Private Class Java
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Private Class Java
Controlling Access to Members of a Class
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The Java Tutorials have been written for JDK 8. Examples and practices described in this page don't take advantage of improvements introduced in later releases and might use technology no longer available. See Java Language Changes for a summary of updated language features in Java SE 9 and subsequent releases. See JDK Release Notes for information about new features, enhancements, and removed or deprecated options for all JDK releases.
Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:
A class may be declared with the modifier public , in which case that class is visible to all classes everywhere. If a class has no modifier (the default, also known as package-private ), it is visible only within its own package (packages are named groups of related classes — you will learn about them in a later
lesson.)
At the member level, you can also use the public modifier or no modifier ( package-private ) just as with top-level classes, and with the same meaning. For members, there are two additional access modifiers: private and protected . The private modifier specifies that the member can only be accessed in its own class. The protected modifier specifies that the member can only be accessed within its own package (as with package-private ) and, in addition, by a subclass of its class in another package.
The following table shows the access to members permitted by each modifier.
The first data column indicates whether the class itself has access to the member defined by the access level. As you can see, a class always has access to its own members. The second column indicates whether classes in the same package as the class (regardless of their parentage) have access to the member. The third column indicates whether subclasses of the class declared outside this package have access to the member. The fourth column indicates whether all classes have access to the member.
Access levels affect you in two ways. First, when you use classes that come from another source, such as the classes in the Java platform, access levels determine which members of those classes your own classes can use. Second, when you write a class, you need to decide what access level every member variable and every method in your class should have.
Let's look at a collection of classes and see how access levels affect visibility.
The following figure shows the four classes in this example and how they are related.
Classes and Packages of the Example Used to Illustrate Access Levels
The following table shows where the members of the Alpha class are visible for each of the access modifiers that can be applied to them.
If other programmers use your class, you want to ensure that errors from misuse cannot happen. Access levels can help you do this.
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In the Java programming language, fields, constructors, methods, and classes can be marked with access modifiers . In this tutorial, we'll talk about the private access modifier in Java.
The private access modifier is important because it allows encapsulation and information hiding, which are core principles of object-oriented programming. Encapsulation is responsible for bundling methods and data, while information hiding is a consequence of encapsulation — it hides an object's internal representation.
The first thing to remember is that elements declared as private can be accessed only by the class in which they're declared .
Now, we'll see some simple code examples to better understand the subject.
First, let's create an Employee class containing a couple of private instance variables:
In this example, we marked the privateId variable as private because we want to add some logic to the id generation. And, as we can see, we did the same thing with manager attribute because we don't want to allow direct modification of this field.
Let's now create a private constructor:
By marking our constructor as private , we can use it only from inside our class.
Let's add a static method that will be our only way to use this private constructor from outside the Employee class:
Now we can get a manager instance of our Employee class by simply writing:
And behind the scenes, of course, the buildManager method calls our private constructor.
Let's now add a private method to our class:
And let's suppose, for some reason, we have an arbitrary rule in our company in which only an employee named “Carl” can be promoted to manager, although other classes aren't aware of this. We'll create a public method with some logic to handle this rule that calls our private method:
Let's see an example of how to use our Employee class from outside:
After executing ExampleClass , we'll see its output on the console:
In this example, we used the public constructor and the public method changeId(customId) because we can't access the private variable privateId directly.
Let's see what happens if we try to access a private method, constructor, or variable from outside our Employee class:
We'll get compilation errors for each of our illegal statements:
There is one special case where we can create a private class — as an inner class of some other class. Otherwise, if we were to declare an outer class as private , we'd be forbidding other classes from accessing it, making it useless:
In this example, we created a private inner class inside our PublicOuterClass by specifying the private access modifier.
Because we used the private keyword, if we, for some reason, try to instantiate our PrivateInnerClass from outside the PublicOuterClass , the code won't compile and we'll see the error:
In this quick tutorial, we've discussed the private access modifier in Java. It's a good way to achieve encapsulation, which leads to information hiding. As a result, we can ensure that we expose only the data and behaviors we want to other classes.
As always, the code example is available over on GitHub .
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