Array

자바의 배열에 대해 정리하였습니다.

 

 

---

< Introduction >

To store a group of data

You can think of array as List in real life.

 

 

---

< Creating an Array Explicitly >

< Syntax >

DataType[] VariableName = {element1, element2, element3, ...};

Array is represented by [ ]
DataType : the type of data it holds.
element : Each content item inside an array.

 

Example)

double[] prices = {13.15, 15.87, 14.22, 16.66};

String[] clothingItems = {"Tank Top", "Beanie", "Funny Socks", "Corduroys"};

public class Newsfeed {
  public Newsfeed(){
  }
  
  public String[] getTopics() {
    String[] topics = {"Opinion", "Tech", "Science", "Health"};
    return topics;
  }

  public static void main(String[] args){
    Newsfeed sampleFeed = new Newsfeed();
    String[] topics = sampleFeed.getTopics();
    System.out.println(topics);
  }
}

// [Ljava.lang.String;@2aae9190

 

 

---

< Importing Arrays >

To have a more descriptive printout of the array itself (not memory address), need a toString( ) method that is provided by the Arrays package in Java.

 

• To import Arrays package, put this code at the top of the file,
  

  import java.util.Arrays;​

• When we import a package in Java, we are making all of the methods of that package available in our code.

 

Example)

import java.util.Arrays;
 
public class Lottery(){
  public static void main(String[] args){
    int[] lotteryNumbers = {4, 8, 15, 16, 23, 42};
    String betterPrintout = Arrays.toString(lotteryNumbers);
    System.out.println(betterPrintout);
  }
}

// [4, 8, 15, 16, 23, 42]

 

 

---

< Get Element By Index >

To access each element in an array using [ ].

 

• the index of an array starts at 0

 

Example)

int[] number = {1, 2, 3, 4, 5, 6};
System.out.println(number[1]);   //   2

 

 

---

< Creating an Empty Array >

We can also create empty arrays and then fill the items one by one. 

 

< Syntax >

new DataType[size]

dataType : the type of data it holds.
size : the size(length) of array.

• Empty arrays have to be initialized with a fixed size.
  
• Once you declare the size, it cannot be changed!

 

Example)

String[] menuItems = new String[5];

menuItems[0] = "Veggie hot dog";
menuItems[1] = "Potato salad";
menuItems[2] = "Cornbread";
menuItems[3] = "Roasted broccoli";
menuItems[4] = "Coffee ice cream";
// also change an item after it has been assigned
menuItems[3] = "Baked cauliflower";

// the array looks like
["Veggie hot dog", "Potato salad", "Cornbread", "Baked cauliflower", "Coffee ice cream"]

 

 

+. When we use new to create an empty array, each element of the array is initialized with a specific value depending on what type the element is:

Data Type	Initialized Value
int	0
double	0.0
boolean	false
Reference	null

 

Example)

String[] my_names = new String[5];
int[] my_ages = new int[5];

: my_names contain five nulls.

: my_ages contain five 0s

 

 

---

< Length >

To get the length of an array using length property of the array.

 

Example)

double[] prices = {13.1, 15.87, 14.22, 16.66};
System.out.println(prices.length);   //   4

 

 

---

< String[] args >

A String[] is an array made up of Strings

The args parameter is another example of a String array.
The array args contains the arguments that we pass in from the terminal when we run the class file.

 

Example : When we run the file 'HelloYou' in the terminal with an argument of "Laura"

java HelloYou Laura

public class HelloYou {
  public static void main(String[] args) {
    System.out.println("Hello " + args[0]);  
  }
}

// Hello Laura