Object-oriented Programming

What is OOP?

It is quite normal to be ask “What is OOP” in the interview.
According to its definition in Wikipedia

Object-oriented programming (OOP) is a programming paradigm based on the concept of “objects”, which can contain data and code: data in the form of fields (often known as attributes or properties), and code, in the form of procedures (often known as methods).

In short, an object has fields and methods.

3 properties of an object:

• Behavior: method and process.
• State: how will it respond when calling the method.
• Identity: identify object with same behavior and state.

Class

The program use Class to contruct an instance of object.

Data stores in instance field and method is used to operate data.

Relationship

3 Types of relationships

• Dependence: uses-a, method in A use the method in B
• Aggregation: has-a, A has object B
• Inheritance: is-a, A is a onject of B

Encapsulation 封装

Protect the field. The field cannot be modified directly using assignment.

To Get & Set

• a private field
• a public acessor / getter
• a public settor

Final

Set a fild to be final will require initialization in constructor. It cannot be modified later.(cannot set)

class Emlpoyee
{
    private final String namd;
}

Constructor

Overloading

Methods have same name but difference perameters.

Static

Static saves memory to make progrm memory efficient.

Field

Every object of this class shares the same static field, whereas every object has its onw copy of non-static field.

When there is no object of this class, the static field still exist as it belong to class, not belong to object.

Every object can set value of this static field, so try not to use public if possible.

Constant / Final

It can be call directly using class without contruct any instance.

public class Math
{
    public static final double PI = 3.14...;
}

Call using

Math.PI;

Method

Call using class without instance.

Math.pow(1,2);

Parameter of Method

Call by

• value: passing value to method
• reference: passing address
• name: not used in java

Call by Value

Java are mainly using call by value.

public void triple(double x)
{
    //x will copy the value of parameter
    x*=3;
}
double init = 10;
triple(init); // init =10, x=30

Call by Reference

To use call by reference, passing an object to parameter

public void triple(Employee x)
{
    //x will be a copy of reference
    x.raiseSalary(3);
}
Employee init = new Employee(); // init is a reference of an instance
triple(init); // both x and init pointing to the same object

Package

import

Use * to import every class under a package. But cannot use java.* to import every package.
Static import allow using method without prename

import java.util.*;
import static java.lang.System.*;

out.println("sample"); //System.out

Construct

Use package packagename at the beginning. Make sure the file structure is the same as the index.

package com.horstmann.corejave; //cd com/horstmann/corejave
public class Employee
{
    ..
}