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By now, you are quite familiar with the public keyword that appears in almost all of our examples
By now, you are quite familiar with the public keyword that appears in almost
all of our examples:
public class Main
The public keyword is an access modifier,
meaning that it is used to set the access level for classes, attributes, methods and
constructors.
We divide modifiers into two groups:
For classes, you can use either public or default:
| Modifier | Description | Try it |
|---|---|---|
public |
The class is accessible by any other class | Try it » |
| default | The class is only accessible by classes in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter | Try it » |
For attributes, methods and constructors, you can use the one of the following:
| Modifier | Description | Try it |
|---|---|---|
public |
The code is accessible for all classes | Try it » |
private |
The code is only accessible within the declared class | Try it » |
| default | The code is only accessible in the same package. This is used when you don't specify a modifier. You will learn more about packages in the Packages chapter | Try it » |
protected |
The code is accessible in the same package and subclasses. You will learn more about subclasses and superclasses in the Inheritance chapter | Try it » |
For classes, you can use either final or abstract:
| Modifier | Description | Try it |
|---|---|---|
final |
The class cannot be inherited by other classes (You will learn more about inheritance in the Inheritance chapter) | Try it » |
abstract |
The class cannot be used to create objects (To access an abstract class, it must be inherited from another class. You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters) | Try it » |
For attributes and methods, you can use the one of the following:
| Modifier | Description |
|---|---|
final |
Attributes and methods cannot be overridden/modified |
static |
Attributes and methods belongs to the class, rather than an object |
abstract |
Can only be used in an abstract class, and can only be used on methods. The method does not have a body, for example abstract void run();. The body is provided by the subclass (inherited from). You will learn more about inheritance and abstraction in the Inheritance and Abstraction chapters |
transient |
Attributes and methods are skipped when serializing the object containing them |
synchronized |
Methods can only be accessed by one thread at a time |
volatile |
The value of an attribute is not cached thread-locally, and is always read from the "main memory" |
If you don't want the ability to override existing attribute values, declare
attributes as final:
public class Main {
final int x = 10;
final double PI = 3.14;
public static void main(String[] args) {
Main myObj = new Main();
myObj.x = 50; // will generate an error: cannot assign a value to a final variable
myObj.PI = 25; // will generate an error: cannot assign a value to a final variable
System.out.println(myObj.x);
}
}
A static method means that it can be
accessed without creating an object of the class, unlike public:
An example to demonstrate the differences between static and public methods:
public class Main {
// Static method
static void myStaticMethod() {
System.out.println("Static methods can be called without creating objects");
}
// Public method
public void myPublicMethod() {
System.out.println("Public methods must be called by creating objects");
}
// Main method
public static void main(String[ ] args) {
myStaticMethod(); // Call the static method
// myPublicMethod(); This would output an error
Main myObj = new Main(); // Create an object of Main
myObj.myPublicMethod(); // Call the public method
}
}
An abstract method belongs to an abstract class, and it does not have a body.
The body is provided by the subclass:
// Code from filename: Main.java
// abstract class
abstract class Main {
public String fname = "John";
public int age = 24;
public abstract void study(); // abstract method
}
// Subclass (inherit from Main)
class Student extends Main {
public int graduationYear = 2018;
public void study() { // the body of the abstract method is provided here
System.out.println("Studying all day long");
}
}
// End code from filename: Main.java
// Code from filename: Second.java
class Second {
public static void main(String[] args) {
// create an object of the Student class (which inherits attributes and methods from Main)
Student myObj = new Student();
System.out.println("Name: " + myObj.fname);
System.out.println("Age: " + myObj.age);
System.out.println("Graduation Year: " + myObj.graduationYear);
myObj.study(); // call abstract method
}
}