Strings, which are widely used in Java programming, are a sequence of characters. In the Java programming language, strings are objects.
The Java platform provides the String class to create and manipulate strings.
Creating Strings:
The most direct way to create a string is to write:
String greeting = “Hello world!”;
Whenever it encounters a string literal in your code, the compiler creates a String object with its value in this case, “Hello world!’.
As with any other object, you can create String objects by using the new keyword and a constructor. The String class has eleven constructors that allow you to provide the initial value of the string using different sources, such as an array of characters.
public class StringDemo{
public static void main(String args[]){
char[] helloArray = { ‘h’, ‘e’, ‘l’, ‘l’, ‘o’, ‘.’};
String helloString = new String(helloArray);
System.out.println( helloString );
}
}
This would produce the following result:
hello.
Note: The String class is immutable, so that once it is created a String object cannot be changed. If there is a necessity to make a lot of modifications to Strings of characters, then you should use String Buffer & String Builder Classes.
String Length:
Methods used to obtain information about an object are known as accessor methods. One accessor method that you can use with strings is the length() method, which returns the number of characters contained in the string object.
After the following two lines of code have been executed, len equals 17:
public class StringDemo {
public static void main(String args[]) {
String palindrome = “Dot saw I was Tod”;
int len = palindrome.length();
System.out.println( “String Length is : ” + len );
}
}
This would produce the following result:
String Length is : 17
Concatenating Strings:
The String class includes a method for concatenating two strings:
string1.concat(string2);
This returns a new string that is string1 with string2 added to it at the end. You can also use the concat() method with string literals, as in:
“My name is “.concat(“Zara”);
Strings are more commonly concatenated with the + operator, as in:
“Hello,” + ” world” + “!”
which results in:
“Hello, world!”
Let us look at the following example:
public class StringDemo {
public static void main(String args[]) {
String string1 = “saw I was “;
System.out.println(“Dot ” + string1 + “Tod”);
}
}
This would produce the following result:
Dot saw I was Tod
Creating Format Strings:
You have printf() and format() methods to print output with formatted numbers. The String class has an equivalent class method, format(), that returns a String object rather than a PrintStream object.
Using String’s static format() method allows you to create a formatted string that you can reuse, as opposed to a one-time print statement. For example, instead of:
System.out.printf(“The value of the float variable is ” +
“%f, while the value of the integer ” +
“variable is %d, and the string ” +
“is %s”, floatVar, intVar, stringVar);
you can write:
String fs;
fs = String.format(“The value of the float variable is ” +
“%f, while the value of the integer ” +
“variable is %d, and the string ” +
“is %s”, floatVar, intVar, stringVar);
System.out.println(fs);
ARRAY
Java provides a data structure, the array, which stores a fixed-size sequential collection of elements of the same type. An array is used to store a collection of data, but it is often more useful to think of an array as a collection of variables of the same type.
Instead of declaring individual variables, such as number0, number1, …, and number99, you declare one array variable such as numbers and use numbers[0], numbers[1], and …, numbers[99] to represent individual variables.
This tutorial introduces how to declare array variables, create arrays, and process arrays using indexed variables.
Declaring Array Variables:
To use an array in a program, you must declare a variable to reference the array, and you must specify the type of array the variable can reference. Here is the syntax for declaring an array variable:
dataType[] arrayRefVar; // preferred way.
or
dataType arrayRefVar[]; // works but not preferred way.
Note: The style dataType[] arrayRefVar is preferred. The style dataType arrayRefVar[] comes from the C/C++ language and was adopted in Java to accommodate C/C++ programmers.
Example:
The following code snippets are examples of this syntax:
double[] myList; // preferred way.
or
double myList[]; // works but not preferred way.
Creating Arrays:
You can create an array by using the new operator with the following syntax:
arrayRefVar = new dataType[arraySize];
The above statement does two things:
It creates an array using new dataType[arraySize];
It assigns the reference of the newly created array to the variable arrayRefVar.
Declaring an array variable, creating an array, and assigning the reference of the array to the variable can be combined in one statement, as shown below:
dataType[] arrayRefVar = new dataType[arraySize];
Alternatively you can create arrays as follows:
dataType[] arrayRefVar = {value0, value1, …, valuek};
The array elements are accessed through the index. Array indices are 0-based; that is, they start from 0 to arrayRefVar.length-1.
Example:
Following statement declares an array variable, myList, creates an array of 10 elements of double type and assigns its reference to myList:
double[] myList = new double[10];
Following picture represents array myList. Here, myList holds ten double values and the indices are from 0 to 9
Processing Arrays:
When processing array elements, we often use either for loop or foreach loop because all of the elements in an array are of the same type and the size of the array is known.
Example:
Here is a complete example of showing how to create, initialize and process arrays:
public class TestArray {
public static void main(String[] args) {
double[] myList = {1.9, 2.9, 3.4, 3.5};
// Print all the array elements
for (int i = 0; i < myList.length; i++) {
System.out.println(myList[i] + ” “);
}
// Summing all elements
double total = 0;
for (int i = 0; i < myList.length; i++) {
total += myList[i];
}
System.out.println(“Total is ” + total);
// Finding the largest element
double max = myList[0];
for (int i = 1; i < myList.length; i++) { if (myList[i] > max) max = myList[i];
}
System.out.println(“Max is ” + max);
}
}
This would produce the following result:
1.9
2.9
3.4
3.5
Total is 11.7
Max is 3.5
The foreach Loops:
JDK 1.5 introduced a new for loop known as foreach loop or enhanced for loop, which enables you to traverse the complete array sequentially without using an index variable.
Example:
The following code displays all the elements in the array myList:
public class TestArray {
public static void main(String[] args) {
double[] myList = {1.9, 2.9, 3.4, 3.5};
// Print all the array elements
for (double element: myList) {
System.out.println(element);
}
}
}
This would produce the following result:
1.9
2.9
3.4
3.5
Passing Arrays to Methods:
Just as you can pass primitive type values to methods, you can also pass arrays to methods. For example, the following method displays the elements in an int array:
public static void printArray(int[] array) {
for (int i = 0; i < array.length; i++) {
System.out.print(array[i] + ” “);
}
}
You can invoke it by passing an array. For example, the following statement invokes the printArray method to display 3, 1, 2, 6, 4, and 2:
printArray(new int[]{3, 1, 2, 6, 4, 2});
Returning an Array from a Method:
A method may also return an array. For example, the method shown below returns an array that is the reversal of another array:
public static int[] reverse(int[] list) {
int[] result = new int[list.length];
for (int i = 0, j = result.length – 1; i < list.length; i++, j–) {
result[j] = list[i];
}
return result;
}
Date Function and use
Getting Current Date & Time:
This is very easy to get current date and time in Java. You can use a simple Date object with toString() method to print current date and time as follows:
import java.util.Date;
public class DateDemo {
public static void main(String args[]) {
// Instantiate a Date object
Date date = new Date();
// display time and date using toString()
System.out.println(date.toString());
}
}
This would produce the following result:
Mon May 04 09:51:52 CDT 2009
Date Comparison:
There are following three ways to compare two dates:
You can use getTime( ) to obtain the number of milliseconds that have elapsed since midnight, January 1, 1970, for both objects and then compare these two values.
You can use the methods before( ), after( ), and equals( ). Because the 12th of the month comes before the 18th, for example, new Date(99, 2, 12).before(new Date (99, 2, 18)) returns true.
You can use the compareTo( ) method, which is defined by the Comparable interface and implemented by Date.
Date Formatting using SimpleDateFormat:
SimpleDateFormat is a concrete class for formatting and parsing dates in a locale-sensitive manner. SimpleDateFormat allows you to start by choosing any user-defined patterns for date-time formatting. For example:
import java.util.*;
import java.text.*;
public class DateDemo {
public static void main(String args[]) {
Date dNow = new Date( );
SimpleDateFormat ft =
new SimpleDateFormat (“E yyyy.MM.dd ‘at’ hh:mm:ss a zzz”);
System.out.println(“Current Date: ” + ft.format(dNow));
}
}
This would produce the following result:
Current Date: Sun 2004.07.18 at 04:14:09 PM PDT
eramarswami.com 