Java中的访问修饰符 | Baeldung中文网

1. Overview

In this tutorial, we’ll discuss access modifiers in Java, which are used for setting the access level to classes, variables, methods, and constructors.

Simply put, there are four access modifiers: public, private, protected, and default (no keyword).

Before we begin, please note that a top-level class can only use public or default access modifiers. At the member level, we can use all four.

2. Default

When we don’t use any keyword explicitly, Java will set a default access to a given class, method, or property. The default access modifier is also called package-private, which means that all members are visible within the same package, but aren’t accessible from other packages:

package com.baeldung.accessmodifiers;

public class SuperPublic {
    static void defaultMethod() {
        ...
    }
}

defaultMethod() is accessible in another class of the same package:

package com.baeldung.accessmodifiers;

public class Public {
    public Public() {
        SuperPublic.defaultMethod(); // Available in the same package.
    }
}

However, it’s not available in other packages.

3. Public

If we add the public keyword to a class, method, or property, then we’re making it available to the whole world (i.e. all other classes in all packages will be able to use it). This is the least restrictive access modifier:

package com.baeldung.accessmodifiers;

public class SuperPublic {
    public static void publicMethod() {
        ...
    }
}

publicMethod() is available in another package:

package com.baeldung.accessmodifiers.another;

import com.baeldung.accessmodifiers.SuperPublic;

public class AnotherPublic {
    public AnotherPublic() {
        SuperPublic.publicMethod(); // Available everywhere. Let's note different package.
    }
}

For more details on how the public keyword behaves when applied to a class, interface, nested public class, or interface and method, see this dedicated article.

4. Private

Any method, property, or constructor with the private keyword is accessible from the same class only. This is the most restrictive access modifier, and is core to the concept of encapsulation. All data will be hidden from the outside world:

package com.baeldung.accessmodifiers;

public class SuperPublic {
    static private void privateMethod() {
        ...
    }
    
     private void anotherPrivateMethod() {
         privateMethod(); // available in the same class only.
    }
}

This more detailed article will show how the private keyword behaves when applied to a field, constructor, method, or inner class.

5. Protected

Between public and private access levels, there’s the protected access modifier.

If we declare a method, property, or constructor with the protected keyword, we can access the member from the same package (as with package-private access level), as well as from all subclasses of its class, even if they lie in other packages:

package com.baeldung.accessmodifiers;

public class SuperPublic {
    static protected void protectedMethod() {
        ...
    }
}

protectedMethod() is available in subclasses (regardless of the package):

package com.baeldung.accessmodifiers.another;

import com.baeldung.accessmodifiers.SuperPublic;

public class AnotherSubClass extends SuperPublic {
    public AnotherSubClass() {
        SuperPublic.protectedMethod(); // Available in subclass. Let's note different package.
    }
}

This dedicated article describes more about the keyword when used in a field, method, constructor, and inner class, as well as the accessibility in the same package or a different package.

6. Comparison

The table below summarizes the available access modifiers. We can see that a class, regardless of the access modifiers used, always has access to its members:

Modifier

Class

Package

Subclass

World

public

protected

default

private

7. Canonical Order of Modifiers

The order of modifiers isn’t strictly enforced in Java. However, the Java Language Specification (JLS) recommends a standard canonical order. This recommended order can ensure consistency across codebases and improve readability.

The canonical order applies to the field, methods, classes, and modules. Here’s a customary recommendation for field modifiers:

Annotation
public/protected/private
static
final
transient
volatile

The annotation comes first, then one of the access modifiers and other keywords. For example, let’s declare a constant with value of 1:

@Id
private static final long ID = 1;

The code above adopts the canonical order as specified in the JLS.

Also, the JLS makes a recommendation for specifying class modifiers:

Annotation
public/protected/private
abstract
static
final
strictfp

Like field modifiers, the annotation comes first, then the access modifier and other keywords.

Finally, here’s the canonical order of modifiers for method declaration as recommended by JLS:

Annotation
public/protected/private
abstract
static
final
synchronized
native
strictfp

Importantly, not all modifiers can be used together. For instance, public, protected, and private are mutually exclusive.

Also, IDEs like IntelliJ can automatically arrange modifiers in the canonical order when we apply formatting. This feature helps maintain consistency and adheres to the JLS recommendation.

8. Conclusion

In this brief article, we focused on access modifiers in Java.

It’s good practice to use the most restrictive access level possible for any given member to prevent misuse. We should always use the private access modifier unless there’s a good reason not to.

Public access level should only be used if a member is part of an API.

As always, the code examples are available over on Github.

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