Class Syntax in Java

A class is a template or prototype for each of many object instances made to the class design. The class specifies the properties (data) and methods (actions) that objects can work with.

Syntax of Class

The syntax for a class is:

["public"] ["abstract"|"final"]"class" Class_name
  ["extends" object_name] ["implements" interface_name]
"{"
// properties declarations
// behavior declarations
"}"

The first optional group indicates the visibility or scope of accessibility from other objects. public means visible everywhere. The default (ie. omitted) is package (aka friendly) or visible within the current package only.
The second optional group indicates the capability of a class to be inherited or extended by other classes. abstract classes must be extended and final classes can never be extended by inheritance. The default (ie. omitted) indicates that the class may or may not be extended at the programmers discretion.
Class_name has initial letter capitalized by Java convention.
The third option of extends is described in the tutorial on inheritance.

The fourth option of implements is described in the tutorial on interfaces.

A simple example of a class specification is a box. The box has length, width and height properties as well as methods for setting dimensions and displaying its volume.

public class Box
{
  // what are the properties or fields
  private int length, width, height;

  // what are the actions or methods
  public void setLength(int p) {length=p;}
  public void setWidth(int p) {width=p;}
  public void setHeight(int p) {height=p;}
  public void showVolume() {System.out.println(length*width*height);}
}

Note 1: There is no main method in a class defining template!
Note 2: Class names begin with a capital. Use lowercase for all other names.
Note 3: It is good programming practice to write separate files for the class templates and the driver or main user program. This allows separate compilation as well as class reuse by other driver programs. A class file can contain more than one associated class but normally its filename is that of the first defined file. A driver program is named the same as the class that contains the main(). Its file may contain other classes as well.

Field Values or Properties
Methods
Types of methods

Getting a class name from static method

So our class's static method must return the name. So consider this class and its static method:

public class MyClass {
    public static String getClassName() {
        String name = ????; // what goes here so the string "MyClass" is returned
        return name;
    }
}

So following are the methods:
Using reflections

MyClass.class.getName();

Also,
If you want the entire package name with it, call:

String name = MyClass.class.getCanonicalName();

If you only want the last element, call:

String name = MyClass.class.getSimpleName();

Using getEnclosingClass method

return new Object() { }.getClass().getEnclosingClass();

Abusing the security manager

System.getSecurityManager().getClassContext()[0].getName();

OR using the static inner class method to return the class name:

public static class CurrentClassGetter extends SecurityManager {
    public String getClassName() {
        return getClassContext()[1].getName(); 
    }
}

Class Objects as Type Literals

Class objects can be used as type specifications too, at runtime. For instance, you can create a generified method like this:

public static <T> T getInstance(Class<T> theClass)
    throws IllegalAccessException, InstantiationException {

    return theClass.newInstance();
}

Here are a few examples of calls to the getInstance() method:

String string   = getInstance(String.class);

MyClass myClass = getInstance(MyClass.class);

As you can see the return type changes depending on what class object you pass in as parameter to the method. This can be quite handy in database API's like Butterfly Persistence where you read objects from a database. Here is an example method definition:

public static <T> T read(Class<T> theClass, String sql)
    throws IllegalAccessException, InstantiationException {

    //execute SQL.

    T o = theClass.newInstance();
    //set properties via reflection.

    return o;
}

Here is how you would call the read() method:

Driver employee   = read(Driver.class, "select * from drivers where id=1");

Vehicle vehicle   = read(Vehicle.class, "select * from vehicles where id=1");

Multiple Inheritance in Java

In Java a class can be derived from only one base class. That is, the following declaration is not allowed:

class C extends A, B // Wrong;
{
}

Nevertheless, it is possible for a class to extend a base class and to implement one or more interfaces:

class C extends A 

implements D, E
{
}

The derived class c inherits the members of A and it implements all the methods defined in the interfaces D and E.

Inheritance among interfaces in java

Static Inner Classes

Consider the following Java code fragment:

public class A
{
    int y;

    public static class B
    {
 int x;

 void f () {}
    }
}

This fragment defines the class A which contains an static inner class B. A static inner class behaves like any ``outer'' class. It may contain methods and fields, and it may be instantiated like this:

A.B object = new A.B ();

This statement creates an new instance of the inner class B.
Given such an instance, we can invoke the f method in the usual way:

object.f();

Note:
It is not necessarily the case that an instance of the outer class A exists even when we have created an instance of the inner class. Similarly, instantiating the outer class A does not create any instances of the inner class B.
The methods of a static inner class may access all the members (fields or methods) of the inner class but they can access only static members (fields or methods) of the outer class. Thus, f can access the field x, but it cannot access the field y.

Various Classes in Java technology

The Object class

All classes in Java^TM technology are directly or indirectly derived from the Object class. Some of the subclasses of Object class are - Boolean, Number, Void, Math, String, StringBuffer etc. Some of the important methods defined in the Object class are given below. These methods are available to all Java classes.

1. boolean equals(Object obj) - The equals method in Object class returns true if two references point to the same object. Some classes like String and Boolean overload this method. 
2. String toString() - The function is used to convert objects to String. If a subclass does not override this method, the method returns a textual representation of the object, which has the following format : @".
3. The following methods related to threads are also defined in Object class -
   void notify()
void notifyall()
void wait(long timeout) throws InteruptedException
void wait(long timeout, int nanosec) throws InteruptedException
void wait() throws InteruptedException


Wrapper classes

Corresponding to all the primitive types Java technology defines wrapper classes. Some examples of these wrapper classes are - Character, Boolean, Integer, Double.

Important methods in the Math class

Some of the methods defined in the Math class are used frequently. These are explained below. Besides the functionality, it is important to understand the arguments and return type of these functions.static double ceil(double(d)) : The method ceil returns the smallest double value equal to a mathematical integer, that is not less than the argument. For example,

ceil(3.4) returns 4.0

ceil(-2.3) returns -2.0

ceil(3.0) returns 3.0

static double floor(double(d)) : The method floor returns the largest double value equal to a mathematical integer, that is not greater than the argument. For example,

floor(3.4) returns 3.0

floor(-2.3) returns -3.0

floor(3.0) returns 3.0

static int round (float f) and static long round(double d) : The method round returns the integer closest to the argument.

round(3.7) returns 4

round(3.2) returns 3

round(3.0) returns 3

round(-3.1) returns -3

String class

The String class is used to implement immutable character strings. This means that the character string cannot be changed once it has been created. Some of the important methods are explained below.int length() - The number of characters in the String class are returned by the length() method. String substring(int startIndex) String substring(int startIndex, int endIndex) The method substring extracts a substring from a string. The method extracts a string from the startIndex to the index endIndex - 1. If endIndex is not specified then string till the end of input string is returned. The example below illustrates this

String str = "I am a string";

int len = str.length();

String str2 = str.substring(2,5);

After the above statements str2 contains the string "am ". The string str still has the same value "I am a string". The variable len has value 13.

StringBuffer class

The StringBuffer class implements mutable strings. This means that the characters stored in the string and the capacity of the string can be changed.

Garbage Collection

Java technology's Garbage collection is complex. In this section I am only giving a brief overview of Garbage Collection. Java technology supports automatic garbage collection. This means that the programmer does not need to free the memory used by objects. Java technology's runtime environment can claim the memory from the objects that are no longer in use. Objects that are not being referred become candidates for garbage collection. It is important to note that these objects are candidates only. Java technology does not guarantee that Garbage collection would happen on these objects. Before actually freeing up the memory, garbage collector invokes the finalize() method of the Object being freed. The System.gc() method can be invoked by the program to suggest to Java technology that the Garbage Collector be invoked. However there is no guarantee when the garbage collection would be invoked. There is also no guarantee on the order in which objects will be garbage collected. The example illustrates when a string Object becomes available for Garbage Collection.

public class GCTest {

public static void main(String args[]) {

String a,b;

String c = new String("test");

a = c;

c = null; // The String "test" is not yet 

//available for GC as a still points to "test"

b = new String("xyz");

b = c; // String "xyz" is now available for GC.

a = null; 

//String "test" is now available for GC.

}

}

Static in java

Static Members
Static members are those that belong to a class as a whole and not to a particular instance (object). A static variable is initialized when the class is loaded. Similarly, a class can have static methods. Static variables and static methods are collectively known as static members, and are declared with a keyword static. Static members in the class can be accessed either by using the class name or by using the object reference, but instance members can only be accessed via object references.

Constants
  Static variables are quite rare. However, static constants are more common. For example, the Math class defines a static constant:

public class Math
{
. . .
   public static final double PI = 3.14159265358979323846;
. . .
}

You can access this constant in your programs as Math.PI.

If the keyword static had been omitted, then PI would have been an instance field of the Math class. That is, you would need an object of the Math class to access PI, and every Math object would have its own copy of PI.

Another static constant that you have used many times is System.out. It is declared in the System class as:

public class System
{
. . .
   public static final PrintStream out = . . .;
. . .
}

As we mentioned several times, it is never a good idea to have public fields, because everyone can modify them. However, public constants (that is, final fields) are ok. Because out has been declared as final, you cannot reassign another print stream to it:
System.out = new PrintStream(. . .); // ERROR--out is final

NOTE:
If you look at the System class, you will notice a method setOut that lets you set System.out to a different stream. You may wonder how that method can change the value of a final variable. However, the setOut method is a native method, not implemented in the Java programming language. Native methods can bypass the access control mechanisms of the Java language. This is a very unusual workaround that you should not emulate in your own programs.

Static Methods
Static methods are methods that do not operate on objects. For example, the pow method of the Math class is a static method. The expression:
Math.pow(x, a)

computes the power x^a. It does not use any Math object to carry out its task. In other words, it has no implicit parameter.

You can think of static methods as methods that don't have a this parameter. (In a non-static method, the this parameter refers to the implicit parameter of the method)

Because static methods don't operate on objects, you cannot access instance fields from a static method. But static methods can access the static fields in their class. Here is an example of such a static method:

public static int getNextId()
{
   return nextId; // returns static field
}

To call this method, you supply the name of the class:
int n = Employee.getNextId();

Could you have omitted the keyword static for this method?
Yes, but then you would need to have an object reference of type Employee to invoke the method.

NOTE :
It is legal to use an object to call a static method. For example, if harry is an Employee object, then you can call harry.getNextId() instead of Employee.getnextId(). However, we find that notation confusing. The getNextId method doesn't look at harry at all to compute the result. We recommend that you use class names, not objects, to invoke static methods.

You use static methods in two situations:
• When a method doesn't need to access the object state because all needed parameters are supplied as explicit parameters (example: Math.pow)
• When a method only needs to access static fields of the class (example: Employee.getNextId).

Also see difference between java and c++ static.
Points to note about static.

Nesting of classes : Introduction

It is possible to define a class within another class; such classes are known as nested classes.

The scope of a nested class is bounded by the scope of its enclosing class. Also a nested class has access to the members, including private members, of the class in which it is nested. However, the
enclosing class does not have access to the members of the nested class.

Consider the case that class B is defined within class A. So it means:

• B is known to A, but not outside of A. 
• B as has access to A's members, including private members.

See types of nested classes .

Abstract classes

As seen from the previous example, the superclass is more general than its subclass(es). The superclass contains elements and properties common to all of the subclasses. The previous example was of a concrete superclass that instance objects can be created from. Often, the superclass will be set up as an abstract class which does not allow objects of its prototype to be created. In this case, only objects of the subclass are used. To do this the reserved word abstract is included in the class definition.
Abstract methods are methods with no body specification. Subclasses must provide the method statements for their particular meaning. If the method was one provided by the superclass, it would require overriding in each subclass. And if one forgot to override, the applied method statements may be inappropriate.

public abstract class Animal  // class is abstract
{
  private String name;
  public Animal(String nm)      // constructor method
  { name=nm; }
  public String getName()       // regular method
  { return (name); }
  public abstract void speak(); // abstract method - note no {}
}


Abstract classes and methods force prototype standards to be followed (ie. they provide templates).
See also - Abstract classes : cpp vs java

Access modifiers in Java

Access modifiers specifies who can access them. There are four access modifiers used in java. They are :

• public
• private
• protected
• no modifer (declaring without an access modifer). Using ‘no modifier’ is also sometimes referred as ‘package-private’ or ‘default’ or ‘friendly’ access.

Usage of these access modifiers is restricted to two levels. The two levels are class level access modifiers and member level access modifiers.

I) Class level access modifiers (java classes only)

Only two access modifiers is allowed, public and no modifier

• If a class is ‘public’, then it CAN be accessed from ANYWHERE.
  
• If a class has ‘no modifer’, then it CAN ONLY be accessed from ‘same package’.

II) Member level access modifiers (java variables and java methods)

All the four public, private, protected and no modifer is allowed.

• public and no modifier – the same way as used in class level.
  
• private – members CAN ONLY access.
  
• protected – CAN be accessed from ‘same package’ and a subclass existing in any package can access.

Understanding by the table below

For better understanding, member level access is formulated as a table:

Access Modifiers	Same Class	Same Package	Subclass	Other packages
public	Y	Y	Y	Y
protected	Y	Y	Y	N
no access modifier	Y	Y	N	N
private	Y	N	N	N

First row {public Y Y Y Y} should be interpreted as:

• Y – A member declared with ‘public’ access modifier CAN be accessed by the members of the ‘same class’.
• Y – A member declared with ‘public’ access modifier CAN be accessed by the members of the ‘same package’.
• Y – A member declared with ‘public’ access modifier CAN be accessed by the members of the ‘subclass’.
• Y – A member declared as ‘public’ CAN be accessed from ‘Other packages’.

Second row {protected Y Y Y N} should be interpreted as:

• Y – A member declared with ‘protected’ access modifier CAN be accessed by the members of the ‘same class’.
• Y – A member declared with ‘protected’ access modifier CAN be accessed by the members of the ‘same package’.
• Y – A member declared with ‘protected’ access modifier CAN be accessed by the members of the ‘subclass’.
• N – A member declared with ‘protected’ access modifier CANNOT be accessed by the members of the ‘Other package’.

similarly interpret the access modifiers table for the third (no access modifier) and fourth (private access modifier) records.

Examples
Public Members:
For a subclass, if a member of its superclass is declared public, the subclass inherits that member regardless of whether both classes are in the same package.

package pack1;
public class Parent {
    public String getName() {
    return “Parent”;
    }
}

package pack2;
Import pack1.Parent;
Public class Child extends Parent{
    Public String getParentsName() {
    return getName();
    }
}

If you see the example above, the child class is able to access the parent class’s method getName() even without instantiating an object of the parent because it inherits the Parent class and all its method as part of the inheritance (extends) feature.

Private Members:
Members marked private can’t be accessed by code in any class other than the class in which the private member was declared. Let’s make a small change to the Parent class from an earlier example.

package pack1;
    public class Parent {
    private String getName() {
    return “Parent”;
    }
}

Now the getName() method is private and is not visible from within the Child class and the same piece of code would throw up the below compilation error when you try to compile the class.

cannot find symbol
symbol : method getName()
This error method looks as if the method getName() does not exist at all. Of course the method exists but unfortunately it is declared private and hence no class (except the class that has the exact code written into it) can access it. This includes a child class that extends the parent class that has the method.
Note: You can write a method inside the child class that has the same name as the private method in the parent. This does not account as Overriding because the parent class method is not even visible and hence it is just another method and is not considered overriding.

Tip: Though you may feel that public is better because all other classes can access your methods, that is seldom the case because a code that is visible to all other classes is not secure and that is not the best way to write your code. It is always best to provide just the appropriate level of access to methods and variables instead of having all of them public.

Protected and Default Members:
The protected and default access control levels are almost identical, but with one critical difference. A default member may be accessed only if the class accessing the member belongs to the same package, whereas a protected member can be accessed (through inheritance) by a subclass even if the subclass is in a different package.
Take a look at the following two classes:

package certification;
public class ClassOne {
    void testIt() { // No modifier means method has default access
    System.out.println("ClassOne");
    }
}

In another source code file you have the following:

package otherCertification;
import certification.ClassOne;
class ClassTwo {
    static public void main(String[] args) {
    ClassOne o = new ClassOne();
    o.testIt();
    }
}

As you can see, the testIt() method in the first file has default (think: package-level) access. Notice also that class OtherClass is in a different package from the AccessClass. When you compile the ClassTwo.java file you will get an error like below:
No method matching testIt() found in class
certification.ClassOne.o.testIt();
From the preceding results, you can see that AccessClass can’t use the OtherClass method testIt() because testIt() has default access, and AccessClass is not in the same package as OtherClass. So AccessClass can’t see it, the compiler complains.
Default and protected behavior differs only when we talk about subclasses. If the protected keyword is used to define a member, any subclass of the class declaring the member can access it through inheritance. It doesn’t matter if the superclass and subclass are in different packages, the protected superclass member is still visible to the subclass. This is in contrast to the default behavior, which doesn’t allow a subclass to access a superclass member unless the subclass is in the same package as the superclass. (See the example above)
Whereas default access doesn’t extend any special consideration to subclasses, the protected modifier respects the parent-child relationship, even when the child class moves away (and joins a new package). So, when you think of default access, think of package restrictions. No exceptions at all. But when you think protected, think package + kids. A class with a protected member is marking that member as having package-level access for all classes, but with a special exception for subclasses outside the package.

Tip: Remember that Default and Protected access is the same as long as inheritance is not involved.

Local Variables and Access Modifiers

Can access modifiers be applied to local variables? A BIG NO!
There is never a case where an access modifier can be applied to a local variable, so watch out for code like the following:

class Test {
    void doTest() {
    private int x = 7; //Not Possible
    this.doSomethingElse(x);
    }
}

The above code will not compile or run. Dont think that the private keyword is legal inside the method doTest().
You can be certain that any local variable declared with an access modifier will not compile. In fact, there is only one modifier that can ever be applied to local variables—final.

 

 

Object references in java

Those readers familiar with C/C++ have probably noticed that object references appear
to be similar to pointers. This suspicion is, essentially, correct. An object reference is
similar to a memory pointer. The main difference—and the key to Java’s safety—is that
you cannot manipulate references as you can actual pointers. Thus, you cannot cause an
object reference to point to an arbitrary memory location or manipulate it like an integer.



The new operator dynamically allocates memory for an object. It has
this general form:
class-var = new classname( );
Here, class-var is a variable of the class type being created. The classname is the name of
the class that is being instantiated. The class name followed by parentheses specifies the
constructor for the class.

Assigning Object Reference Variables

Box b1 = new Box();
Box b2 = b1;



b1 and b2 will

both refer to the same object.

Box b1 = new Box();
Box b2 = b1;
// ...
b1 = null;
Here, b1 has been set to null, but b2 still points to the original object.
Note : When you assign one object reference variable to another object reference variable, you
are not creating a copy of the object, you are only making a copy of the reference.

this - same as cpp



Garbage Collection
Since objects are dynamically allocated by using the new operator, you might be
wondering how such objects are destroyed and their memory released for later
reallocation. In some languages, such as C++, dynamically allocated objects must
be manually released by use of a delete operator. Java takes a different approach; it
handles deallocation for you automatically. The technique that accomplishes this is
called garbage collection. It works like this: when no references to an object exist, that
object is assumed to be no longer needed, and the memory occupied by the object can
be reclaimed. There is no explicit need to destroy objects as in C++. Garbage collection
only occurs sporadically (if at all) during the execution of your program. It will not
occur simply because one or more objects exist that are no longer used. Furthermore,
different Java run-time implementations will take varying approaches to garbage
collection, but for the most part, you should not have to think about it while writing
your programs.







The finalize( ) Method
Sometimes an object will need to perform some action when it is destroyed. For
example, if an object is holding some non-Java resource such as a file handle or
window character font, then you might want to make sure these resources are freed
before an object is destroyed. To handle such situations, Java provides a mechanism

called finalization. By using finalization, you can define specific actions that will occur
when an object is just about to be reclaimed by the garbage collector.

To add a finalizer to a class, you simply define the finalize( ) method. The Java run
time calls that method whenever it is about to recycle an object of that class. Inside the
finalize( ) method you will specify those actions that must be performed before an
object is destroyed. The garbage collector runs periodically, checking for objects that
are no longer referenced by any running state or indirectly through other referenced
objects. Right before an asset is freed, the Java run time calls the finalize( ) method on
the object.
The finalize( ) method has this general form:
protected void finalize( )
{
// finalization code here
}
Here, the keyword protected is a specifier that prevents access to finalize( ) by code
defined outside its class. This and the other access specifiers are explained in Chapter 7.
It is important to understand that finalize( ) is only called just prior to garbage
collection. It is not called when an object goes out-of-scope, for example. This means
that you cannot know when—or even if—finalize( ) will be executed. Therefore, your
program should provide other means of releasing system resources, etc., used by the
object. It must not rely on finalize( ) for normal program operation.
If you are familiar with C++, then you know that C++ allows you to define a destructor
for a class, which is called when an object goes out-of-scope. Java does not support this
idea or provide for destructors. The finalize( ) method only approximates the function
of a destructor. As you get more experienced with Java, you will see that the need for
destructor functions is minimal because of Java’s garbage collection subsystem.