Lecture 7


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Notes on core java

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Lecture 7

  1. 1. Core Java Debasish Pratihari String:  Strings are sequence of character  In java strings are object of String,StringBuffer or StringBuilder class.  Strings in Java are 16-bit Unicode.  The Java language provides special support for the string concatenation operator ( + ).  All String representation classes of java are final Type of String :  Immutable     Mutable   They are read-only, once created can’t be changed The String class implements immutable character Strings. All string literals in Java programs are immutable strings Are dynamic character string. i.e they can be changed unlike mutable string. StringBuffer and StringBuilder class in java implements Mutable String. Example String :          System.out.println("Lakshya"); String place = "Bhubaneswar"; System.out.println("Lakshya " + place); String c = "Lakshya".substring(3,6); String d = place.substring(3, 6); String name= new String(“Debasish”); String s1 = new String(); String s3 = new String( name ); char lan[] = { 'j', 'a', 'v',’a’ }; String s4 = new String( lan ); Lecture/core/String/07 Page #1 feel the Technology…
  2. 2. Core Java Debasish Pratihari String Literal Pool :  String allocation, like all object allocation, proves costly in both time and memory. The JVM performs some trickery while instantiating string literals to increase performance and decrease memory overhead. To cut down the number of String objects created in the JVM, the String class keeps a pool of strings.  Each time your code create a string literal, the JVM checks the string literal pool first. If the string already exists in the pool, a reference to the pooled instance returns. If the string does not exist in the pool, a new String object instantiates, then is placed in the pool. Java can make this optimization since strings are immutable and can be shared without fear of data corruption. String concatenation:  Java Strings can be concatenated (joined) using the + and += operators to create new Strings.  Every time an operation modifies a String object, a new read-only String object is created. String language = "Java"; String course = "Introduction to " + language; course += ", @ Lakshya"; Comparing String : Java provides a variety of methods to compare String objects.  public int compareTo( String str )  public int compareToIgnoreCase( String str )  public boolean equals( Object obj )  public boolean equalsIgnoreCase( String str )  Use of the == operator Lecture/core/String/07 Page #2 feel the Technology…
  3. 3. Core Java Debasish Pratihari Useful String methods :         public public public public public public public public char charAt( int index ) String concat( String str ) int length() String toUpperCase() String toLowerCase() String trim() String substring(int beginIndex) static String valueOf( xxx b ) String representation of Objects :  All Java objects implement the method toString() which returns a String object that best "describes" that object.  you do not even need to explicitly call toString(). The println() method has a version that takes an Object as its argument. This version will implicitly call toString() for you and print the result. Color color = new Color( 255, 0, 0 ); String colorStr = color.toString(); System.out.println( colorStr ); This will printout: java.awt.Color[r=255,g=0,b=0] String Interning:     It ensures that no two objects encapsulates the same string. This means that if two string variables reference strings that are identical, the reference must be identical. The intern() method will check a string reference against all the String objects currently in existence. If it already exists, the current object will discarded, and reference to the existing object will be given. Lecture/core/String/07 Page #3 feel the Technology…
  4. 4. Core Java Debasish Pratihari Creating Mutable String :  StringBuffer str= new StringBuffer(“Exlore java @ Lakshya”);  StringBuffer duplicate= new StringBuffer(str);  StringBuffer str= new StringBuffer(20); Manipulating StringBuffer Object: 25%     Setting a Length o Str.setLength(30); Adding to existing String o str.apppend(“new string”); Finding Position of a substring o str.lastIndexOf(“new”); Replacing substring o str.replace(0,4, “java”); Things to Remember:  A String object stores a fixed character string that can’t be changed.  StringBuffer and StringBuilder objects can store a string of character that you can modify.  Number of character present in a string can be found using length().  A String object can be created from StringBuffer by using toString(). Lecture/core/String/07 Page #4 feel the Technology…