/belajar-java

Primary LanguageJava

Belajar Java

Installation

ref:

Numeric Primitive Data Type

Data Type Bits Minimum Maximum
byte 8 -128 127
short 16 -32,768 32,768
int 32 -2,147,483,648 2,147,483,648
long 64 -9.22337E+18 9.22337E+18
float 32 See the docs
double 64 See the docs
  • Java class library includes helper classes for each primitive.
  • Helper classes support conversion and formatting tools.
Data Type Helper Class
byte Byte
short Short
int Integer
long Long
float Float

Helper Class Example

  • java.lang.Double support double valuse
double doubleValue = 156.3d;
Double doubleObj = new Double(doubleValue);
byte byteValue = doubleObj.byteValue();
int intValue = doubleObj.intValue();
float floatValue = doubleObj.floatValue();
String strValue = doubleObj.toString();

Primitive Numeric Default

  • Primitive numeric variables default to 0
int myInt;
System.out.println("the value of myInt is " + myInt);

Using Java Operators

  • Assignment
  • Incrementing/Decrementing
  • Postfix vs Previx Incrementing
  • Comparing Values
Operator Purpose
== equal
!= not equal
> greater than
< less than
>= greater than or equal
<= less than or equal
instanceof class membership

example instanceof:

String s = "Hello";
if (s instanceof java.lang.String) {
    System.out.println("s is a string");
}
  • Comparing strings
String s1 = "Hello";
String s2 = "Hello";
if (s1.equal(s2)) {
    System.out.println("they are match");
} else {
    System.out.println("no match!");
}
  • Logical Operators
Operator Purpose
&& And
|| Or
?: Ternary (short hand for if-then)

example ternary:

String s = condition ? trueValue : falseValue;

Java Classes Define Objects

  • An object is an instance of a class
  • Nonprimitive variables are references to objects
  • Objects can have multiple references

Example:

public class ClothingItem {

    // an instance variable a.k.a. a field
    // since this doesn't have static keyword
    // is NOT a member of the class itself
    // it's a member of the instance of the class
    public String type;

    public static void main(String[] args) {
        // this line has datatype 'ClothingItem' -> the name of the class
        // the identifier -> the name of the variable
        // the keyword 'new'
        // and the constructor method
        // this is called Instantiation
        ClothingItem item = new ClothingItem();
        item.type = "Hat";
        item.displayItem();
    }

    // this is a method without static keyword
    // called instance method
    private void displayItem() {
        System.out.println("this is a " + this.type);
    }

    private void displayItem() {
        System.out.println("this is a " + this.type);
    }
}

String is an object

  • String values are instances of java.lang.String

Example:

String str = new String("Hello!");
// shortcut
String str = "Hello!";
  • String objects are immutable
String str = "Halo";
// re-assign like this is not changing object
// but in fact, in the background, java direfrencing
// the original object. the object can now be cleared
// from memory in the process know as garbage collection
String str = "Hai";

Char Array to String

public class Main {
    public static void main(String[] args) {
        char[] chars = {'H', 'a', 'l', 'o'};
        String s = new String(chars);
        System.out.println(s);
    }
}