SJCP Exam Preparation: Language Fundamentals, Part Two

This series will prepare you for the Sun Java certification exam. To begin, your instructor introduces you to the fundamentals of the Java language.

Objectives

• Identifiers and Keywords
• Primitive Types
• Literals
• Arrays
• Variable Types and Initialization

If you have any questions after studying this document, do not hesitate to drop me an e-mail, and I will try to clarify things as best as I can.

Koray Guclu

You can read more information about SCJP certification from the SJCP Web page [2], and you can download the .pdf file called "Sun Certified Programmer for Java 2 Platform Success Guide" [3].

Identifiers and Keywords

¹ 'goto' and 'cost' are reserved words but they do not have any use in Java. Since 'goto' and 'cost' are reserved words, they cannot be used as identifiers.
² 'true, 'false', 'null' might appear to be keywords but they are technically literals [1]['3.10.3]. 'true, 'false' are Boolean literals and 'null' is the null literal (they cannot be used as identifiers).

Primitive Types and their Wrapper Classes

Java has eight primitive types; each primitive type has a matching wrapper class. A variable declared as type primitive cannot store an object reference. The primitive types are used to store primitive values. All numeric types are signed, except 'boolean' and 'char'. The 'boolean' literal has exactly two values: 'true', and 'false'. There are two kinds of numeric types: the integral and the floating-point types. The integral types are:

• byte
• short
• int
• long
• char

• float
• double

• Float.NaN
• Float.POSITIVE_INFINITY
• Float.NEGATIVE_INFINITY
• Double.NaN
• Double.POSITIVE_INFINITY
• Double.NEGATIVE_INFINITY

Literals

A literal is a value, which can be assigned to a primitive type or a String. For example:

'null' and 'boolean' literals

'char' literals

String literals

Integral literals

An integral literal is decimal and its type is 32 bit 'int' by default. For that reason, the compiler will throw an exception if we define an integral literal exceeding the range of 'int' primitive type. The 'long' primitive type is defined by an 'l' or 'L' suffix of the literal. For example, 254L or 254l is a long literal. Examples are:

Floating-point literals

It is also possible to specify the 'double' type explicitly by adding a 'd' or 'D' suffix. We can define float-point literals, which are type of 'float', by adding a 'f' or 'F' suffix. For example, both 3.14f, and 3.14F are valid floating-point literals. Floating point literals can also have an exponential part. The exponential part has an ' e' or 'E' prefix and optional '+' or '-' signs followed by digits. For example, 0.23E-2 is a valid floating-point number with exponential part. The following table shows the four possible floating-point representations.

Arrays

An array is a storage place of a list of items of the same type. An array can be either a collection of primitive types or object references. We cannot define two different type in an array. Every element of an array must be of the same type. A Java array is type Object. Therefore, methods of type Object can be called on an array. The size of an array cannot be changed once the array is created. The elements of an array can be accessed by an index value that is greater or equal to zero. The index value is type 'int' and cannot be a negative number. If the array size is specified by n then the elements of the array can be accessed by an index value between the range of 0 and n-1. It is possible to create an array with zero elements. Once an array is declared and allocated, array elements are automatically initialized to a default value, wherever the array is defined. It does not matter whether the array is declared at class level (member) or method level (local), the elements of the array will always be initialized to the default value of the element. Reference variables are initialized to 'null' and primitive types are initialized to the default values. However, there will not be an automatic initialization if the array is only declared (not allocated). Arrays should be declared, allocated, and initialized. The following table shows a couple of array examples that have been declared, allocated, and set to either a specific or a default value.

As you can see, we do not specify the size of the array in the declaration part. Alternatively, it is possible to declare, allocate, and define a list of custom elements in one step. That is done as follows.

Example

Variable Types and Variable Initialization

We can classify variables according to their lifetimes. In this respect, there are two types of variables : member and method local. Member variables are declared at class level (like methods). A member variable is automatically initialized to a default value. The list of default values is mentioned and a table is given in the previous section. A member variable is implicitly initialized to null if declared as an Object reference, and initialized to a default value if declared as a primitive type. Method local variables are declared in a method. Method local variables are not initialized to a default value; therefore, they must be initialized to a value explicitly.

class myClass {
    // Member variables
    int k; // Implicitly set to default value 0
    double d; // Implicitly set to default value 0.0
    public void myMethod(){
        // Method local variable
        int local_t = 0; 
        // we must initialize 
        // 'local_t' explicitly
    }
}

Sample Code

final class Singleton {
    // M e m b e r  V a r i a b l e s
    // Instance variables
    private int defaultint;
    private double defaultdouble;
    private char defaultchar;
    private String defaultString;
    // Static ( Class ) variables
    private static Singleton singleton;
    private static String ARRAYFACTORY = "ArrayFactory";
    private static String NUMBERFACTORY = "NumberFactory";
    // C o n s t r a c t o r
    private Singleton() {
    };
    // M e t h o d s
    // Static method
    public static Singleton getReference() {
        if (singleton == null)
            singleton = new Singleton();
        return singleton;
    }
    public void test() {
        // Method local variable
        String factory_msg = " is called";
        println(NUMBERFACTORY + factory_msg);
        // default values 
        println(" int    = " + defaultint);
        println(" double = " + defaultdouble);
        println(" char   = " + defaultchar);
        println(" String = " + defaultString);
  
	println(ARRAYFACTORY + factory_msg);
        // int array 
        int sequence[] = {1, 2, 3, 4, 5};
	println(" Array size of 'sequence' is " 
	                + sequence.length);
        for (int k = 0; k < sequence.length; k++)
	    println(" Array element sequence[" + k + "] = " 
	                + sequence[k]);
        // double array 
        double defaultvalue[] = new double[3];
	println(" Array size of 'defaultvalue' is " 
	                + defaultvalue.length);
        for (int k = 0; k < defaultvalue.length; k++)
	    println(" Array element defaultvalue[" + k + "] = " 
	                + defaultvalue[k]);
    }
    public static void println(String str) {
        System.out.println(str);
	}
    // M a i n  M e t h o d
    public static void main(String[] args) {
        Singleton.getReference().test();
    }
}

Questions

1. Which of the followings are valid array declarations? (Select one or more answers)
   a.) int k[][] = new int[5,3];
   b.) int i[] = new i[];
   c.) int int[] = new int[];
   d.) int [][]i= new int[3][4];
   e.) int []a = {1,2,3,4,5};
2. What is the hexadecimal representation of 12 as a valid Java literal?
   a.) 12
   b.) C
   c.) c
   d.) 0C
   e.) 0xc
3. What are the legal identifers in Java? (Select one or more answers)
   a.) %factory
   b.) $factory
   c.) object-factory
   d.) for
   e.) _byte
   f.) 3rdfactory
   g.) Int
4. What is the output of the following program?

   class Example{
       static int sequence[] = {1, 2, 3, 4, 5};
       public static void main(String args[]){
           System.out.println(" The value of sequence[4] is " 
                              + sequence[4]);
       }
   }

   a.) Null Pointer exception is raised
   b.) Runtime error, because the class is not instantiated
   c.) The value of sequence[4] is 0
   d.) The value of sequence[4] is 5
   e.) The value of sequence[4] is null
5. What is the range of an int?
   a.) -2^-32 , 2³²
   b.) -2^-31 , 2³¹
   c.) -2^-31-1 , 2³¹
   d.) -2³¹ , 2³¹-1
   e.) -2^-31 , 2³¹-1
   f.) 0, 2³²

Answers

1. d,e
2. e
3. b,e,g
4. d
5. d

References

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