Review:   Java class

  • Review:

    • A class in Java is used to:

      • Group related variables and methods together

Review:   Java class

  • Review:

    • A class in Java is used to:

      • Group related variables and methods together

  • Schematic representation of a class:

Java String class

  • String is a class in the Java library that implement strings

    • A string consists of a sequence of characters   →   the variables

    • A string can be manipulated in certain ways   →   the methods

  • Schematic representation of the String class:

  • In this chapter, we learn to how to use the String class in the Java library

Defining and using String typed variables

  • Syntax to define a String typed variable: 

     Method 1: uninitialized

           String varName;

           // Later, you can assign a string
	   // to this variable with:
	   //
	   //       varName = "Some string...";

 Method 2: initialized
 
           String varName = "initial string...";

    Example: 

      String var1;

  String var2 = "abcd";

DEMO: 04-Math+String/04-string/DefineStringVar.java

Reference data types and reference variables

  • Reference data type:

      • All Java classes are reference data types (and vice versa)

  • Since String is a Java class:

    • The String data type is not a primitive data type

    • The String data type is a reference data type

  • Reference variables:

      • Reference variables = a variable defined using a class name

    Example: 

      String  var1;  // var1 is a reference variable

  • A reference variable always stores a reference (= address) of an "object" in Java (next)

What happens when you define and assign to a String typed variable

  • Computer code that creates a String typed variable and then assigns to it: 

      String  var1;

  var1 = "ABC";

    What happens inside the computer when this computer code is executed ?
     

     

What happens when you define and assign to a String typed variable

  • Computer code that creates a String typed variable and then assigns to it: 

      String  var1;

  var1 = "ABC";

    When the " String var1" is executed, the computer will create the variable var1:
     

    The variable var1 will contain the default value null (which means: illegal reference)

What happens when you define and assign to a String typed variable

  • Computer code that creates a String typed variable and then assigns to it: 

      String  var1;

  var1 = "ABC";

    When the assignment " var1 = "ABC" is executed, the computer will (1) create a String object and (2) assign its address (= reference) to variable var1:

    We can now use the address in the variable var1 to find the string (stored in memory)

What happens when you define and assign to a String typed variable

  • Computer code that creates a String typed variable and then assigns to it: 

      String  var1;

  var1 = "ABC";

    We typically use an arrow to represent the fact that a (reference) variable stores the address of an object as follows:

    The arrow indicates that we will use the object pointed to by the reference variable

DEMO: 04-Math+String/04-string/DefineStringVar.java --- click on ObjRef icon and show content

Reference data types and objects     Summary

  

 (1) String    is  a class or a reference data type in Java

 (2) var1      is a reference variable
               always contains a reference (= address) of an object

 (3) "ABC"     is a (String typed) object

 (4) var1      references (points to) the "ABC" String object

How to use String objects: instance methods

  • Recall that classes contains methods that can manipulate the data stored in the class:

  • The String class in the Java library contains many methods that manipulate the string:

Sample methods inside the String class

  • Sample methods defined inside the String class that manipulate a String object: 

       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

 (Many more, see: )

How to apply methods inside the String class on String objects

  • Important fact:

    • A class in Java is used to:

      • Create "object" with certain properties         and also

      • Provide a set of methods to manipulate the "object"

  • How to apply a method inside the String class (on a String object): 

       String var1;  // (1) Define a String variable 
               //     (to reference to a String object)

 var1 = "abc"; // (2) Create a String object
               //     (we can use var1 to refer to this String object)

 var1.length() // (3) Applies the length() method on the String object
                      that var1 is referencing

Example applying some methods on String objects

  • In this program, we create 2 String objects and apply some methods on each of them: 

    public class ExampleString 
{
   public static void main(String[] args) 
   {
       String s1 = "Hello 123";  // Create the first string
       String s2 = " Good-bye "; // Create the second string

       int    x;
       char   c;
       String result;

       x = s1.length();  // the length of string s1
       x = s2.length();  // the length of string s2
       c = s1.charAt(1); // the char at pos 1 in string s1
       c = s2.charAt(1); // the char at pos 1 in string s2
       result = s1.toUpperCase(); // string s1 in uppercase
       result = s2.toUpperCase(); // string s2 in uppercase
       result = s1.concat(s2); // String s1 concatenated with string s2
       result = s2.concat(s1); // String s2 concatenated with string s1
       result = s2.trim(); // remove spaces before and after string s2
   }
}

DEMO: 04-Math+String/04-string/UseString.java