Strings in Java

Core Java Training
Handling Strings in Java

Classification: Restricted
Agenda
• Memory Allocation & Garbage Collection
• Strings in Java

Memory Allocation & Garbage
Collection

Memory Allocation in Java and Garbage Collection
• Java Heap Space
• Used by Java runtime to allocate memory to Objects and JRE classes.
• Any new Object is always created in Heap Space.
• Garbage Collection runs on the heap memory to free the memory used by objects that
doesn’t have any reference.
• All instance and class variables are also stored in the heap.
• Java Stack Memory
• Used for execution of a thread.
• Store method specific values, and “references” to Objects being used in the method.
• Stack memory is LIFO (Last-In-First-Out)
• Whenever a method is invoked, a new block is created in the stack memory for the
method to hold local primitive values and reference to other objects in the method. As
soon as method ends, the block becomes unused and become available for next method.
• Stack memory size is very less compared to Heap memory.

Memory Allocation in Java and Garbage Collection

Creating String in Java
There are two ways to create a String in Java
• String literal
• Using “new” keyword
Does it make any difference? Well, yes!

String Pool Concept in Java (String Interning)
 String Intern Pool maintained in Java Heap Space

String Pool Concept in Java (String Interning)
• String is immutable in Java
• All Strings are stored in String Pool (also called String Intern Pool) allocated
within Java Heap Space
• It is implementation of String Interning Concept.
• String interning is a method of storing only one copy of each distinct string
value, which must be immutable.
• Interning strings makes some string processing tasks more time- or space-
efficient at the cost of requiring more time when the string is created or
interned.
• The distinct values are stored in a string intern pool.
• String pool is an example of Flyweight Design Pattern.
• Using new operator, we force String class to create a new String object in
heap space.

Discussion: How many Strings are getting created here?

java.lang.String API – Important methods

java.lang.String API – Examples

Converting String to numbers and vice versa
• String to Number
int i = Integer.parseInt(str);
Integer i = Integer.valueOf(str);
double d = Double.parseDouble(str);
Double d = Double.valueOf(str);
Note: Both throw NumberFormatException If the String is not valid for conversion
• String to Boolean
boolean b = Boolean.parseBoolean(str);
• Any Type to String
String s = String.valueOf(value);

String vs StringBuffer/StringBuilder
Prefer StringBuffer or StringBuilder when performing multiple
concatenations in your code.

Exercise: What is the output?
public static String someMethod( String str) //”Hello “
{
if( str == null)
return null;
int len = str.length();
for( ; len > 0; len--)
{
if( ! Character.isWhitespace( str.charAt( len - 1)))
break;
}
return str.substring( 0, len);
}

Exercise
• Write a program to input 2 strings – string1 and string2, and concatenate
the first 5 characters of string1 with last 5 characters of string2.

Exercise
• Write a program to input 3 strings and check if at least any two strings are
equal.

Exercise
• Write a program to sort an array of Strings in ascending order.

Topics to be covered in next session
• Practice session
• Object Oriented Programming (OOPs) Concepts

Thank You!

Strings in Java

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