Core Java
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The document discusses Java classes, methods, and constructors. It explains that classes define data objects and operations that can be applied to those objects. Methods and variables can be declared as static, existing independently of any object. Constructors are used to create instances of a class and automatically invoke the superclass constructor.
![ Person mary = new Person ( );
int myArray[ ] = new int[5];
int myArray[ ] = {1, 4, 9, 16, 25};
String languages [ ] = {"Prolog", "Java"};
Since arrays are objects they are allocated dynamically
Arrays, like all objects, are subject to garbage collection
when no more references remain
◦ so fewer memory leaks
◦ Java doesn’t have pointers!](https://image.slidesharecdn.com/corejavaconcepts-110807041828-phpapp01-150920043622-lva1-app6891/85/Core-Java-10-320.jpg)
![ C:UMBC331java>type echo.java
// This is the Echo example from the Sun tutorial
class echo {
public static void main(String args[]) {
for (int i=0; i < args.length; i++) {
System.out.println( args[i] );
}
}
}
C:UMBC331java>javac echo.java
C:UMBC331java>java echo this is pretty silly
this
is
pretty
silly
C:UMBC331java> NSIT ,Jetalpur](https://image.slidesharecdn.com/corejavaconcepts-110807041828-phpapp01-150920043622-lva1-app6891/85/Core-Java-12-320.jpg)
![/* This program computes the factorial of a number
*/
public class Factorial { // Define a class
public static void main(String[] args) { // The program starts
here
int input = Integer.parseInt(args[0]); // Get the user's
input
double result = factorial(input); // Compute the
factorial
System.out.println(result); // Print out the
result
} // The main() method
ends here
public static double factorial(int x) { // This method
computes x!
if (x < 0) // Check for bad
input
return 0.0; // if bad, return 0
double fact = 1.0; // Begin with an
initial value
while(x > 1) { // Loop until x
equals
fact = fact * x; // multiply by x
each time NSIT ,Jetalpur](https://image.slidesharecdn.com/corejavaconcepts-110807041828-phpapp01-150920043622-lva1-app6891/85/Core-Java-13-320.jpg)
Core Java
- 1. Khasimsaheb
- 2. The javadoc program generates HTML API documentation from the “javadoc” style comments in your code. /* This kind comment can span multiple lines */ // This kind is of to the end of the line /* This kind of comment is a special * ‘javadoc’ style comment */
- 3. The class is the fundamental concept in JAVA (and other OOPLs) A class describes some data object(s), and the operations (or methods) that can be applied to those objects Every object and method in Java belongs to a class Classes have data (fields) and code (methods) and classes (member classes or inner classes) Static methods and fields belong to the class itself Others belong to instances
- 4. class Person { Variable String name; int age; Method void birthday ( ) { age++; System.out.println (name + ' is now ' + age); } }
- 5. { int x = 12; /* only x available */ { int q = 96; /* both x and q available */ } /* only x available */ /* q “out of scope” */ } { int x = 12; { int x = 96; /* illegal */ } } This is ok in C/C++ but not in Java.
- 6. Java objects don’t have the same lifetimes as primitives. When you create a Java object using new, it hangs around past the end of the scope. Here, the scope of name s is delimited by the {}s but the String object hangs around until GC’d { String s = new String("a string"); } /* end of scope */
- 7. Java methods and variables can be declared static These exist independent of any object This means that a Class’s ◦ static methods can be called even if no objects of that class have been created and ◦ static data is “shared” by all instances (i.e., one rvalue per class instead of one per instance class StaticTest {static int i = 47;} StaticTest st1 = new StaticTest(); StaticTest st2 = new StaticTest(); // st1.i == st2.I == 47 StaticTest.i++; // or st1.I++ or st2.I++ // st1.i == st2.I == 48
- 8. public class Circle {public class Circle { // A class field// A class field public static final double PI= 3.14159; // A usefulpublic static final double PI= 3.14159; // A useful constantconstant // A class method: just compute a value based on the// A class method: just compute a value based on the argumentsarguments public static double radiansToDegrees(double rads) {public static double radiansToDegrees(double rads) { return rads * 180 / PI;return rads * 180 / PI; }} // An instance field// An instance field public double r; // The radius of thepublic double r; // The radius of the circlecircle // Two methods which operate on the instance fields of// Two methods which operate on the instance fields of an objectan object public double area() { // Compute the area ofpublic double area() { // Compute the area of the circlethe circle return PI * r * r;return PI * r * r; }} public double circumference() { // Compute thepublic double circumference() { // Compute the circumference of the circlecircumference of the circle return 2 * PI * r;return 2 * PI * r; }} }}
- 9. Subscripts always start at 0 as in C Subscript checking is done automatically Certain operations are defined on arrays of objects, as for other classes ◦ e.g. myArray.length == 5
- 10. Person mary = new Person ( ); int myArray[ ] = new int[5]; int myArray[ ] = {1, 4, 9, 16, 25}; String languages [ ] = {"Prolog", "Java"}; Since arrays are objects they are allocated dynamically Arrays, like all objects, are subject to garbage collection when no more references remain ◦ so fewer memory leaks ◦ Java doesn’t have pointers!
- 11. Example Programs
- 12. C:UMBC331java>type echo.java // This is the Echo example from the Sun tutorial class echo { public static void main(String args[]) { for (int i=0; i < args.length; i++) { System.out.println( args[i] ); } } } C:UMBC331java>javac echo.java C:UMBC331java>java echo this is pretty silly this is pretty silly C:UMBC331java> NSIT ,Jetalpur
- 13. /* This program computes the factorial of a number */ public class Factorial { // Define a class public static void main(String[] args) { // The program starts here int input = Integer.parseInt(args[0]); // Get the user's input double result = factorial(input); // Compute the factorial System.out.println(result); // Print out the result } // The main() method ends here public static double factorial(int x) { // This method computes x! if (x < 0) // Check for bad input return 0.0; // if bad, return 0 double fact = 1.0; // Begin with an initial value while(x > 1) { // Loop until x equals fact = fact * x; // multiply by x each time NSIT ,Jetalpur
- 14. Classes should define one or more methods to create or construct instances of the class Their name is the same as the class name ◦ note deviation from convention that methods begin with lower case Constructors are differentiated by the number and types of their arguments ◦ An example of overloading If you don’t define a constructor, a default one will be created. Constructors automatically invoke the zero argument constructor of their superclass when they begin (note that this yields a recursive process!) NSIT ,Jetalpur
- 15. Java methods are like C/C++ functions. General case: returnType methodName ( arg1, arg2, … argN) { methodBody } The return keyword exits a method optionally with a value int storage(String s) {return s.length() * 2;} boolean flag() { return true; } float naturalLogBase() { return 2.718f; } void nothing() { return; } void nothing2() {}