SCJP – Quick Notes

DECLARATIONS AND ACCESS CONTROL

ACCESS MODIFIERS

Modifier Used with Description
public Classes
Interfaces
Constructors
Inner Classes
Methods
Field variables
A Class or Interface may be accessed from outside its package.
Constructors, Inner Classes, Methods and Field variables may be accessed from wherever their class is accessed.
protected Constructors
Inner Classes
Methods
Field variables
May be accessed by other classes in the same package or from any subclasses of the class in which they are declared.
private Constructors
Inner Classes
Methods
Field variables
May be accessed only from within the class in which they are declared.
no modifier Classes
Interfaces
Constructors
Inner Classes
Methods
Field variables
May only be accessed from within the package in which they are declared.

SPECIAL MODIFIERS

Modifier Used with Description
abstract Classes
Interfaces
Methods
Declares a Class or Method that is incomplete.
All Interfaces are implicitly abstract so the modifier is redundant.
A Class which has an abstract Method must be declared abstract.
final Classes
Field variables
Methods
Method parameters
Local variables
Indicates a definition is complete and cannot be changed.
Classes may not be extended.
Field variables may not be modified once a value is assigned.
Methods cannot be overridden.
Required for Method parameters and Local variables if they are to be used by an Inner Class.
Note: A Class may not be both final and abstract.
native Methods Indicates a platform-specific method written in another language.
Note: a method cannot be both native and abstract
static Initializers
Methods
Variables
Indicates an initializer, method or variable belongs to a class vs an instance (object) of the class.
Static initializers are processed once, when the class is loaded.
Static methods are used to access static variables. They may not be used to access non-static variables unless they specify an instance of the class.
synchronized Methods Indicates a method acquires a lock on an object before it executes.
Used to control access to objects shared by multiple threads.
transient Variables Indicates the variable is not part of the permanent state of an object and may not be serialized (written to a stream).
volatile Variables Indicates a variable may be changed by more than one thread.
Each thread has it’s own copy of a working variable. Volatile ensures the variable is compared to the master copy each time it is accessed.

this and super

this

  • this is an Object-Oriented Programming (OOP) operator
  • it is used to refer to the current instance of an object
  • it can be used in the body of a class constructor to refer to the object being created
  • it can be used in the body of an instance method or initializer to refer to the object whose method is being executed
  • it cannot be used in a static method or initializer
  • when used in a constructor it must be the first statment in the constructors body

super

  • super is an Object-Oriented Programming (OOP) operator
  • used to call a constructor declared in a classes superclass
  • if used in a constructor, must be the first statment in constructor body
  • You cannot use this() and super() in the same constructor( Important )

Shadowing

  • Because of the way identifiers are looked up; shadowing declarations can occur
  • For example, a field declaration can be shadowed by a local variable declaration

Hiding

  • Shadowing is not the same as hiding
  • hiding applies to members that would normally be inherited but are not because of a declaration of the same identifier in a subclass
  • method can hide a method in the superclass by overriding it
  • a method cannot override a static method in the superclass; however, it can hide it by using the same declaration
  • static methods are hidden vs overridden as the JLS states they “cannot be overridden” so the compiler never compares subclass method declarations to static superclass method declarations.
  • static method in a subclass cannot hide an instance method in the superclass

class SuperA {

void method1() { }

}

class SubA extends SuperA() {

static void method1() { // compile-error }

}

  • a hidden method can be accessed by using super() , casting to the superclass or using the methods fully qualified name

ACCESS CONTROL

  • declared protected , accessible from other code within the same package or from subclasses in other packages if the outside code is involved in the implementation of the class. For example, the following produces a compile-error

package point;

class Point {

protected int x, y;

}

package threepoint;

import point.Point;

class ThreePoints extends Point {

protected int z;

public void delta(Point p) {

p.x += this.x; // compile-error: cannot access p.x

p.y += this.y; // compile-error: cannot access p.y

}

}

Even though ThreePoints is a subclass of Point, it cannot access the protected fields in Point. The subclass must be involved in the implementation of Point. The fact that the code is within the body of a subclass is irrelevant. To the compiler, Point is a type reference and p.x and p.y are declared protected in the type Point. If the parameter is changed to ThreePoints p the code will compile as the type ThreePoints inherits the protected fields x and y from Point.

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Tomcy John

Tomcy John

Blogger & Author at javacodebook
He is an Enterprise Java Specialist holding a degree in Engineering (B-Tech) with over 10 years of experience in several industries. He's currently working as Principal Architect at Emirates Group IT since 2005. Prior to this he has worked with Oracle Corporation and Ernst & Young. His main specialization is on various web technologies and acts as chief mentor and Architect to facilitate incorporating Spring as Corporate Standard in the organization.
Tomcy John

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