Java, SCJP, Core Java Programming

Declarations and Access Control

2006-04-01T11:19:00.000-08:00

Write code that declares, constructs and initializes arrays of any base type using any of the permitted forms both for declaration and for initialization.

Declaring arrays:

For example,use either

 int[] x;

 int x[];

Both are legal positions for the brackets. To declare a multidimensional array, use multiple sets of brackets, e.g. int i[][]; declares a two dimensional array. Note that, for example, int[]i[]; is also legal, and means the same thing. In Java, multidimensional arrays can be "not square" i.e. they are just arrays of arrays, and each of those constituent arrays can be of a different size.

Construct arrays:

Arrays are objects. Use the new keyword to construct them. For example having declared an array i:

int[] i;

you then construct the array and assign it to i as follows:

i = new int[10];

The size of the array is in the brackets (in this example 10). It is common to do both operations in one statement:

int i[] = new int[10];

To initialize an array using loop iteration:

An example:

 int array[] = new int[15];
 for(int j=0; jWrite code to initialize an array using the combined declaration and initialization format:          
          An example:
char c[]= new char[] {'a','b','c','d','e'};
or you can use
char c[]= {'a','b','c','d','e'};
                             Declare classes, nested classes, methods, instance variables, static variables and automatic (method local)                      variables making appropriate use of all permitted modifiers (such as public, final,                      static, abstract etc.). State the significance of each of these modifiers both singly and in combination and state the effect of package relationships on declared items qualified by these modifiers.              
          
          This is fundamental Java. If you are unsure of this topic, see a textbook.
          A good  but technical reference for these kinds of issues is the Java Language Specification (not for beginners).             It is available at the following URL:            
http://java.sun.com/docs/books/jls/html/index.html
          Some terms and their synonyms:          
          Scope/Visibility: Where something can be seen / is accessable from.          
          Nested/Inner Class: A class whose code sits inside the body of another 'outer' class. It exists 'inside' the class in that it             can see the private methods and variables.          
          Instance/Member: Means the method/variable/nested class belongs to each object which is an instance of the class.             The term 'a member'             is often used to refer to both methods and variables of this type.             Cannot be accessed by statically i.e. without having an instance of the class.          
          Class/Static: The method/variable/nested class belongs to the class as opposed to the instances of the class. Can be used without creating an instance of the class, but static methods/nested class cannot use instancts variables/methods. 
          Local/Automatic variable: A variable which is declared within a method or as a parameter to the method.             Cannot be seen outside the method.          
          Scoping Types          
          "default" or "friendly" - this is where no modifier is used. Means something is only visible to classes in the same package.
          protected - can only be accessed by classes in the same package or subclasses of this class. This frequently surprises experienced developers, especially those with a prior backround in the mutant hybrid of object orientated programming and assembly language known as 'C++' :-) To fair the name is misleading, as it sounds like it restricts access, where as it in fact it adds subclasses outside the package to the list of things that can access the item in question, as compared to using "default" access. 
          public - can be accessed by any other class.                       
          private - can only be accessed from inside the class.           
          Declare classes using the modifiers public, abstract or final:          
          public - is visible outside of its package. Without this modifier, the class cannot be accessed outside             its package.                       
          abstract - cannot be instantiated, is allowed to contain abstract methods.                       
          final - cannot be subsclassed.          
          Using the modifiers private, protected, public, static,             final, native or abstract:          
          private - can only be accessed from inside the class. Private members are not inherited by subclasses. Inner classes             can be declared private.                       
          protected - can only be accessed by classes in the same package or subclasses of this class.                       
          public - can be accessed by any other class.                       
          static - belongs to the class rather than any particular instance of the class. For variables, effectively, there is just one copy of this variable for all instances of the class, and if an instance changes the value, the other instances see that new value. For methods, it means the method can be called without having created an instance, but within the methodd you cannot use the this keyword, or refer to instance variables and methods directly (without creating an instance and referring to the variable/class in that instance). For inner classes, it means they can be instantiated without having an instance of the enclosing class, but as with static methods, the methods of the inner class cannot refer to instance variables or methods of the enclosing class directly. 
          final - cannot be changed. Variables are constants, methods cannot be overridden, classes cannot be subclassed. Since Java1.1 you can declare the variable without assigning a value. Once you assign a value, you cannot change it. The are known as 'blank' finals, are frequently used for things like constants you wish to initialise from a configuration file. 
          native - a method which is not written in java and is outside the JVM in a library.                       
          abstract - a method which is not implemented. Must be implemented in a subclass if that subclass is to be             'concrete' and allow people to instantiate it.          
          Nested Classes          
          To define a non-static nested class either in a class or method scope:          
          Place the class definition (for the nested class) inside another class definition (the outer class) or a method.
          To define, in method scope, an anonymous nested class that implements a specified interface:          
          An anonymous nested class is defined where is it instantiated (in a method). An anonymous nested class must either             implement an interface or extend a class, but the implements or extends keywords are not used. For example the following line causes the method to return an object which is an instance of an anonymous nested class: 
return new SomeClass() { /*body of the anonymous class goes here*/ };
You might like to think of an anonymous nested class as part of a really long new statement, which happens to             contain a class definition, and which is why it has a ";" at the end.              The following example calls someMethod(), passing an instance of the anonymous nested class:          
someMethod(new SomeClass() { /*body of the anonymous class goes here*/ });
In both cases SomeClass() is not the name of the anonymous class (anonymous means it has no name) rather is it the name of the class that you are extending or the interface you are implementing. These classes cannot define a constructor, as they do not have a name that you can use to declare the constructor method. If SomeClass() is a class, the default constructor of that class is called, if you want to use a non-default constructor instead, you supply arguments e.g.: 
return new SomeClass(12) { /*body of the anonymous class goes here*/ };
will call the SomeClass constructor which takes an int.
          Write code in a non-static method of the outer class to construct an instance of the nested class.          
Inner x = new Inner(); constructs an instance of Inner where Inner is a nested class defined in the current class.                              Write code to construct an instance on a nested class where either no this object exists,             or the current this object is not an instance of the outer class.          
          You must create an instance of the outer class first. Given a class, Outer, containing a nested class Inner:          Outer.Inner y = new Outer().new Inner();
The above creates an instance of Inner called y, but it had to construct an instance of Outer first. The following example creates the Outer instance on a separate line, the syntax in the second line is the one you use when you already have an instance of the outer class. 
Outer x = new Outer();
Outer.Inner y = x.new Inner();
          If Inner is static, you can use:          Outer.Inner I= new Outer.Inner();
State which variables and methods in enclosing scopes are accessible from methods of the inner class.          
           A non-static inner class has access to all member variables and methods of the containing class. If the inner class is defined inside a method, it has access to those method (a.k.a. automatic or local) variables which are declared final, in addition to the above.                       
          A static inner class is restricted in the same way as a static method: it cannot refer to instance variables and methods of the containing class directly (without creating an instance and referring to the variable/class in that instance). 
                             For a given class, determine if a default constructor will be created and if so state the prototype of that constructor.              
          
          The default constructor takes no arguments e.g. classname() where classname is the name of you class.              A default constructor is automatically created only if you do not create any constructors in your class.              If you create a non-default constructor (i.e. one that takes an argument), then you may have to create a default one             yourself, if you want there to be one.                        
          All constructors call the default constructor of its parents class (if there is one), and so on up the hierarchy to Object,             unless you specify a different constructor using super(...) (to use a constructor in the parent) or this(...).              If you use such calls, they must be the first thing in the constructor.                       
                        
©1999, 2000, 2002 Dylan Walsh.  Under free documentation liscence.

This link is to get listed in a directory.

2006-02-18T13:14:00.000-08:00

Human-Anatomy.net Software

2005-12-28T18:43:00.000-08:00

Written by Dylan Walsh. Released under FDL.

Declarations and Access Control

Write code that declares, constructs and initializes arrays of any base type using any of the permitted forms both for declaration and for initialization.

Declaring arrays:

For example,use either

 int[] x;

 int x[];

Construct arrays:

Arrays are objects. Use the new keyword to construct them. For example having declared an array i:

int[] i;

you then construct the array and assign it to i as follows:

i = new int[10];

The size of the array is in the brackets (in this example 10). It is common to do both operations in one statement:

int i[] = new int[10];

To initialize an array using loop iteration:

An example:

 int array[] = new int[15];
 for(int j=0; j  Write code to initialize an array using the combined  declaration and initialization format: 
  An example:
 char c[]= new char[] {'a','b','c','d','e'};
  or you can use
 char c[]= {'a','b','c','d','e'};
     Declare classes, nested  classes, methods, instance variables, static variables and automatic (method  local) variables making appropriate use of all permitted modifiers (such as  public, final, static,  abstract etc.). State the significance of each of these modifiers  both singly and in combination and state the effect of package relationships on  declared items qualified by these modifiers. 
 
  This is fundamental Java. If you are unsure of this topic, see a  textbook.
  A good but technical reference for these kinds of issues is the Java Language  Specification (not for beginners). It is available at the following URL:
http://java.sun.com/docs/books/jls/html/index.html
  Some terms and their synonyms: 
  Scope/Visibility: Where something can be seen / is accessable from.  
  Nested/Inner Class: A class whose code sits inside the body of another  'outer' class. It exists 'inside' the class in that it can see the private  methods and variables. 
  Instance/Member: Means the method/variable/nested class belongs to  each object which is an instance of the class. The term 'a member' is often used  to refer to both methods and variables of this type. Cannot be accessed by  statically i.e. without having an instance of the class. 
  Class/Static: The method/variable/nested class belongs to the class as  opposed to the instances of the class. Can be used without creating an instance  of the class, but static methods/nested class cannot use instancts  variables/methods. 
  Local/Automatic variable: A variable which is declared within a method  or as a parameter to the method. Cannot be seen outside the method. 
  Scoping Types 
  "default" or "friendly" - this is where no modifier is used. Means something  is only visible to classes in the same package.
  protected - can only be accessed by classes in the same package  or subclasses of this class. This frequently surprises experienced developers,  especially those with a prior backround in the mutant hybrid of object  orientated programming and assembly language known as 'C++' :-) To fair the name  is misleading, as it sounds like it restricts access, where as it in fact it  adds subclasses outside the package to the list of things that can access the  item in question, as compared to using "default" access. 
  public - can be accessed by any other class. 
  private - can only be accessed from inside the class. 
  Declare classes using the modifiers public,  abstract or final: 
  public - is visible outside of its package. Without this  modifier, the class cannot be accessed outside its package. 
  abstract - cannot be instantiated, is allowed to contain  abstract methods. 
  final - cannot be subsclassed. 
  Using the modifiers private,  protected, public, static,  final, native or abstract: 
  private - can only be accessed from inside the class. Private  members are not inherited by subclasses. Inner classes can be declared  private. 
  protected - can only be accessed by classes in the same package  or subclasses of this class. 
  public - can be accessed by any other class. 
  static - belongs to the class rather than any particular  instance of the class. For variables, effectively, there is just one copy of  this variable for all instances of the class, and if an instance changes the  value, the other instances see that new value. For methods, it means the method  can be called without having created an instance, but within the methodd you  cannot use the this keyword, or refer to instance variables and  methods directly (without creating an instance and referring to the  variable/class in that instance). For inner classes, it means they can be  instantiated without having an instance of the enclosing class, but as with  static methods, the methods of the inner class cannot refer to instance  variables or methods of the enclosing class directly. 
  final - cannot be changed. Variables are constants, methods  cannot be overridden, classes cannot be subclassed. Since Java1.1 you can  declare the variable without assigning a value. Once you assign a value, you  cannot change it. The are known as 'blank' finals, are frequently used for  things like constants you wish to initialise from a configuration file. 
  native - a method which is not written in java and is  outside the JVM in a library. 
  abstract - a method which is not implemented. Must be  implemented in a subclass if that subclass is to be 'concrete' and allow people  to instantiate it. 
  Nested Classes 
  To define a non-static nested class either in a class or  method scope: 
  Place the class definition (for the nested class) inside another class  definition (the outer class) or a method.
  To define, in method scope, an anonymous nested class  that implements a specified interface: 
  An anonymous nested class is defined where is it instantiated (in a method).  An anonymous nested class must either implement an interface or extend a class,  but the implements or extends keywords are not used.  For example the following line causes the method to return an object which is an  instance of an anonymous nested class: 
 return new SomeClass() { /*body of the anonymous class goes here*/ };
  You might like to think of an anonymous nested class as part of a really long  new statement, which happens to contain a class definition, and  which is why it has a ";" at the end. The following example calls  someMethod(), passing an instance of the anonymous nested class:  
 someMethod(new SomeClass() { /*body of the anonymous class goes here*/ });
  In both cases SomeClass() is not the name of the anonymous class  (anonymous means it has no name) rather is it the name of the class that you are  extending or the interface you are implementing. These classes cannot define a  constructor, as they do not have a name that you can use to declare the  constructor method. If SomeClass() is a class, the default constructor of that  class is called, if you want to use a non-default constructor instead, you  supply arguments e.g.: 
 return new SomeClass(12) { /*body of the anonymous class goes here*/ };
  will call the SomeClass constructor which takes an int.
  Write code in a non-static method of the outer class to  construct an instance of the nested class. 
 Inner x = new  Inner(); constructs an instance of Inner where Inner is a nested class  defined in the current class.  Write code to construct an instance on a nested class  where either no this object exists, or the current  this object is not an instance of the outer class. 
 You must  create an instance of the outer class first. Given a class, Outer, containing a  nested class Inner:  Outer.Inner y = new Outer().new Inner();
  The above creates an instance of Inner called y, but it had to construct an  instance of Outer first. The following example creates the Outer instance on a  separate line, the syntax in the second line is the one you use when you already  have an instance of the outer class. 
 Outer x = new Outer();
Outer.Inner y = x.new Inner();
 If Inner is static, you can use:  Outer.Inner I= new Outer.Inner();
  State which variables and methods in enclosing scopes are  accessible from methods of the inner class. 
  A non-static inner class has access to all member variables and methods of  the containing class. If the inner class is defined inside a method, it has  access to those method (a.k.a. automatic or local) variables which are declared  final, in addition to the above. 
  A static inner class is restricted in the same way as a static method: it  cannot refer to instance variables and methods of the containing class directly  (without creating an instance and referring to the variable/class in that  instance). 
     For a given class, determine  if a default constructor will be created and if so state the prototype of that  constructor. 
 
  The default constructor takes no arguments e.g. classname()  where classname is the name of you class. A default constructor is automatically  created only if you do not create any constructors in your class. If you  create a non-default constructor (i.e. one that takes an argument), then you may  have to create a default one yourself, if you want there to be one. 
  All constructors call the default constructor of its parents class (if there  is one), and so on up the hierarchy to Object, unless you specify a different  constructor using super(...) (to use a constructor in the parent)  or this(...). If you use such calls, they must be the first thing  in the constructor.

Should I use an Ide while learning java.

2005-10-25T09:28:00.000-07:00

Hmmmmmm. IDE's making coding really easy. And they save a lot of time. So If you are a serious Java developer coding without an IDE is realy foolish. But if you are trying to learn Java, and SCJP is an entry level certification, then dont even think about using an advanced IDE like Eclipse. In fact, I gave my SCJP exam after I had learnt a lot of Java and was using Eclipse for some time. I really had forgotten how to use the basic features, the features you are tested on in SCJP. So I had to leave using Eclipse for sometime. But a basic IDE with syntax highlighting, code explorer but certainly without code completion is ok. I would personaly recommend JCreator Lite. I did use it when I was beginning to use java.

2005-10-25T08:45:00.000-07:00

This site has some extremely good java resources. And it is completely free.

SCJP study notes:Chapter 8 java.lang package

2005-08-31T02:32:00.000-07:00

Originally written by Velmurugan Periasamy. Copyright belongs to him.

Chapter 8 java.lang package

· Object class is the ultimate ancestor of all classes. If there is no extends clause, compiler inserts ‘extends object’. The following methods are defined in Object class. All methods are public, if not specified otherwise.

Method	Description
boolean equals(Object o)	just does a == comparison, override in descendents to provide meaningful comparison
final native void wait() final native void notify() final native void notifyAll()	Thread control. Two other versions of wait() accept timeout parameters and may throw InterruptedException.
native int hashcode()	Returns a hash code value for the object. If two objects are equal according to the equals method, then calling the hashCode method on each of the two objects must produce the same integer result.
protected Object clone() throws CloneNotSupportedException CloneNotSupportedException is a checked Exception	Creates a new object of the same class as this object. It then initializes each of the new object's fields by assigning it the same value as the corresponding field in this object. No constructor is called. The clone method of class Object will only clone an object whose class indicates that it is willing for its instances to be cloned. A class indicates that its instances can be cloned by declaring that it implements the Cloneable interface. Also the method has to be made public to be called from outside the class. Arrays have a public clone method. int ia[ ][ ] = { { 1 , 2}, null }; int ja[ ][ ] = (int[ ] [ ])ia.clone(); A clone of a multidimensional array is shallow, which is to say that it creates only a single new array. Subarrays are shared, so ia and ja are different but ia[0] and ja[0] are same.
final native Class getClass()	Returns the runtime class of an object.
String toString	Returns the string representation of the object. Method in Object returns a string consisting of the name of the class of which the object is an instance, the at-sign character `@', and the unsigned hexadecimal representation of the hash code of the object. Override to provide useful information.
protected void finalize() throws Throwable	Called by the garbage collector on an object when garbage collection determines that there are no more references to the object. Any exception thrown by the finalize method causes the finalization of this object to be halted, but is otherwise ignored. The finalize method in Object does nothing. A subclass overrides the finalize method to dispose of system resources or to perform other cleanup.

· Math class is final, cannot be sub-classed.

· Math constructor is private, cannot be instantiated.

· All constants and methods are public and static, just access using class name.

· Two constants PI and E are specified.

· Methods that implement Trigonometry functions are native.

· All Math trig functions take angle input in radians.

Angle degrees * PI / 180 = Angle radians

· Order of floating/double values:

-Infinity --> Negative Numbers/Fractions --> -0.0 --> +0.0 --> Positive Numbers/Fractions --> Infinity

· abs – int, long, float, double versions available

· floor – greatest integer smaller than this number (look below towards the floor)

· ceil – smallest integer greater than this number (look above towards the ceiling)

· For floor and ceil functions, if the argument is NaN or infinity or positive zero or negative zero or already a value equal to a mathematical integer, the result is the same as the argument.

· For ceil, if the argument is less than zero but greater than –1.0, then the result is a negative zero

· random – returns a double between 0.0(including) and 1.0(excluding)

· round returns a long for double, returns an int for float. (closest int or long value to the argument)

The result is rounded to an integer by adding ½ , taking the floor of the result, and casting the result to type int / long.

(int)Math.floor(a + 0.5f)

(long)Math.floor(a + 0.5d)

· double rint(double) returns closest double equivalent to a mathematical integer. If two values are equal, it returns the even integer value. rint(2.7) is 3, rint(2.5) is 2.

· Math.min(-0.0, +0.0) returns –0.0, Math.max(-0.0, +0.0) returns 0.0, -0.0 == +0.0 returns true.

· For a NaN or a negative argument, sqrt returns a NaN.

· Every primitive type has a wrapper class (some names are different – Integer, Boolean, Character)

· Wrapper class objects are immutable.

· All Wrapper classes are final.

· All wrapper classes, except Character, have a constructor accepting string. A Boolean object, created by passing a string, will have a value of false for any input other than “true” (case doesn’t matter).

· Numeric wrapper constructors will throw a NumberFormatException, if the passed string is not a valid number. (empty strings and null strings also throw this exception)

· equals also tests the class of the object, so even if an Integer object and a Long object are having the same value, equals will return false.

· NaN’s can be tested successfully with equals method.

Float f1 = new Float(Float.NaN);

Float f2 = new Float(Float.NaN);

System.out.println( ""+ (f1 == f2)+" "+f1.equals(f2)+ " "+(Float.NaN == Float.NaN) );

The above code will print false true false.

· Numeric wrappers have 6 methods to return the numeric value – intValue(), longValue(), etc.

· valueOf method parses an input string (optionally accepts a radix in case of int and long) and returns a new instance of wrapper class, on which it was invoked. It’s a static method. For empty/invalid/null strings it throws a NumberFormatException. For null strings valueOf in Float and Double classes throw NullPointerException.

· parseInt and parseLong return primitive int and long values respectively, parsing a string (optionally a radix). Throw a NumberFormatException for invalid/empty/null strings.

· Numeric wrappers have overloaded toString methods, which accept corresponding primitive values (also a radix in case of int,long) and return a string.

· Void class represents void primitive type. It’s not instantiable. Just a placeholder class.

· Strings are immutable.

· They can be created from a literal, a byte array, a char array or a string buffer.

· Literals are taken from pool (for optimization), so == will return true, if two strings are pointing to the same literal.

· Anonymous String objects (literals) may have been optimized even across classes.

· A String created by new operator is always a different new object, even if it’s created from a literal.

· But a String can specify that its contents should be placed in a pool of unique strings for possible reuse, by calling intern() method. In programs that do a lot of String comparisons, ensuring that all Strings are in the pool, allows to use == comparison rather than the equals() method, which is slower.

· All string operations (concat, trim, replace, substring etc) construct and return new strings.

· toUpperCase and toLowerCase will return the same string if no case conversion was needed.

· equals takes an object (String class also has a version of equals that accepts a String), equalsIgnoreCase takes a string.

· Passing null to indexOf or lastIndexOf will throw NullPointerException, passing empty string returns 0, passing a string that’s not in the target string returns –1.

· trim method removes all leading and trailing white-space from a String and returns a new String. White-space means, all characters with value less than or equal to the space character – ‘\u0020’.

· String class is final.

· + and += operators are overloaded for Strings.

· reverse, append, insert are not String methods.

· String Buffers are mutable strings.

· StringBuffer is a final class.

· They can be created empty, from a string or with a capacity. An empty StringBuffer is created with 16-character capacity. A StringBuffer created from a String has the capacity of the length of String + 16. StringBuffers created with the specified capacity has the exact capacity specified. Since they can grow dynamically in size without bounds, capacity doesn’t have much effect.

· append, insert, setCharAt, reverse are used to manipulate the string buffer.

· setLength changes the length, if the current content is larger than specified length, it’s truncated. If it is smaller than the specified length, nulls are padded. This method doesn’t affect the capacity.

· equals on StringBuffer does a shallow comparison. (same like ==) Will return true only if the objects are same. Don’t use it to test content equality

· trim is not a StringBuffer method.

· There is no relationship between String and StringBuffer. Both extend Object class.

· String context means, ‘+’ operator appearing with one String operand. String concatenation cannot be applied to StringBuffers.

· A new String buffer is created.

· All operands are appended (by calling toString method, if needed)

· Finally a string is returned by calling toString on the String Buffer.

· String concatenation process will add a string with the value of “null”, if an object reference is null and that object is appearing in a concatenation expression by itself. But if we try to access its members or methods, a NullPointerException is thrown. The same is true for arrays, array name is replaced with null, but trying to index it when it’s null throws a NullPointerException.

Java, SCJP, Core Java Programming

Declarations and Access Control

Declaring arrays:

Construct arrays:

To initialize an array using loop iteration:

Write code to initialize an array using the combined declaration and initialization format:

Some terms and their synonyms:

Scoping Types

Declare classes using the modifiers `public`, `abstract` or `final`:

Using the modifiers `private`, `protected`, `public`, `static`, `final`, `native` or `abstract`:

Nested Classes

To define a non-static nested class either in a class or method scope:

To define, in method scope, an anonymous nested class that implements a specified interface:

Write code in a non-static method of the outer class to construct an instance of the nested class.

Write code to construct an instance on a nested class where either no `this` object exists, or the current `this` object is not an instance of the outer class.

State which variables and methods in enclosing scopes are accessible from methods of the inner class.

This link is to get listed in a directory.

Declarations and Access Control

Declaring arrays:

Construct arrays:

To initialize an array using loop iteration:

Write code to initialize an array using the combined declaration and initialization format:

Some terms and their synonyms:

Scoping Types

Declare classes using the modifiers `public`, `abstract` or `final`:

Using the modifiers `private`, `protected`, `public`, `static`, `final`, `native` or `abstract`:

Nested Classes

To define a non-static nested class either in a class or method scope:

To define, in method scope, an anonymous nested class that implements a specified interface:

Write code in a non-static method of the outer class to construct an instance of the nested class.

Write code to construct an instance on a nested class where either no `this` object exists, or the current `this` object is not an instance of the outer class.

State which variables and methods in enclosing scopes are accessible from methods of the inner class.

Should I use an Ide while learning java.

SCJP study notes:Chapter 8 java.lang package

Chapter 8 java.lang package