Java SE: Programming Complete - 4

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

Classes and Objects

Designing Classes

• Description of the package that this class is a member fo
• Description of imports of classes from different packages that this class may need to reference
• This class access modifer, keyward class followed by the class name.
• Class and method bodies are enclosed with {}
• Uninitialized primitives are defaulted to 0, boolean to false.
• Uninitialized object references are defaulted to null.

package <PackageName>;

import <PackageName>.<ClassName>;

<AccessModifer> <VariableType> <VariableName> = <VariableValue>;
<AccessModifer> <VariableType> <VariableName>;

<AccessModifer> class <ClassName> {
  <AccessModifer> <ReturnType> <MethodName>(<ParameterType> <ParameterName>, ...) {

    return <ReturnValue>
  }
}

Object Creation and Access

• The new operator creates an Object, allocating memory to store this object
• You may assign an object reference to a variable of the appropriate type.
• Access variable or methods of the object using . operator.

Local Variables and Recursive Object Reference

• Variables declared inside methods are known as Local.
• Method parameters are essentially local variables
• The local variable can “shadow” the instance variable if their names coincide
• Use the this keyword to refer to current instance.

Local Variable Type Inference (from Java 10)

• There is no need to describe a variable type if it can be unambiguously inferred from the context
• Inter types of local variables with initializers.
• No need to explicitly declare local variable type if it can be inferred from the assigned value.
• This feature is limited to
  • Local variables with initializers
  • Indexes in the enhanced for-loops
  • Local variables declared in a traditional for-loops
• Overuse can reduce code readability.

Define Constants

• Constants represent data that is assigned once and cannot be changed.
• The keyword final is used to make a variable as a constant.
• Instance final variable must be either initialized immediately or via all constructors. (No default value here, because why you need a variable if it’s always default value?)

Static Context

• Each Class has its own memory context
• Class memory context (also known as static context) is shared by all instances of this class
• The keyword static is ued to mark variables or methods that belong to the class context.
• To access static context, ojbect reference is not required (but can be used)
• Attempt to access current instance methods or variable from the static context will result in compiler error.

• Static initializer runs once, before any other operation (when class is loaded)

  class example {
    static {
      xxxx
    }
  }
  
  

Combining Static and Final

• Shared constants can be defined as static and final variables
• It provides a simple way of defining globally visible constatns.
• Encapsulation (private access modifier) not necessary because value is read-only
• All Math class operations are static
• Factory methods are static methods that create and return a new instance.
• static import enables referencing static variables and methods of another class as if they are in this class.