data fields are set to a zero value. If no constructors are provided for a class, Java will provide a default constructor that will initialize all data to a zero value.
Triangle triangle = new Triangle();
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34. Initialization Constructor
public Triangle(int a, int b, int c) {
sideA=a;
sideB=b;
sideC=c; }
Constructors often have parameters. The parameters allow data to be passed into the class so that it can be assigned to the instance variables / data fields.
Initialization constructors have a parameter list and will receive parameters when called. The number of parameter and types passed in must match up with the parameter list following the method name.
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35. Initialization Constructor Triangle triangle = new Triangle(3,4,5);
class Triangle {
private int sideA, sideB, sideC;
public Triangle(int a, int b, int c)
sideA=a;
sideB=b;
sideC=c; }
Initialization constructors have a parameter list and will receive parameters when called. The number of parameter and types passed in must match up with the parameter list following the method name.
Triangle triangle = new Triangle(3,4,5);
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36. return methods
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37. Return Methods int num = keyboard.nextInt();
Return methods perform some action
and return a result back to the
calling location.
int num = keyboard.nextInt();
Return methods typically take in some type of data, do something to the data, and then send back a result. nextInt() is a Scanner return method. nextInt() retrieves the next integer value from the keyboard and sends it back to num.
nextInt() returns an int back to the calling location. The value returned is assigned to num.
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38. Return Methods INPUT 1 OUTPUT 1 return method num 1 Scanner keyboard =
new Scanner(System.in);
int num = keyboard.nextInt();
out.println(num);
INPUT 1
OUTPUT 1
nextInt() gets the next int entered on the keyboard and returns it. The int returned by nextInt() is placed in num.
return method
num 1
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39. Return Method access return type name params code
Return methods have a return type specified before the method name.
public int getNum()
getNum() returns an integer value.
public double getStuff()
getStuff() returns a double/decimal value.
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40. Math
return methods
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41. frequently used methods
Math
frequently used methods
Name
Use
floor(x)
rounds x down
ceil(x)
rounds x up
pow(x,y)
returns x to the power of y
abs(x)
returns the absolute value of x
sqrt(x)
returns the square root of x
round(x)
rounds x to the nearest whole number
min(x,y)
returns smallest of x and y
max(x,y)
returns biggest of x and y
random()
returns a double >=0.0 and < 1.0
The Math class contains many useful math related methods.
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42. Math Methods INPUT 3.45 OUTPUT 4.0 num 3.45 return methods
Scanner keyboard =
new Scanner(System.in);
double num = keyboard.nextDouble();
out.println(Math.ceil(num));
INPUT 3.45
OUTPUT
4.0
Math methods are return methods. First, a value is passed to a math method. Second, the math method performs some action. Finally, the math method returns a result.
num 3.45
return methods
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43. Math Methods
OUTPUT
out.println(Math.floor(3.254));
out.println(Math.ceil(2.45));
out.println(Math.pow(2,7));
out.println(Math.abs(-9));
out.println(Math.sqrt(256));
out.println(Math.sqrt(144));
out.println(Math.round(3.6));
out.println(Math.max(5,7));
out.println(Math.max(5,-7));
out.println(Math.min(5,7));
out.println(Math.min(5,-7));
floor(val) returns val decreased to the nearest interger.
ceil(val) returns val increased to the nearest interger.
pow(x,y) reutrns x raised to the power of y
abs(val) returns the absolute value of val
sqrt(val) returns the square root of val
round(val) returns val rounede to the nearest integer
max(one, two) returns the largest of one and two
min(one, two) returns the smallest of one and two
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44. Math Methods
out.println(Math.random()*10);
int num = (int)(Math.random()*10);
out.println(num);
OUTPUT
random() returns a random number between 0.0 and 1.0 not including 1.0.
random() returns a double in the range 0.0 to 1.0, not including 1.0.
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45. Modifier methods are methods that change the properties of an object.
Modifier methods make changes to the instance variables of the class.
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46. modifier methods OUTPUT 13 public class Calc { private int one, two;
private int answer;
public void setNums( int n1, int n2 ){
one=n1;
two=n2; }
public void add(){
answer = one + two;
public void print(){
System.out.println(answer);
test.setNums(4,9); test.add(); test.print();
Modifier methods make changes to the instance variables of the class.
Method setNums() assigns parameter n1 to instance variable one and parameter n2 to instance variable two.
The purpose of method setNums() is to modify the instance variables / instance fields.
OUTPUT 13
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47. Pieces of the OOP Puzzle Part Two
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48. constructors Default Constructor public Triangle() { sideA=0; sideB=0;
sideC=0; }
Default
Constructor
Constructors are similar to methods. Constructors set the properties of an object to an initial state.
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49. constructors Initialization Constructor
public Triangle(int a, int b, int c) {
sideA=a;
sideB=b;
sideC=c; }
Initialization
Constructor
Constructors are similar to methods. Constructors set the properties of an object to an initial state.
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50. modifier methods public void setSides(int a, int b, int c) { sideA=a;
sideB=b;
sideC=c; }
Modifier methods are methods that change the properties of an object.
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51. accessor methods public void print() {
out.println(sideA + " " + sideB + " " + sideC); }
Accessor methods are methods that retrieve or grant access to the properties of an object, but do not make any changes.
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