Summary: difference between public, default, protected, and private key words
4150 ワード
According to Java Tutorial :
Controlling Access to Members of a Class
Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control: At the top level— At the member level—
A class may be declared with the modifier
At the member level, you can also use the
The following table shows the access to members permitted by each modifier.
Access Levels
Modifier
Class
Package
Subclass
World
Y
Y
Y
Y
Y
Y
Y
N
no modifier
Y
Y
N
N
Y
N
N
N
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.
Java Array sort descending order:
Controlling Access to Members of a Class
Access level modifiers determine whether other classes can use a particular field or invoke a particular method. There are two levels of access control:
public
, or package-private (no explicit modifier). public
, private
, protected
, or package-private (no explicit modifier). 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.
Access Levels
Modifier
Class
Package
Subclass
World
public
Y
Y
Y
Y
protected
Y
Y
Y
N
no modifier
Y
Y
N
N
private
Y
N
N
N
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.
Java Array sort descending order:
sort(T[] a, Comparator<? super T> c)
Arrays.sort(a, Collections.reverseOrder());