These are highlights from a Java 8 developer with years of experience trying to highlight only the important details that we easily miss throughout the time. You might think that sometimes is silly to highlight some things but even those things might help you to remember the entire context. —

  • Remember that from left to right, once you concatenate a string everything else will be considered a String like the examples below:

System.out.println(2 + 2 + "C"); // Result in 4c
System.out.println("C" + 2 + 2); // Result in C22

Remember for the exam the following String methods

  • length()
  • charAt(…)
    • chartAt(int position)
  • indexOf(…)
    • indexOf(int ch, int fromIndex)
    • indexOf(String str)
    • indexOf(String str, int fromIndex)

var name = "animals";
System.out.println(name.indexOf('a')); // 0
System.out.println(name.indexOf("al")); // 4
System.out.println(name.indexOf('a',4)); // 4
System.out.println(name.indexOf("al",5)) // 1
  • substring(…)
    • substring(int beginIndex)
    • substring(int beginIndex, int endIndex)

var name = "animals";
System.out.println(name.substring(3)); //mals
System.out.println(name.substring(name.indexOf('m'))); //mals
System.out.println(name.substring(3,4)); //m
System.out.println(name.substring(3,7)); //mals
System.out.println(name.substring(3,3)); //Empty string
System.out.println(name.substring(3,2)); //Exception
System.out.println(name.substring(3,8)); //Exception
  • equalsIgnoreCase(String str) - Remember it only applied to String
  • startsWith(String prefix)
  • endsWith(String suffix)
  • contains(CharSequence charSeq)
  • replace(…)
    • replace(char oldChar, char newChar)\
    • replace(CharSequence target, CharSequence replacement)
  • trim()\
  • strip() - It does the same as the trim() but also supports unicode
  • stripLeading() - Removes whitespaces only from the beginning
  • stripTrailing() - Removes whitespaces only from the end
  • ident() - adds and removes spaces from the beginning of the string.

// To better visualize the code below will generate an output of

var block = """
                             d
                              e
                             f""";

        System.out.println(block);
        System.out.println("------");
        System.out.println(block.indent(1)); // Note that the result will be 10 because the ident always add a a line break at the end of text blocks for normalization if there is none.
        System.out.println("------");
        System.out.println(block.indent(-1)); 
        System.out.println("------");
        System.out.println(block.length());
        System.out.println(block.indent(1).length());
        System.out.println(block.indent(-1).length());
        
        
        
/**
 *
 * 
d
 e
f
 ------
  d
   e
  f

 ------
 d
 e
 f

 ------
 6
 10
 6
 * 
 * */
  • translateEscapes()

// What it does is translate the scape in case there is one.. better to check the example below

var str = "1\\t2";
System.out.println(str) // 1\t2
System.out.println(str) // 1 2
  • String.format(…)
    • String.format(String format, Object args)
    • String.format(Locale loc, String format, Object args…)
  • formatted(Object…) this is like the format directly in the string
  • symbols
    • %s Applies to any type
    • %d Integer values
    • %f float and double
    • %n insert a line break
  • char is also a numeric type therefore you can do something like below

for(char current = 'a'; current <= 'z'; current++)

StringBuilder important methods

  • substring(int start, int end)
  • length()
  • charAt(int position)
  • insert(int offset, String str)
  • delete(int start, int end)

var sb = new StringBuilder("abcdef");

// Remember that the end position is righ before the index applied.
sb.delete(1,3) // sb = adef
  • replace(int start, int end, String newValue) - This is used to replace a portion of the string
  • reverse() – this method will reverse the entire String
  • String pool is an intern pool where the strings are stored and shared so if you have somehting like below it will be the same object

var x = "Hello World";
var y = "Hello World";

System.out.println(x == y);
// This returns true
  • On the other hand if you evaluate something in runtime like below they will not be the same String

var x = "Hello World";
var y = "Hello World".trim();

System.out.println(x == y);
// This returns false

//This also return false
var x = "Hello World";
var y = new String("Hello World");

  System.out.println(x == y);
  • intern() - If the literal is not in the pool java will add in runtime

var name = "Hello World";
var name2 = new String("Hello World").intern();
System.out.println(name == name2)

// This will return true
  • Important examples for String pool

var first = "dog" + 1; // This is added in the String pool making it a **compile time constant**
var second = "d" + "o" + "g" + "1"; // This is added in as well
var third = "r" + "a" + "t" + new String("1");
System.out.println(first == second);

Arrays

  • Ways of declaring an array variable;

// All of the declaration below have the same result

int[] numDogs;
int [] numDogs2;
int []numDogs3;
int numDogs4[];
int numDogs5 [];
  • Instantiating array

int[] numbers = new int[3];

// Instantiating and initializing
int[] numbers = new int[3]{3,4,5};

//Or using the short way 
int[] numbers = {3,4,5};

// You cannot use var to declare and instantiate an array. It will not work
var numbers = {3,4,5}; // Does not compile.

  • Remember that the array does not allocate space for the String objects. instead, it allocates space for a reference to where the objects are really stored.

  • Binary search is an optimized way of searching in the array, but to make it work your array must be sorted.


int[] numbers = {};
  • Arrays.compare() can be used to compare arrays but there are some rules that you need to remember to make sure that you will remember them.


int[] arr1 = {1,2};
        int[] arr2 = {1};
        //This will return 1 since first array is longer;
        System.out.println(Arrays.compare(arr1,arr2));


        int[] arr3 = {1,2};
        int[] arr4 = {1,2};
        //This will return 0 since this is an exact match
        System.out.println(Arrays.compare(arr3,arr4));

        String[] arr5 = {"a"};
        String[] arr6 = {"aa"};
        //This will return a negative number since the first is  a substring of the second
        System.out.println(Arrays.compare(arr5,arr6));


        String[] arr7 = {"a"};
        String[] arr8 = {"A"};
        //This will return a positive number since lower case in the compare is higher
        System.out.println(Arrays.compare(arr7,arr8));



        String[] arr9 = {"a"};
        String[] arr10 = {null};
        //Null is smaller than letter therefore it will be positive
        System.out.println(Arrays.compare(arr9,arr10));
  • mismatch() if the array is equals it returns -1 otherwise it returns the first index where it differs.
  • Multidimensional arrays can be declared in several ways in the exam and you have to differ them.

int[][] vars1; // 2D array
int vars2 [][]; // 2D array
int[] vars3[]; // 2D array
int[] vars4[], space [][]; // This is a 2D and a 3D array

Math api

  • Math.round()
    • Math.round(123.50); // Will return 124
    • Math.round(123.45f); // Will return 123
  • Math.ceil(3.14); 4.0
  • Math.floor(3.14); 3.0

  • Math.random()

    Date and time

  • LocalDate.now();
  • LocalTime.now();
  • LocalDateTime.now();
  • ZonedDateTime.now();
  • LocalDate.of(…)
    • LocalDate.of(2022, Month.JANUARY, 20)
    • LocalDate.of(2022, 1, 20)
  • LocalTime.of(…)
    • LocalTime.of(6,15) - hour and minute
    • LocalTime.of(6,15,30) + seconds
    • LocalTime.of(6,15,30,200) + nanoseconds
  • LocalDateTime.of(…)
    • LocalDateTime.of(int year, int month, int dayOfMonth, int hour, int minute)
    • LocalDateTime.of(int year, int month, int dayOfMonth, int hour, int minute, int second)
    • LocalDateTime.of(int year, int month, int dayOfMonth, int hour, int minute, int second, int nanos)
    • LocalDateTime.of(int year, Month month, int dayOfMonth, int hour, int minute)
    • LocalDateTime.of(int year, Month month, int dayOfMonth, int hour, int minute, int second)
    • LocalDateTime.of(int year, Month month, int dayOfMonth, int hour, int minute, int second, int nanos)
    • LocalDateTime.of(LocalDate date, LocalTime time)
  • ZonedDateTime.of(…)
    • ZonedDateTime.of(int year, int month, int dayOfMonth, int hour, int minute, int second, int nanos, ZoneId zone)
    • ZonedDateTime.of(LocalDate date, LocalTime time, ZoneId zone)

    • ZonedDateTime.of(LocalDateTime dateTime, ZoneId zone)

  • Remember that these date classes are Immutable

var date = LocalDate.now();
date = date.plusDays(2); // This will return a new isntance
  • Period class has the following methods
    • Period.ofYears(long)
    • Period.ofMonths(long)
    • Period.ofDays(long)
    • Period.ofWeeks(long)
    • Period.of(1,0,7)
  • Period class can be used In LocalDate and LocalDateTime since it takes only years months and days

var     myDate = LocalDate.now();

      var myPeriod = Period.ofMonths(1);

      System.out.println(myDate.plus(myPeriod));
  • Duration works similarly as Period but it has also different behavior since it is intendent for smaller periods of time
    • Duration.ofDays(long)
    • Duration.ofHours(long)
    • Duration.ofMinutes(long)
    • Duration.ofSeconds(long)
    • Duration.ofMillis(long)
    • Duration.ofNanos(long)
    • Duratino.of(long,ChornoUnit)
 // Using ChronoUnit your code would look like more or less like below

    Duration.of(1, ChronoUnit.DAYS)
  • You can also use ChronoUnit to get the difference
var firstHour = LocalTime.now();
var secondHour = LocalTime.now();
        

ChronoUnit.HOURS.between(firstHour,secondYour)
  • You have to remember that in 13 of march at 2 pm it is not Daylight saving time and the hour 2:30 will no exist for example
  • Considering the explanation above, remember that java is smart enough to do all the adjustments
  • When you call substring with only one parameter it will go from there until the end.

var numbers
  • dateTime.toInstant() does not exists