How do you to store content in a class array?

Question

I'm trying to code an array of date objects called january, and fill with Jan 1, 2017 - Jan 31, 2017.

I have already created a class called Date and stored String months, int days, and int years. But I need a faster way to store all this data. Also, I was wondering if you could create a function called ToString() that returns a nicely formatted string containing the given date.

This is what I have so far:

Date[]january = new Date[31];
january[0].months = "January";
january[0].days = 1;
january[0].years =2017;
january[1].months = "January";
january[1].days = 2;
january[1].years =2017;

Answer 1

First of all, that code will crash. When you create a Date[31] you're creating an empty array, which will be filled with nulls. When you try to set a property on a null object, your program will crash. Before setting properties of january[0], you need to initialize it to new Date() (or whatever constructor is appropriate).

Instead of writing out all the days manually, you can use a for loop to generate them all.

Date[] january = new Date[31];
for (int i = 0; i < january.length; i++) {
    january[i] = new Date();
    january[i].months = "January";
    january[i].days = i + 1;
    january[i].years = 2017;
}

Note that when setting the days, 1 must be added to i because the array is zero-indexed but days of the month are not.

Edit: Here's my simple implementation of Date based on the code in the question:

public class Date {
    private String months;
    private int days;
    private int years;

    @Override
    public String toString() {
        return months + " " + days + ", " + years;
    }
}

Answer 2

If this were real work rather than homework, I would:

• Use the java.time objects for date-time work rather than make my own.
• Use an ArrayList rather than a array.

The YearMonth class represents, well, the year-month.

Month is an enum, with a dozen pre-defined objects, one for each month of the year January-December.

A LocalDate is a date-only value, without time-of-day and without time zone.

List is an interface for an ordered collection, and ArrayList is a particular implementation.

YearMonth ym = YearMonth.of( 2017 , Month.JANUARY ) ;
int monthLength = ym.lengthOfMonth() ;
List< LocalDate > daysInJanuary = new ArrayList<>( monthLength ) ;
for( int i = 1 ; i <= monthLength ; i ++ ) {
    LocalDate ld = ym.atDay( i ) ;
    daysInJanuary.add( ld ) ;
}

daysInJanuary.toString(): [2017-01-01, 2017-01-02, 2017-01-03, 2017-01-04, 2017-01-05, 2017-01-06, 2017-01-07, 2017-01-08, 2017-01-09, 2017-01-10, 2017-01-11, 2017-01-12, 2017-01-13, 2017-01-14, 2017-01-15, 2017-01-16, 2017-01-17, 2017-01-18, 2017-01-19, 2017-01-20, 2017-01-21, 2017-01-22, 2017-01-23, 2017-01-24, 2017-01-25, 2017-01-26, 2017-01-27, 2017-01-28, 2017-01-29, 2017-01-30, 2017-01-31]

To generate a nicely formatted string representing each date's value, I would let java.time localize automatically.

Locale locale = Locale.CANADA_FRENCH ;  // Or Locale.US, etc.
DateTimeFormatter f = DateTimeFormatter.ofLocalizedDate( FormatStyle.FULL ).withLocale( locale ) ;

for( LocalDate ld : daysInJanuary ) {
    String output = ld.format( f ) ;
    System.out.println( output ) ;
}

Output is something like this, depending on the Locale:

Sunday, January 1, 2017

Monday, January 2, 2017

Tuesday, January 3, 2017

…

See this code run live at IdeOne.com. But be aware that IdeOne.com is limited to only a single Locale, Locale.US.

Answer 3

import java.util.*;
import java.text.SimpleDateFormat;

Date[]january = new Date[31];
for(int i=0;i<31;i++){ // or use january.length in place of 31
    january[i] = new Date(2017, 0, (i+1));
    System.out.println(formatDate(january[i]));
}

public static String formatDate(Date date){
       return (new SimpleDateFormat("MMMM").format(date) + " "+ new SimpleDateFormat("DD").format(date)  + ","+ date.getYear());
}

Use for loop or another loop because only date is changing in your code.