Private Class Java

Private Class Java
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The keyword ‘private’ in Java is used to establish the limitation of accessibility of the class, method or a variable in the java code block. If a class, method or variable is entitled as private in the program, that means that a particular class, method or variable cannot be accessed by outside the class or method, unlike the public method. The Private keyword is typically used in Java in a fully encapsulated class.
Private Keyword in Java works within a particular class. It can’t be accessed outside of the class. It doesn’t work outside the class/classes and interface/ interfaces. Private Keyword works well if the members of the class are PRIVATE and that too in a fully encapsulated class. Private Keyword or variable or method can also be overridden to the sub-class/classes using some access modifiers to invoke PRIVATE METHOD outside of the class. With this, Private Keyword also works outside of the class only using the Private Access Modifiers.
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Here are some examples of private modifiers, which are as follows:
Here we are illustrating the example of Private Access Modifier, which shows compilation error because of private data member accessing from the class AB, which is shown in the below example. Private methods or Private members can only be accessible within a particular class.
This is an example of illustrating the use of the PRIVATE keyword with the program below:
Here in this example, you can see how the PRIVATE METHOD is overridden to the sub-class using the access modifier, which is the default. We are not even allowed to invoke the parent class method/methods from the sub-class.
Here in this example, I am illustrating that the PRIVATE METHOD cannot be invoked/called outside of the class. Here now, the private method is calling from the Outside class by changing class runtime behavior.
This is an example of a private method and field in Java Programming Language . Here private method/methods use static binding at the compile-time, and it even can’t be overridden. Don’t confuse with the Private variable output because the Private variable is actually accessible inside the inner class/classes. If PRIVATE variable/variables are invoked/called outside of the class, definitely the compiler will produce an error.
Here we are going to explain the advantages of using Private methods/fields in Java below.
Here are some rules and regulations for private that you should know.
Coming to the end of the main topic, we actually happy to know how helpful and easy using Private Keyword in Java. In this article, I hope you understand the private keyword, private variable, access modifier, private constructor, and how to use these private keywords in programs.
This is a guide to a Private constructor in java. Here we discuss the basic concept, working, advantages, rules, and private regulations in java and their examples and implementation. You may also look at the following articles to learn more –
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Exception in thread "main" java.lang.Error: Unresolved compilation problem:
The field A.msg is not visible

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
The method display() from the type A is not visible

Exception in thread "main" java.lang.Error: Unresolved compilation problem:

Exception in thread "main" java.lang.Error: Unresolved compilation problem:
The constructor A(String) is not visible

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A Java private keyword is an access modifier. It can be assigned to variables, methods, and inner classes. It is the most restricted type of access modifier.
Let's see an example to determine whether the private variable is accessible or not outside the class.
Let's see an example to determine whether we can assign the private modifier to the outer class.
In the above example, we learn that the private method can't be invoked outside the class. Here, we call the private method from outside the class by changing the runtime behavior of that class.
Let's see an example to determine whether we create the instance of private constructor outside the class.
Let's see the real use of private keyword with the help of an example.
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class ClassWithPrivateField {
#privateField ;
}

class ClassWithPrivateMethod {
#privateMethod ( ) {
return 'hello world' ;
}
}

class ClassWithPrivateStaticField {
static # PRIVATE_STATIC_FIELD ;
}

class ClassWithPrivateStaticMethod {
static #privateStaticMethod ( ) {
return 'hello world' ;
}
}

class ClassWithPrivateField {
#privateField ;

constructor ( ) {
this . #privateField = 42 ;
delete this . #privateField ; // Syntax error
this . #undeclaredField = 444 ; // Syntax error
}
}

const instance = new ClassWithPrivateField ( )
instance . #privateField === 42 ; // Syntax error

class ClassWithPrivateField {
#privateField ;

constructor ( ) {
this . #privateField = 42 ;
}
}

class SubClass extends ClassWithPrivateField {
#subPrivateField ;

constructor ( ) {
super ( ) ;
this . #subPrivateField = 23 ;
}
}

new SubClass ( ) ;
// SubClass {#subPrivateField: 23}

class ClassWithPrivateStaticField {
static # PRIVATE_STATIC_FIELD ;

static publicStaticMethod ( ) {
ClassWithPrivateStaticField . # PRIVATE_STATIC_FIELD = 42 ;
return ClassWithPrivateStaticField . # PRIVATE_STATIC_FIELD ;
}

publicInstanceMethod ( ) {
ClassWithPrivateStaticField . # PRIVATE_STATIC_FIELD = 42 ;
return ClassWithPrivateStaticField . # PRIVATE_STATIC_FIELD ;
}
}

console . log ( ClassWithPrivateStaticField . publicStaticMethod ( ) ) ; // 42
console . log ( new ClassWithPrivateStaticField ( ) . publicInstanceMethod ( ) ) ; // 42

class BaseClassWithPrivateStaticField {
static # PRIVATE_STATIC_FIELD ;

static basePublicStaticMethod ( ) {
this . # PRIVATE_STATIC_FIELD = 42 ;
return this . # PRIVATE_STATIC_FIELD ;
}
}

class SubClass extends BaseClassWithPrivateStaticField { } ;

let error = null ;

try {
SubClass . basePublicStaticMethod ( ) ;
} catch ( e ) {
error = e ;
}

console . log ( error instanceof TypeError ) ;
// true
console . log ( error ) ;
// TypeError: Cannot write private member #PRIVATE_STATIC_FIELD
// to an object whose class did not declare it

class ClassWithPrivateMethod {
#privateMethod ( ) {
return 'hello world' ;
}

getPrivateMessage ( ) {
return this . #privateMethod ( ) ;
}
}

const instance = new ClassWithPrivateMethod ( ) ;
console . log ( instance . getPrivateMessage ( ) ) ;
// hello world

class ClassWithPrivateAccessor {
#message ;

get #decoratedMessage ( ) {
return ` 🎬 ${ this . #message } 🛑 ` ;
}
set #decoratedMessage ( msg ) {
this . #message = msg ;
}

constructor ( ) {
this . #decoratedMessage = 'hello world' ;
console . log ( this . #decoratedMessage ) ;
}
}

new ClassWithPrivateAccessor ( ) ;
// 🎬hello world🛑

class ClassWithPrivateStaticMethod {
static #privateStaticMethod ( ) {
return 42 ;
}

static publicStaticMethod1 ( ) {
return ClassWithPrivateStaticMethod . #privateStaticMethod ( ) ;
}

static publicStaticMethod2 ( ) {
return this . #privateStaticMethod ( ) ;
}
}

console . log ( ClassWithPrivateStaticMethod . publicStaticMethod1 ( ) === 42 ) ;
// true
console . log ( ClassWithPrivateStaticMethod . publicStaticMethod2 ( ) === 42 ) ;
// true

class Base {
static #privateStaticMethod ( ) {
return 42 ;
}
static publicStaticMethod1 ( ) {
return Base . #privateStaticMethod ( ) ;
}
static publicStaticMethod2 ( ) {
return this . #privateStaticMethod ( ) ;
}
}

class Derived extends Base { }

console . log ( Derived . publicStaticMethod1 ( ) ) ;
// 42
console . log ( Derived . publicStaticMethod2 ( ) ) ;
// TypeError: Cannot read private member #privateStaticMethod
// from an object whose class did not declare it

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Frequently asked questions about MDN Plus

Class fields are public by default, but private class members can be created
by using a hash # prefix. The privacy encapsulation of these class features is
enforced by JavaScript itself.

Private members are not native to the language before this syntax existed. In prototypical inheritance, its behavior may be emulated with WeakMap objects or closures , but they can't compare to the # syntax in terms of ergonomics.
Private fields include private instance fields and private static fields.

Private instance fields are declared with # names (pronounced
" hash names "), which are identifiers prefixed with # . The
# is a part of the name itself. Private fields are accessible on
the class constructor from inside the class
declaration itself. They are used for declaration of field names as well
as for accessing a field's value.

It is a syntax error to refer to # names from out of scope.
It is also a syntax error to refer to private fields
that were not declared before they were called, or to attempt to remove
declared fields with delete .

Note: Use the in operator to check for potentially missing private fields (or private methods). This will return true if the private field or method exists, and false otherwise.
Like public fields, private fields are added at construction time in a base class, or at the point where super() is invoked in a subclass.
Note: #privateField from the ClassWithPrivateField base class is private to ClassWithPrivateField and is not accessible from the derived Subclass .
Private static fields are added to the class constructor at class evaluation time. Like their public counterparts, private static fields are only accessible on the class itself or on the this context of static methods, but not on the this context of instance methods.

There is a restriction on private static fields: Only the class which
defines the private static field can access the field. This can lead to unexpected behavior when using this .
In the following example, this refers to the SubClass class (not
the BaseClassWithPrivateStaticField class) when we try to call
SubClass.basePublicStaticMethod() , and so causes a TypeError .

Private instance methods are methods available on class instances whose access is
restricted in the same manner as private instance fields.

Private instance methods may be generator, async, or async generator functions. Private
getters and setters are also possible, although not in generator, async, or
async generator forms.

Like their public equivalent, private static methods are called on the class itself,
not instances of the class. Like private static fields, they are only accessible from
inside the class declaration.

Private static methods may be generator, async, and async generator functions.

The same restriction previously mentioned for private static fields holds
for private static methods, and similarly can lead to unexpected behavior when using
this .
In the following example, when we try to call Derived.publicStaticMethod2() ,
this refers to the Derived class (not
the Base class) and so causes a TypeError .

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The private keyword is an access modifier used for attributes, methods and constructors, making them only accessible within the declared class.
Read more about modifiers in our Java Modifiers Tutorial .
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Private Class Java

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