SPECIAL STRING OPERATIONS INJAVA

SPECIAL STRING OPERATIONS

• Strings are a common and important part of programming, Java has added special support for several string operations within the syntax of the language.
•  These operations include the automatic creation of new String instances from string literals, concatenation of multiple String objects by use of the + operator, and the conversion of other data types to a string representation.
•  There are explicit methods available to perform all of these functions, but Java does them automatically as a convenience for the programmer and to add clarity.

String Literals

• A String instance from an array of characters by using the new operator. However, there is an easier way to do this using a string literal.
• Java automatically constructs a String object. Thus, you can use a string literal to initialize a Stringobject.

Example:

char chars[] = { 'a', 'b', 'c' };

String s1 = new String(chars);

String s2 = "abc"; // use string literal

A String object is created for every string literal, you can use a string literal any place you can use a Stringobject. For example, you can call methods directly on a quoted string as if it were an object reference, as the following statement shows. It calls the length( ) method on the string “abc”.As expected, it prints “3”.

System.out.println("abc".length());

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String Concatenation

• Java does not allow operators to be applied to String objects. The one exception to this rule is the + operator, which concatenates two strings, producing a String object as the result. This allows you to chain together a series of + operations.

Example:

String age = "9";

String s = "He is " + age + " years old.";

System.out.println(s);

Output:

“He is 9 years old.”

One practical use of string concatenation is found when you are creating very long strings. Instead of letting long strings wrap around within your source code, you can break them into smaller pieces, using the + to concatenate them.

Example:

class ConCat {

public static void main(String args[]) {

String longStr = "This could have been " +

"a very long line that would have " +

"wrapped around. But string concatenation " +

"prevents this.";

System.out.println(longStr);

}

}

String Concatenation with Other Data Types

• It is possible to concatenate strings with other types of data.

int age = 9;

String s = "He is " + age + " years old.";

System.out.println(s);

Here age is an int rather than another String, but the output produced is the same as before. This is because the int value in age is automatically converted into its string representation within a String object. This string is then concatenated as before. The compiler will convert an operand to its string equivalent whenever the other operand of the + is an instance of String.

Example:

String s = "four: " + 2 + 2;

System.out.println(s);

Output:

four: 22

rather than the

four: 4

Operator precedence causes the concatenation of “four” with the string equivalent of 2 to take place first. This result is then concatenated with the string equivalent of 2 a second time. To complete the integer addition first, you must use parentheses, like this:

String s = "four: " + (2 + 2);

Now s contains the string “four: 4”.

String Conversion and toString( ) 

• When Java converts data into its string representation during concatenation, it does so by calling one of the overloaded versions of the string conversion method valueOf( ) defined by String. valueOf( ) is overloaded for all the simple types and for type Object.
•  valueOf( ) returns a string that contains the human-readable equivalent of the value with which it is called. For objects, valueOf( ) calls the toString( ) method on the object. 
• Every class implements toString( ) because it is defined by Object. However, the default implementation of toString( ) is seldom sufficient. For most important classes that you create, you will want to override toString( ) and provide your own string representations.

String toString( )

• To implement toString( ), simply return a Stringobject that contains the humanreadable string that appropriately describes an object of your class.
• By overriding toString( ) for classes that you create, you allow them to be fully integrated into Java’s programming environment. For example, they can be used in print( ) and println( ) statements and in concatenation expressions.
• The following program demonstrates this by overriding toString( ) for the Box class:

class Box {

double width;

double height;

double depth;

Box(double w, double h, double d) {

width = w;

height = h;

depth = d;

}

public String toString() {

return "Dimensions are " + width + " by " +

depth + " by " + height + ".";

}

}

class toStringDemo {

public static void main(String args[]) {

Box b = new Box(10, 12, 14);

String s = "Box b: " + b; // concatenate Box object

System.out.println(b); // convert Box to string

System.out.println(s);

}

}

Output:

Dimensions are 10.0 by 14.0 by 12.0

Box b: Dimensions are 10.0 by 14.0 by 12.0

 Box’s toString( ) method is automatically invoked when a Box object is used in a concatenation expression or in a call to println( )

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