SCJP 5 : Chapter 3. API Contents (Part-1)

SCJP 5 : Chapter 3. API Contents (Part-1)

Develop code that uses the primitive wrapper classes (such as Boolean, Character, Double, Integer, etc.), and/or autoboxing & unboxing. Discuss the differences between the String, StringBuilder, and StringBuffer classes.

Autoboxing/unboxing of primitive types

Manual conversion between primitive types (such as an int) and wrapper classes (such as Integer) is necessary when adding a primitive data type to a collection. As an example, consider an int being stored and then retrieved from an ArrayList:

list.add(0, new Integer(59));
int n = ((Integer)(list.get(0))).intValue();

The new autoboxing/unboxing feature eliminates this manual conversion. The above segment of code can be written as:

list.add(0, 59);
int total = list.get(0);

However, note that the wrapper class, Integer for example, must be used as a generic type:

List< Integer > list = new ArrayList< Integer >();

The autoboxing and auto-unboxing of Java primitives produces code that is more concise and easier to follow.

..... int i = 10; Integer iRef = new Integer(i); // Explicit Boxing int j = iRef.intValue(); // Explicit Unboxing iRef = i; // Automatic Boxing j = iRef; // Automatic Unboxing .....

Automatic boxing and unboxing conversions alleviate the drudgery in converting values of primitive types to objects of the corresponding wrapper classes and vice versa.

Boxing conversion converts primitive values to objects of corresponding wrapper types: if p is a value of a primtiveType, boxing conversion converts p into a reference ref of corresponding WrapperType, such that ref.primitiveTypeValue() == p.

Unboxing conversion converts objects of wrapper types to values of corresponding primitive types: if ref is a reference of a WrapperType, unboxing conversion converts the reference ref into ref.primitiveTypeValue(), where primitiveType is the primitive type corresponding to the WrapperType.

Assignment conversions on boolean and numeric types:

..... boolean boolVal = true; byte b = 2; short s = 2; char c ='2'; int i = 2; // Boxing Boolean boolRef = boolVal; Byte bRef = 88; Short sRef = 2; Character cRef = '2'; Integer iRef = 2; // Integer iRef1 = s; // WRONG ! short not assignable to Integer // Unboxing boolean boolVal1 = boolRef; byte b1 = bRef; short s1 = sRef; char c1 = cRef; int i1 = iRef; .....

Method invocation conversions on actual parameters:

..... ... flipFlop("(String, Integer, int)", new Integer(4), 2004); ... private static void flipFlop(String str, int i, Integer iRef) { out.println(str + " == > (String, int, Integer)"); } .....

The output:

(String, Integer, int) == > (String, int, Integer)

Casting conversions:

..... Integer iRef = (Integer) 2; // Boxing followed by identity cast int i = (int) iRef; // Unboxing followed by identity cast // Long lRef = 2; // WRONG ! Type mismatch: cannot convert from int to Long // Long lRef = (Long) 2; // WRONG ! int not convertible to Long Long lRef_1 = (long) 2; // OK ! explicit cast to long Long lRef_2 = 2L; // OK ! long literal .....

Numeric promotion: unary and binary:

..... Integer iRef = 2; long l1 = 2000L + iRef; // binary numeric promotion int i = -iRef; // unary numeric promotion .....

In the if statement, condition can be Boolean:

..... Boolean expr = true; if (expr) { out.println(expr); } else { out.println(!expr); // Logical complement operator } .....

In the switch statement, the switch expression can be Character, Byte, Short or Integer:

..... // Constants final short ONE = 1; final short ZERO = 0; final short NEG_ONE = -1; // int expr = 1; // (1) short is assignable to int. switch works. // Integer expr = 1; // (2) short is not assignable to Integer. switch compile error. Short expr = 1; // (3) OK. Cast is not required. switch (expr) { // (4) expr unboxed before case comparison. .....case ONE: ..........out.println("ONE"); break; .....case ZERO: ..........out.println("ZERO"); break; .....case NEG_ONE: ..........out.println("NEG_ONE"); break; .....default: ..........assert false; } .....

The output:

ONE

In the while, do-while and for statements, the condition can be Boolean:

..... Boolean expr = true; while (expr) { .....expr = !expr; } .....

..... Character[] version = { '5', '.', '0' }; // Assignment: boxing for (Integer iRef = 0; // Assignment: boxing ..........iRef < version.length; // Comparison: unboxing ..........++iRef) { // ++: unboxing and boxing .....out.println(iRef + ": " + version[iRef]); // Array index: unboxing } .....

Output:

0: 5
1: .
2: 0

Boxing and unboxing in collections/maps:

..... String[] words = new String[] {"aaa", "bbb", "ccc", "aaa}; Map< String, Integer > m = new TreeMap< String, Integer >(); for (String word : words) { Integer freq = m.get(word); m.put(word, freq == null ? 1 : freq + 1); } out.println(m); .....

The output:

{aaa=2, bbb=1, ccc=1}

In the next example an int is being stored and then retrieved from an ArrayList. The J2SE 5.0 leaves the conversion required to transition to an Integer and back to the compiler.

Before:

..... ArrayList< Integer > list = new ArrayList< Integer >(); list.add(0, new Integer(42)); int total = (list.get(0)).intValue(); .....

After:

..... ArrayList< Integer > list = new ArrayList< Integer >(); list.add(0, 42); int total = list.get(0); .....

An Integer expression can have a null value. If your program tries to autounbox null, it will throw a NullPointerException. The == operator performs reference identity comparisons on Integer expressions and value equality comparisons on int expressions. Finally, there are performance costs associated with boxing and unboxing, even if it is done automatically.

Here is another sample program featuring autoboxing and unboxing. It is a static factory that takes an int array and returns a List of Integer backed by the array. This method provides the full richness of the List interface atop an int array. All changes to the list write through to the array and vice-versa:

..... // List adapter for primitive int array public static List< Integer > asList(final int[] a) { .....return new AbstractList< Integer >() { ..........public Integer get(int i) { return a[i]; } ..........// Throws NullPointerException if val == null ..........public Integer set(int i, Integer val) { ...............Integer oldVal = a[i]; ...............a[i] = val; ...............return oldVal; ..........} ..........public int size() { return a.length; } .....}; } .....

Wrappers and primitives comparison:

..... public class WrappersTest { .....public static void main(String[] s) { ..........Integer i1 = new Integer(2); ..........Integer i2 = new Integer(2); ..........System.out.println(i1 == i2); // FALSE ..........Integer j1 = 2; ..........Integer j2 = 2; ..........System.out.println(j1 == j2); // TRUE ..........Integer k1 = 150; ..........Integer k2 = 150; ..........System.out.println(k1 == k2); // FALSE ..........Integer jj1 = 127; ..........Integer jj2 = 127; ..........System.out.println(jj1 == jj2); // TRUE ..........int jjj1 = 127; ..........Integer jjj2 = 127; ..........System.out.println(jjj1 == jjj2); // TRUE ..........Integer kk1 = 128; ..........Integer kk2 = 128; ..........System.out.println(kk1 == kk2); // FALSE ..........Integer kkk1 = 128; ..........int kkk2 = 128; ..........System.out.println(kkk1 == kkk2); // TRUE ..........Integer w1 = -128; ..........Integer w2 = -128; ..........System.out.println(w1 == w2); // TRUE ..........Integer m1 = -129; ..........Integer m2 = -129; ..........System.out.println(m1 == m2); // FALSE ..........int mm1 = -129; ..........Integer mm2 = -129; ..........System.out.println(mm1 == mm2); // TRUE .....} } .....

NOTE, certain primitives are always to be boxed into the same immutable wrapper objects. These objects are then CACHED and REUSED, with the expectation that these are commonly used objects. These special values are:

• The boolean values true and false.
• The byte values.
• The short and int values between -128 and 127.
• The char values in the range '\u0000' to '\u007F'.

..... Character c1 = '\u0000'; Character c2 = '\u0000'; System.out.println("c1 == c2 : " + (c1 == c2)); // TRUE !!! Character c11 = '\u00FF'; Character c12 = '\u00FF'; System.out.println("c11 == c12 : " + (c11 == c12)); // FALSE .....

c1 == c2 : true
c11 == c12 : false

The following example gives NullPointerException:

Integer i = null;
int j = i; // java.lang.NullPointerException !!!

This example demonstrates methods resolution when selecting overloaded method:

..... public static void main(String[] args) { .....doSomething(1); .....doSomething(2.0); } public static void doSomething(double num) { .....System.out.println("double : " + num); } public static void doSomething(Integer num) { .....System.out.println("Integer : " + num); } .....

The output is:

double : 1.0
double : 2.0

Java 5.0 will always select the same method that would have been selected in Java 1.4.

The method resolution inludes three passes:

1. Attempt to locate the correct method WITHOUT any boxing, unboxing, or vararg invocations.
2. Attempt to locate the correct method WITH boxing, unboxing, and WITHOUT any vararg invocations. 3. Attempt to locate the correct method WITH boxing, unboxing, or vararg invocations.

String

The String class represents character strings. All string literals in Java programs, such as "abc", are implemented as instances of this class.

All implemented interfaces: Serializable, CharSequence, Comparable< String >.

This method returns the number of characters in the string as in:

int length ()

This example results in variable x holding the value 8:

String str = "A string";
int x = str.length ();

Removes whitespace from the leading and trailing edges of the string:

String trim ()

This results in the variable str referencing the string "14 units":

String string = " 14 units ";
String str = string.trim ();

The following methods return the index, starting from 0, for the location of the given character in the string. (The char value will be widened to int):

int indexOf (int ch)
int lastIndexOf (int ch)

For example:

String string = "One fine day";
int x = string.indexOf ('f');

This results in a value of 4 in the variable x. If the string holds NO such character, the method returns -1.

To continue searching for more instances of the character, you can use the method:

indexOf(int ch, int fromIndex)

This will start the search at the fromIndex location in the string and search to the end of the string.

The methods:

indexOf (String str)
indexOf (String str, int fromIndex)

provide similar functions but search for a sub-string rather than just for a single character.

Similarly, the methods:

lastIndexOf (int ch)
lastIndexOf (int ch, int fromIndex)

lastIndexOf (String str)
lastIndexOf (String str, int fromIndex)

search backwards for characters and strings starting from the right side and moving from right to left. (The fromIndex second parameter still counts from the left, with the search continuing from that index position toward the beginning of the string).

These two methods test whether a string begins or ends with a particular substring:

boolean startsWith (String prefix)
boolean endsWith (String str)

For example:

..... String [] str = {"Abe", "Arthur", "Bob}; for (int i=0; i < str.length (); i++) { .....if (str1.startsWith ("Ar")) doSomething (); } .....

The first method return a new string with all the characters set to lower case while the second returns the characters set to upper case:

String toLowerCase ()
String toUpperCase ()

Example:

..... String [] str = {"Abe", "Arthur", "Bob}; for (int i=0; i < str.length(); i++){ .....if (str1.toLowerCase ().startsWith ("ar")) doSomething (); } .....

StringBuilder

J2SE5.0 added the StringBuilder class, which is a drop-in replacement for StringBuffer in cases where thread safety is not an issue. Because StringBuilder is NOT synchronized, it offers FASTER performance than StringBuffer.

In general, you should use StringBuilder in preference over StringBuffer. In fact, the J2SE 5.0 javac compiler normally uses StringBuilder instead of StringBuffer whenever you perform string concatenation as in:

System.out.println ("The result is " + result);

All the methods available on StringBuffer are also available on StringBuilder, so it really is a drop-in replacement.

Instances of StringBuilder are not safe for use by multiple threads. If such synchronization is required then it is recommended that StringBuffer be used.

StringBuffer

String objects are immutable, meaning that once created they cannot be altered. Concatenating two strings does not modify either string but instead creates a new string object:

String str = "This is";
str = str + " a new string object";

Here str variable now references a completely new object that holds the "This is a new string object" string.

This is not very efficient if you are doing extensive string manipulation with lots of new strings created through this sort of append operations. The String class maintains a pool of strings in memory. String literals are saved there and new strings are added as they are created. Extensive string manipulation with lots of new strings created with the String append operations can therefore result in lots of memory taken up by unneeded strings. Note however, that if two string literals are the same, the second string reference will point to the string already in the pool rather than create a duplicate.

The class java.util.StringBuffer offers more efficient string creation. For example:

StringBuffer strb = new StringBuffer ("This is");
strb.append(" a new string object");
System.out.println (strb.toString());

The StringBuffer uses an internal char array for the intermediate steps so that new strings objects are not created. If it becomes full, the array is copied into a new larger array with the additional space available for more append operations.

The StringBuffer is a thread-safe, mutable sequence of characters. A string buffer is like a String, but can be modified. At any point in time it contains some particular sequence of characters, but the length and content of the sequence can be changed through certain method calls.

String buffers are safe for use by multiple threads. The methods are synchronized where necessary so that all the operations on any particular instance behave as if they occur in some serial order that is consistent with the order of the method calls made by each of the individual threads involved.

The principal operations on a StringBuffer are the append and insert methods, which are overloaded so as to accept data of any type. Each effectively converts a given datum to a string and then appends or inserts the characters of that string to the string buffer. The append method always adds these characters at the end of the buffer; the insert method adds the characters at a specified point.

For example, if z refers to a string buffer object whose current contents are "start", then the method call z.append("le") would cause the string buffer to contain "startle", whereas z.insert(4, "le") would alter the string buffer to contain "starlet".

In general, if sb refers to an instance of a StringBuffer, then sb.append(x) has the same effect as sb.insert(sb.length(), x).

Whenever an operation occurs involving a source sequence (such as appending or inserting from a source sequence) this class synchronizes only on the string buffer performing the operation, not on the source.

Every string buffer has a capacity. As long as the length of the character sequence contained in the string buffer does not exceed the capacity, it is not necessary to allocate a new internal buffer array. If the internal buffer overflows, it is automatically made larger. As of release JDK 5, this class has been supplemented with an equivalent class designed for use by a single thread, StringBuilder. The StringBuilder class should generally be used in preference to this one, as it supports all of the same operations but it is faster, as it performs no synchronization.

StringBuffer class DOES NOT override the equals() method. Therefore, it uses Object class' equals(), which only checks for equality of the object references. StringBuffer.equals() does not return true even if the two StringBuffer objects have the same contents:

..... StringBuffer sb = new StringBuffer("ssss"); StringBuffer sb_2 = new StringBuffer("ssss"); out.println("sb equals sb_2 : " + sb.equals(sb_2)); .....

The output:

sb equals sb_2 : false

NOTE, String's equals() method checks if the argument if of type string, if not it returns false:

..... StringBuffer sb = new StringBuffer("ssss"); String st = new String("ssss"); out.println("sb equals st : " + sb.equals(st)); // Always 'false' out.println("st equals sb : " + st.equals(sb)); // Always 'false' .....

The output:

sb equals st : false
st equals sb : false

______________

Author: Mikalai Zaikin. Please Click Here to visit Authors site for any updates and changes to the study notes.