Misc

Advanced Programming

SUT • Spring 2019

Outline

  • Package

    • Package access

  • Static

  • this

  • Method overloading

  • toString

  • equals

Access Specifiers

Access Specifiers

  • public

    • Interface access

  • private

    • Don’t touch!

Example

public class Person {
    public String name;
    private Integer age;

    public String getName() {
        return this.name;
    }

    private Integer age() {
        return this.age;
    }
}

public class Main {
    public static void main(String[] args) {
        Person p = new Person();
        p.name = "Ali Alavi";
        p.age = 12; // Compile Error
    }
}

Public classes

  • You can declare a class which is not public

  • The class is visible within the file

  • There can be only one public class per file

  • The name of the public class must exactly match the name of the file

    • including capitalization

  • It is possible to have a java file with no public class

    • not typical

Object Access or Class Access

  • Access specifiers are declared for classes

    • Not for objects

  • When a member is declared as private

    • It is not visible in methods outside this class

    • It is visible by other objects of the same class

init() Method

public class Access {
	private String name;
	public Access(String name) {
		this.name = name;
	}
	public void check(Access access){
		access.name = name;
	}
	public static void main(String[] args) {
		Access a = new Access("Ali");
		Access b = new Access("Taghi");
		a.check(b);
		System.out.println(a.name);
		System.out.println(b.name);
	}
}

  • a can touch private property (name) of b

  • Because a and b has the same class

  • name is not private for b

  • name is private for Access

Package

  • A package contains a group of classes

  • Organized together under a single namespace

  • Packages organize classes belonging to the same category or providing similar functionality

  • A package provides a unique namespace for the types it contains

  • Classes in one package has the same folder
    Packages may contain other packages

    • Hierarchy of packages

Packages and Folders

Packages and Folders

Some famous java packages

  • java.util

  • java.lang

  • java.io

Package Access

  • Remember public and private access specifiers

  • The default access has no keyword

  • It is commonly referred to as package access

    • friendly

  • Other classes in the current package have access to that member

  • To classes outside of this package, the member appears to be private

Java Packages

  • import keyword

  • Class Qualified Name = package-name + class-name

  • For example

java .lang.String

java.lang.Math
    double sqrt = Math.sqrt(123);

java.util.Scanner

java.awt.Event

org.w3c.dom.events.Event

Using packages

import java.util.Arrays;
import java.util.*;

public class Swapping {
    public static void main(String[] args) {
        Scanner scanner = new Scanner(System.in);
        int nextInt = scanner.nextInt();
        System.out.println(nextInt);
        
        int[] original = new int[5];
        int[] copy = Arrays.copyOf(original, 10);
    }
}

Using packages

public class Swapping {
    public static void main(String[] args) {
        Integer a = 5;
        java.lang.Integer b = 5;
        java.lang.String s = "asd";
        java.lang.Boolean bb = true; 
    }
}

  • java.lang is implicitly imported

No import in Byte Code

  • Unused imports have a trivial impact on the compiler

  • But there are no imports in the byte code or at runtime

  • Unused import have no impact in runtime

    • But it is better to remove them

  • Organize Imports

Static Members and this

Writing eBay for Cars

class Peykan{...}
public class Pride {
    int color;
    int price;
    boolean isHatchBack;

    int designYear;
    int length, width;
}

Properties of objects

Properties of class

Static

  • When you create a class

    • You describe how objects of that class look

    • And how they will behave

  • You don’t actually get an object until you create one

    • using new

  • At that point storage is allocated and methods become available.

  • There are two situations in which this approach is not sufficient.

    • Class data or static property

    • Class method or static method

Class Data

  • You want to have only a single piece of storage for a particular field

  • regardless of how many objects of that class are created

  • or even if no objects are created

  • Example:

    • Pride.designYear

    • Person.?
       

  • Class data = static properties

Class Methods

  • if you need a method that isn’t associated with any particular object of this class.

  • You need a method that you can call even if no objects are created

  • Class methods = static methods

Static properties

  • static properties are shared among all the objects

  • static properties are properties of classes

    • Not objects

  • Example:

    • Integer.MAX_VALUE

    • Player.NumberOfObjects

Static methods

  • Static methods can access only static properties
  • Static methods are actually class operations
  • If a method uses only static fields, make it static!
  • Static methods are accessible via classes
    • double d = Double.parseDouble("12");
    • String s = String.valueOf(12);
    • public static void main(String[] args)
  • Static properties and methods are invoked on class name
    • Are not invoked on objects

Sample

public class Person {
    private String name;
    private int age;
    public static int MAX_AGE = 150;

    public void setAge(int age) {
        if (age < MAX_AGE)
            this.age = age;
    }

    public static int getMaxAge() {
        return MAX_AGE;
        //no access to age and name
    }
}

Static Initialization

  • Static properties are class data, not object data

  • Constructors are created for initializing object data

  • How to initialize class data?

  • Two ways:

    • Inline values

    • Static block

  • Static initialization is done when Class Loader loads the class

Inline initialization

public static int MAX_AGE = 150;
private static double PI = 3.14;

static String defaultName = theDefaultName(); 
private static String theDefaultName() {
    return "Ali Alavi";
}

Static Block

public class Person {
    public static int MAX_AGE;
    private static double PI;
    static String defaultName;

    private static String theDefaultName() {
        return "Ali Alavi";
    }

    static {
        MAX_AGE = 150;
        PI = 3.14;
        String s = theDefaultName();
        if (s != null)
            defaultName = theDefaultName();
    }
}

Initialization Block

public class SomeClass {
    private int value = 0;
    {
        System.out.println("initialization block: value = " + value + ".");
        value = 1;
    }

    public SomeClass(){
        System.out.println("SomeClass(): value = " + value + ".");
        value = 2;
    }

    public SomeClass(int param){
        System.out.println("SomeClass(int param): value = " + value + ".");
        value = 3;
    }

    public static void main(String[] args){
        SomeClass c1 = new SomeClass();
        SomeClass c2 = new SomeClass(2);
    }
}

Order of initialization

  • Once per class

    • Static variable declaration

    • Static block

  • Once per object

    • variable declaration

    • Initialization block

    • Constructor

Sample

public class Person {
    public static int MAX_AGE;
    private static double PI = 3.14;

    static {
        MAX_AGE = 150;
    }

    private String nation = "Iran";
    private int age;
    private String name;

    {
        name = "Ali";
    }

    public Person() {
        age = 10;
    }

    public Person(int a, String n) {
        age = a;
        name = n;
    }
}
public static void main(String[] args) {
   Person p1 = new Person();
   Person p2 = new Person(20, "Taghi");
}

this

Method-property access

  • How does getArea() know where radius is?

public class Circle {
    private double radius;
    
    public Circle(double r) {
        this.radius = r;
    }

    public double getArea() {
        return this.radius * this.radius * 3.14;
    }

    public static void main(String[] args){
        Circle c = new Circle(2);
        System.out.println(c.getArea());
    }
}

Compiler solution

  • A reference to the object is implicitly passed to methods

    •  circle.getArea()

  • is converted (by compiler) to something like:

    • Circle.getArea(circle)

  • What if you want to access this circle object?

    • Use this keyword

  • this is available within non-static methods

Sample Application of this

public class Lead {
    int i = 0;
    Leaf increment() {
        i++;
        return this;
    }

    void print(){
        System.out.println("i = " + i);
    }

    public static void main(String[] args){
        Leaf x = new Leaf();
        x.increment().increment().increment().print();
    }
}

Sample Application of this (2)

public class Book {
    private String name;
    private Person author;

    public void setName(String name) {
        this.name = name;
    }

    public void setAuthor(Person author) {
        this.author = author;
    }
}

Static and this

  • What are static methods?

  • Methods without implicit this

  • Static methods are bound to classes, not objects

Overloading

Method Overloading

  • Methods with the same name in the same class

  • With different parameters

  • Different Signatures

Sample

class Circle {
    void f() {
        System.out.println("f is called");
    }
    
    void f(int number) {
        System.out.println("f is called with number = " + number);
    }
    
    void f(String s) {
        System.out.println("f is called with s = " + s);
    }
    
    void f(String s, int number) {
        System.out.println("f is called with s = " + s + 
                            " and number = " + number);
    }
    
    public static void main(String[] args){
        Circle circle = new Circle();
        circle.f();
        circle.f(5);
        circle.f("salam");
        circle.f("salam", 5);
    }
}

No Return type method overloading

Why this is not permitted?

class Circle {
    void f() {
        System.out.println("f is called");
    }
    
    int f() {
        System.out.println("f is called");
        return 0;
    }
}

Type Operations

Type Casting

  • You can change the type by type casting
  • Casting in reference types and other objects is not so simple
    • See It Later
long longValue = 98127391827L;
int iad = longValue; // Compiler Error
int i = (int) longValue;


Integer intValue = 391827;
Long lad = (Long) intValue; // Runtime Error

Type Conversion

  • Sometimes we want to convert an object to another type

  • Type casting is not the solution here

  • We should write some methods to convert the types

Type Conversion (2)

Scanner scanner = new Scanner(System.in);
String s = scanner.next();
int intValue = Integer.parseInt(s);

Integer i = 12;
String str = i.toString();

toString

  • toString() is a special method

  • You can write it for every class

Sample

public class Rectangle {
    private int length, width;
    public Rectangle(int length, int width) {
        this.length = length;
        this.width = width;
    }

    public String toString() {
        return "Retangle[length=" + length + ", width=" + width + "]";
    }

    public int getArea(){
        return width + length;
    }

    public static void main(String[] args){
        Rectangle r = new Rectangle(10, 2);
        System.out.println(r.getArea());
        System.out.println(r.toString());
        System.out.println("Here: " + r.toString());
    }
}

Checking equality in java

int i = 5;
int j = 4+1;
System.out.println("i is equal to j: " + (i == j))

String i = new String("ali");
String j = new String("ali");
System.out.println("i is equal to j: " + (i == j))

String i = "ali";
String j = "ali";
System.out.println("i is equal to j: " + (i == j))

String i = new String("ali");
String j = new String("ali");
System.out.println("i is equal to j: " + (i.equals(j)))

Creating Your Own Equals()

public class Person {
    private String nationalID;
    private String name;
    private String email;
    private int age;

    public boolean equals(Person other) {
	return nationalID.equals(other.nationalID);
    }
}

Person p1 = new Person("1290786547", "Ali Alavi");
Person p2 = new Person("1290786547", "Taghi Taghavi");
Person p3 = new Person("0578905672", "Taghi Taghavi");
System.out.println(p1.equals(p2));
System.out.println(p2.equals(p3));

Creating Your Own Equals()

  • In fact, equals() is more complicated

  • The parameter should be an Object

    • Person as the parameter was incorrect

  • More checks are required

  • IDE Support for writing equals

    • Use your IDE for

      • getters, setters, constructors, …

Sample

public boolean equals(Object obj) {
  if (this == obj)
    return true;

  if (obj == null)
    return false;

  if (getClass() != obj.getClass())
    return false;

  Person other = (Person) obj;
  if (nationalID == null) {
    if (other.nationalID != null)
      return false;
  } else if (!nationalID.equals(other.nationalID))
      return false;

  return true;
}

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