Java

20220531

Fist notes on java programming.

public class HelloWorld {
	public static void main(String args[] ) {
		String text = "hello world";
		System.out.println( text );
	}
}

If a class implements an interface all methods in that interface must be defined in the class

Variables

int var1 = 0

Variables must be declared and initialized

private int var1;

Private variables in classes can only be accessed from within the class (same for methods).

Access levels

• Public all can use it. Public attributes can be changes by all
• Protected, can be used within the class, by subclasses and by classes in the same package (A package is a group of related classes in one namespace)
• Default, like protected but no access from subclasses outside the package. If nothing is specified it is Default.
• Private can only be used within the class

Methods

• Every program must have a main method, that is executed when the program starts
• A 'void' method is a method with no return value, else the return value type must be specified
• Private methods in classes can only be accessed from within the class (same for variables).

Classes

abstract classes cannot be instantiated by itself it is only a superclass for its subclasses.

public class <subclass> extends <superclass>

Instantiate a subclass object

public class <subclass> extends <superclass> {
    public <subclass> (<types and attributes from superclass>)
        super(<attributes defined in superclass>)
    }
    private <type> <subclass specific attribute>

   public void <subclass specific method>() {
   }

Superclasses can have 'protected' attributes and methods making them available for subclasses and in the same package.

Methods and attributes in subclasses can override the superclass methods and attributes.

Interfaces

A bit like subclasses but only describing the methods used from the superclass. It does not implement the method but only declares it. The implementation is in the subclass. A subclass can implement multiple interfaces unlike inheritance that can have only 1 superclass.

Interface can extend other interfaces.

public class <subclass> implements <interface> {
    <attribute declarations>
    
    public void <method from interface>() {
    }
}