Day 1 - 1Z0-829

sudo apt install openjdk-17-jdk -y

The above command installs java 17 in Ubuntu

Naming Conflicts

java.util.Date and java.sql.Date

• Explicit mention always gets the precedence over any wildcard

• If you want to use both, then use a fully qualified name like this may be:

  •     public class Conflicts {
            java.util.Date date;
            java.sql.Date date;
        }
    

Argument Options

• 

Compiling with Jar file

on Win:

java -cp ".;C:\temp\someOtherLocation;c:\temp\myJar.jar" myPackage.MyClass

on Unix based:

java -cp ".:/tmp/someOtherLocation:/tmp/myJar.jar" myPackage.MyClass

In the above commands,

• . - means to look for class files in the current dir

• : - used in Unix based to separate paths

• ; - used by Win to separate paths

Order of Initialization

• Fields → Initializer Block → Constructor

• Initializer block cannot initialize a field before its declaration → ❌Doesn’t Compile

Data Types

• Primitive

  

  • Without l/L all non-decimals are int

  • Without f/F all the decimals are double

  • Bit size of boolean depends on the JVM

• Literals:

    double notAtStart = _100.00;     // Doesn't Compile
    double notAtEnd = 10000.00_;     // Doesn't Compile
    double notByDecimal = 10_.5;     // Doesn't Compile
    double rufrbro = 1_0_0.0__0;     // Doesn't Compile
    double idgafnow = 6_______9;     // Doesn't Compile
  

• Wrapper Class:

  • WRAPPER_CLASS.valueOf($PRIMITIVE_VALUE) → Create the Wrapper Object

  • int primitive = Integer.parseInt("123"); → Conv String to int primitive

  • Integer wrapper = Integer.valueOf("123"); → Conv String to Integer (Wrapper of int)

Text Blocks

• 

• Example:

String block = """
    "doe\"\"\"
    \"deer\"""
   """;
System.out.print("*" + block + "*");

// The Output is:
* "doe"""
 "deer"""
*

• Note: Text blocks require a line break after the opening """

P. S.

• Any variable used inside a text block is a string and not a variable

• var is not a reserved word

• All Wrapper Classes defaults to null

• boolean defaults to false

• There is no default value for method params