main, abs, read, and other static Methods

• main - execution consists of objects invoking methods of other objects, but how does the process begin?
  • Where does the first object come from?
  • Who invokes its method?
• abs - operates on int which is a primitive type
  • no classs definition
  • no methods
• read - no object yet
  • We're trying to read in value of an object
  • Object isn't created yet-- how dow we invoke a read method on it?

static Methods to the Rescue

Static methods

• No receiver
• No access to instance variables
• Class method
• Standard invocation syntax, but class name is used instead of receiving object

Static Data - State for Static Methods

Static variables

• One copy per class (as opposed to one copy per object)
• Often used when maintaining info about the class as a whole
  • Assigning id's to objects (employee id's, student id's)
  • Constants that need nbot be repeated for each instance
    • Color constants, Mode constants

read Methods

Need to create the object before can invoke its methods

class Name {
	Name(String last, String first) {
		this.last = last;
		this.first = first;
	}
	
	static Name read(BufferedReader br) {
		String line = br.readLine();
		if (line == null) return null;	// No more names in file
		line = line.trim();
		
		int pos = line.indexOf(",");
		String last = line.substring(0, pos);
		line = line.substring(pos+1).trim();
		
		pos = line.indexOf(",");
		String first = line.substring(0, pos);
		line = line.substring(pos+1).trim();

		return new Name(last, first);
	}