Java classes, fields, constructors and methods can have one of four different access modifiers:
- private
- default
- protected
- public
private
If a method or variable is marked as
private, then only code inside the same class can access the variable, or call the method. Code inside subclasses cannot access the variable or method, nor can code from any external class.
If a class is marked as private then no external class an access the class. This doesn't really make so much sense for classes though. Therefore, the access modifier
private is mostly used for fields, constructors and methods.
Here is an example of a
private field:public class Clock {
private long time = 0;
}
The member variable
time inside the Clock class cannot be accessed from code outside theClock class. But code inside the class can access the time variable. For instance:public class Clock {
private long time = 0;
public long getTime() {
return this.time;
}
public void setTime(long theTime) {
this.time = theTime;
}
}
In the above example the two methods
getTime() and setTime() can access the time member variable. The two methods are declared public, meaning they can be called from code anywhere in your application.default
The default access level is declared by not writing any access modifier at all. Default access levels means that code inside the class itself + code inside classes in the same package as this class, can access the class, field, constructor or method. Therefore, the
default access modifier is also sometimes called a package access modifier.
Subclasses cannot access methods and member variables in the superclass, if they have default accessibility declared, unless the subclass is located in the same package as the superclass.
Here is an example:
public class Clock {
long time = 0;
}
public class ClockReader {
Clock clock = new Clock();
public long readClock{
return clock.time;
}
}
The
ClockReader class above can read the time member variable of the Clock object, provided that ClockReader and Clock are located in the same package.protected
The
protected acces modifier does the same as the default access, except subclasses can also access protected methods and member variables of the superclass. This is true even if the subclass is not located in the same package as the superclass.
Here is an example:
public class Clock {
protected long time = 0; // time in milliseconds
}
public class SmartClock() extends Clock{
public long getTimeInSeconds() {
return this.time / 1000;
}
}
In the above example the subclass
SmartClock has a method called getTimeInSeconds()which accesses the time variable of the superclass Clock. This is possible even if Clock andSmartClock are not located in the same package.public
The
public access modifier means that all code can access the class, field, constructor or method, regardless of where the accessing code is located.
Here is a public field example:
public class Clock {
public long time = 0;
}
public class ClockReader {
Clock clock = new Clock();
public long readClock{
return clock.time;
}
}
The
ClockReader class can access the time field in the Clock no matter what package theClockReader is located in, because both the Clock class and the time field are declared public
