Basics of Java (1)

January 27, 1999 CS536

Notes by Ho-Seop Kim

Java can be thought as "C+++" : it throws away bads and inherits goods; more straightforward than C++

Primitive data types: int, short, long, char, byte, float, double, boolean; everything else is a class object

"There's no pointer in Java" <-> "Everything in Java is a pointer"

First declare an object, then create it

C++:

int a[100]; // declare and allocate space (on stack)

int[] a = new int[100];
int a[] = new int[100];

Simple example

class point{

private static int howmany = 0;

private int x;

private int y;

point(){x = 0; y = 0; howmany++;}

point(int xin, int yin){x = xin; y = yin; howmany++;}

void clear(){x = 0; y = 0;}

}

point p; // just creates reference
point p = new point(); // constructor

If no user-defined constructor is given, Java will run default constructor; it sets fields to some default values (false for boolean, 0 for int etc.)

All dynamically allocated memory spaces are automatically garbage-collected when it is not referenced any more

Classes have

Members

Members are

Methods (code/functions) // unlike C/C++, method must have a return type

Constructor // no return type

Visibility: public, private, package (default; similar to public), protected

Static vs instance

point p = new point();
point q = new point(1, 2);
       +-----+        point
p ---> |x = 0|       +-----------+
       |y = 0|       |howmany = 2|
       +-----+       +-----------+
       +-----+
q ---> |x = 1|
       |y = 2|
       +-----+

Sometimes we need variable generic to class. But there's no global variable in Java. All data should be in a class. With public static int howmany = 0;we can use point.howmany.

Static function: You don't need to create class instance to use it. For example, Math.sin(). Static function should not use any non-static member.

Simple code in C++ vs Java

int a = 1;        public class test{
main(){               static public void main(String[] args){
   a = 0;                 a = 0;
}                     }
                  }

To compile and run a Java program in CS environment (JDK),

javac test.java ---> test.class

java test // no ".class"

Some useful methods inherited from Object class

clone()

point p = new point(1, 2);

point q = p; // shallow copy; p & q point the same point object

point q = (point)p.clone(); // deep copy

toString() - useful for print out

System.out.print(p); // automatically calls point.toString()

equals() - boolean equals(Object o)

p == q // compares references

p.equals(q) // compares contents