Java String class

A string is a sequence of characters
String is a class in the Java library that represents strings
We will discuss:
In this chapter, we discuss how to use the Java String class

Defining and using String typed variables

Syntax to define a String typed variable:

String message = "Hello"; String text; text = "Welcome to Java";

The String data type is not a primitive data type
String is known as a reference data type
Let's study what reference data types are next

Reference data types and objects

Reference data type:
Note: we have not yet learn how to creat classes used to define variables yet !

Reference variables:

Variables defined with a reference data type is known as a reference variable.
Reference variable can reference objects of the corresponding data type

Reference data types and objects Illustrated

String is a reference data type message is a reference variable "Hello" is a (String typed) object message references (points to) the "Hello" object

How to use String variables

Reference data types (= classes) will be discussed later in detail in Chapter 9 ("Objects and Classes").
For the time being, you need to know only:

How to use String variables

Reference data types (= classes) can define methods that can be used to operate on the objects

Sample methods defined inside the String class:

Method Description --------------------------------------------------------------- length() Returns the number of characters in this string charAt(index) Returns the character at the specified index from this string concat(s1) Returns a new string that concatenates this string with string s1 toUpperCase() Returns a new string with all letters in uppercase toLowerCase() Returns a new string with all letters in lowercase trim() Returns a new string with whitespace characters trimmed on both sides

How to use String variables Example

Example program showing how to use the methods in the String class:

public class MyProg { public static void main(String[] args) { String s1 = "Hello"; // s1 is reference variable int len = s1.length(); // Return the length of s1 char c = s1.charAt(0); // "Hello" c = s1.charAt(1); // 01234 s1 = s1.concat("Abc"); s1 = s1 + "Xyz"; // + ALSO concatenates ! s1 = s1.toUpperCase(); s1 = s1.toLowerCase(); String s2 = " ABC "; s1 = s2.trim(); } }

Java's methods instance and static methods

There are 2 kinds of methods in Java:

Instance methods:

String s1; // s1 references to an object s1.toUpperCase(); // Instance methods works on an object

Static methods:

Math.pow(5,2); // Static methods do not works on objects ^^^^ Class name

Objects and instance methods

Strings are objects in Java
Methods that are invoked (used) on objects are called:

Syntax to invoke an instance method on an object is:

referenceVar.instanceMethodName( arguments ) Example: s1.concat("Abc");

Static methods (a.k.a. class methods)

A non-instance method is called a static method or a class method
A static method is invoked without using an object

Syntax to invoke a static method is:

ClassName.staticMethodName( arguments ) Example: Math.pow(2,3);

All methods inside the Math class are static methods !
The main( ) method of a Java program is always a static method !

Mixing other data types with String data

Java allows values from other type of values to be concatenated to a String:

public class MyProg { public static void main(String[] args) { String s = "Hello"; s = s + 3.14; // s = "Hello3.14" s = s + 999; // s = "Hello3.14999" s = s + 'A'; // s = "Hello3.14999A" s = s + (3 < 4); // s = "Hello3.14999Atrue" s = 777 + s ; // s = "777Hello3.14999Atrue" } }

Quiz: What is assigned in the variable s ?

public class MyProg { public static void main(String[] args) { String s; s = "A" + 1; s = "1" + 1; s = "1" + 1 + 1; // Careful: + is left associative ! s = 1 + "1"; s = 1 + 1 + "1"; // Careful: + is left associative ! } }

An easier way to print messages with variables

Instead of using multiple print statements:

double radius = 20, area; area = 3.14159 * radius * radius; System.out.print("Area of circle with radius "); System.out.print(radius); System.out.print(" is "); System.out.println(area);

We can use 1 print statement with concatenation:

double radius = 20, area; area = 3.14159 * radius * radius; System.out.println("Area of circle with radius " + radius + " is " + area);

Reading in words from console

The next() method in a Scanner object reads in a string (word) that ends with a whitespace

Example:

import java.util.Scanner; public class MyProg { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter three words separated by spaces: "); String s1 = input.next(); String s2 = input.next(); String s3 = input.next(); System.out.println("s1 is " + s1); System.out.println("s2 is " + s2); System.out.println("s3 is " + s3);

Reading in lines from console

The nextLine() method in a Scanner object reads in one line that ends with a NEWLINE (RETURN)

Example:

import java.util.Scanner; public class MyProg { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a line: "); String s1 = input.nextLine(); System.out.println("s1 is " + s1); } }

Warning: do not mix next( ) and nextLine( )

You may get unexpected results:

import java.util.Scanner; public class MyProg { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter at least 1 word separated by spaces: "); String s1 = input.next(); // Read 1st word String s2 = input.nextLine(); // Read rest of the line (2 words) ! System.out.println("s1 is " + s1); System.out.println("s2 is " + s2); } }

Reading in a character from console

Java does not have methods that reads in a character

Instead: (1) use the next() method and (2) extract the character at index 0 with charAt(0)

import java.util.Scanner; public class MyProg { public static void main(String[] args) { Scanner input = new Scanner(System.in); System.out.print("Enter a character: "); String s1 = input.next(); // (1) read in a word char c = s1.charAt(0); // (2) extract character System.out.println("c is " + c); } }