1. CSC240 Computer Science III
Zhen Jiang
Dept. of Computer Science
West Chester University
West Chester, PA 19383
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2. Table of Contents Introduction Declaration and call Access Scope
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3. Introduction Why? Why loop Why array Why method
Why object (the host of instance method)?
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4. Every thing under the control inside of object (which must be declared in the class, as a general template to cover all possible changes/statuses)
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5. state: String name, breed int age
Dog class
state: String name, breed
int age
behavior: writeOutput ( ) getAgeInHumanYears ( )‏
Dog object
balto
state: “Balto” (name)
8 (age)
“Siberian Husky” (breed)
behavior: getAgeInHumanYears ( )
writeOutput ( )
Dog object
scooby
state: “Balto” (name)
8 (age)
“Siberian Husky” (breed)
behavior: getAgeInHumanYears ( )
writeOutput ( )

6. Declaration and Call No more static How to use instance method m()?
public class A {
public void m() { … }
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7. Access Where is possible to call? Call at each different scenario
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8. From another instance method in the same or different class
public class A {
public void m() { … }
public void m2() {
m();
public class B { public void m() { … } public void m3() { m() A a; //a.m is not supported A b = new A (); b.m();
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9. From static method such as void main
public class A {
public void m() { … }
public static void m2() {
A a = new A();
a.m();
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10. Call of a static method by instance methods, for a comparison
public class A {
public static void m() { … }
public void m2() {
m();
public class B { public void m() { … } public void m3() { m() A.m();
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11. Self-test
public class A { public void m1(){ System.out.println("A's m1"); m2(); } public static void m2(){ System.out.println("A's static m2"); } }
public class B { public void m1(){ A a = new A(); a.m1(); A.m2(); } public static void m2(){ System.out.println("B's static m2"); B b2 = new B(); b2.m1(); } public static void main(String args[]){ A a = new A(); B b = new B(); b.m1(); B.m2(); } }
A's m1
A's static m2
B's static m2
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12. Constructor Name No return Not static Related to the use of new
Overloading
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13. Mutator Accessor toString Argument and parameter
Type (compatible & overloading)
Primitive type and reference type (array & object)
Accessor
Return
toString
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14. Constructor public class A { public A() { … } public A(int x) {
public class B { public void m3() { A a = new A(); A b = new A(2); }
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15. Mutator and accessor public class A { public A() { … }
public void a() {
public void A() {
public class B { public void m3() { A a = new A(); a.a(); a.A(); // but very confused! }
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16. Note: Every class has a toString method.
When an object is printed or concatenated with a String, Java calls the object's toString method.
System.out.println("p1 is " + p1);
is equivalent to:
System.out.println("p1 is " + p1.toString());
Note: Every class has a toString method.
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17. toString public class A { private int x = 2; public A() { … }
public String toString () {
// no parameter!
return “a” + x;
public class B { public void m3() { A a = new A(); System.out.println(a); }
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18. Scope Attribute (property, data field, etc) Parameter What is “this”
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19. Attribute what is this public class A { private int x;
public A(int y) {
x = y; }
public void m(int x) {
// int x;
this.x = x;
public String toString(){
return x+””;
public class B { public void m3() { A a = new A(2); a.m(3); System.out.println(a); }
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20. Parameter public class A { public int x; public A(int y) { x = y; }
public void n() {
// another method
// other than m
public void m(A a) {
// what attribute can be used: x vs. a.x
// What refers, by the call a.m(b): a.x vs. b.x
// What method can be used: n(); vs. a.n();
// What they refer, by a.m(b): a.n() and b.n()
public class B { public void m3() { A a = new A(2); A b = new A(3); a.m(b); }
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21. What is local a, is it different from that in a.m(b)?
Parameter
public class A {
private int x;
public A(int y) {
x = y; }
public void n() {
// another method
// other than m
public int getX(){
return x;
public void m(A a) {
// How to access a.x and b.x inside m
// upon the call a.m(b):
x vs. a.getx()
// Call methods a.n and b.n insider m
// upon the call a.m(b)
n(); vs. a.n();
public class B { public void m3() { A a = new A(2); A b = new A(3); a.m(b); }
What is local a, is it different from that in a.m(b)?
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22. Why b. x is used in slide 20 but b. getx(), i. e
Why b.x is used in slide 20 but b.getx(), i.e., a method is used in slide 21?
Public vs. private
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