Java SE: Programming Complete - 3

https://learn.oracle.com/ols/course/java-se-programming-complete/82508/85200

Text, Date, Time and Numberic Objects

String

• String is a class (not a primitive), can be instantiated by using the new keyword.
• However, String is the only Java object that allows simplified instantiation as a text value enclosed with double quotes, and it’s a recommened approach.
• JVM can optimize memory allocated to store String objects by maintaining a single copy of each String literal in the String Pool memory area, regardless of how many variables reference this copy (This process is called interning)
• Using new keyword disables String interning.
• The intern() method returns a reference to an interned (single) copy of a String literal.

  String a = new String("Hello");
  String b = new String("Hello");
  String c = "Hello";
  String d = "Hello";
  System.out.println(a == b);       // false
  System.out.println(a.intern() == b.intern());   // true
  System.out.println(c == d);       // true

• String is immutable, String operations such as trim(), contact(), toLowerCase(), toUpperCase() ould always return a new String, would not modify the original String.
• String contains a sequence character indexed by integer, starts from 0.
• When getting a substring of a string, the begin index is inclusive of the result, but end index is out.

StringBuilder

• StringBuilder is mutable, allowing modifcation of the character sequences they store.

Wrapper Classes for Primitives

• Wrapper classes apply object-oriented capabilities to primitives.
• Construct wrapper object out of primitive or string using the valueOf() methods
• Extract primitive values out fo the wrapper using the xxxValue() methods.
• Instead of formal conversion of wrapper to primitive and back, you can use direct assignment known as auto-boxing and auto-unboxing
• Create wrapper of primitive out of the string using the parseXXX() methods: float f = strObj.parseFloat(1.23F)
• Convert a primitive to string using the String.valueOf() method: String strObj = String.valueOf(3)
• Wrapper classes provide constants, such as min and max values for every type. for example Short.MIN_VALUE

Representing Numbers Using BigDecimal Class

• The java.math.BigDecimal class is useful in handling decimal numbers that require exact precision

Method Chaining

• When an operation returns an object, you may invoke next operation upon this object immediately.
• Without method chainning, code appears to be cluttered with unnecessary intermediate variables

Local Date and Time

• Classes LocalDate, LocalTime and LocalDateTime are introduced in Java SE 8, to address the shortcomings of the older java.util.Date and java.util.Calendar. (Thread Safety, API design and timezone)
• Date and time objects can be created using methods now() to get current date and time, or using of() for specific date and time.
• Local Date and Time objects are immutable
• Operations isBefore() and isAfter() check if a date or time is before or after another.

Durations, Periods and Instants

• The java.time.Duration class can represent an anoumnt of time in nanoseconds.
• The java.time.Period class can represent an amount of time in units such as years or days.
• The java.time.Instant class can represent an instantaneous point on the time-line (time-stamp)
• Duration, Period and Instant are immutable.

Zoned Date and Time

• Time zones can be applied to local date and time values java.time.ZonedDateTime
• The java.time.ZoneId class defines time zones.
• ZonedDateTime provides time zone specific operations such as withZoneSameInstant
• Accounts for daylight saving time and time zone differences.

Represent Languages and Countries

• The java.util.Locale class represents lauguages and countries
• Local can represent just language or a combination of language plus country or area.

Format and Parse Numeric Values

• The java.text.NumberFormat class is used to parse and format numeric values.

Doulbe d = 0.2;
Locale locale = new Locale("en", "GB");
NumberFormat currencyFormat = NumberFormat.getCurrencyInstance(locale);
NumberFormat percentageFormat = NumberFormat.getPercentInstance(locale);
String c = currencyFormat.format(d);
String p = percentageFormat.format(d);

Format and Parse Date and Time Values

• The java.time.format.DateTimeFormatter class is used to parse and format date and time values.
• You can set custom format pattern or use standard format patterns defined by java.time.format.FormatStyle enum.

Localizable Resources

• Resource bundles contain localizable resources.
• Resource bundles can be represented as plain text file with the extension .properties
• Resources are placed into resource bundles as =
• The java.util.ResourceBUndle class loads bundles and retrieves resources.
• Default bundle can be used if no locale is specified

Format Message Patterns

• Formatter classes parse and format messages, numbers, date and time values.
• The java.text.MessageFormat class substitutes values into message patterns.
• Message patterns can be stored in resource bundles. for examle product={0}, price {1}, quantity {2}, best before {3} // {x} will be replaced by the real vaule with MessageFormat.format(pattern, args...)