Java Unit 3 (2025) PDF

Unit 3 – Basic Concepts of Strings and Exceptions 3.1 Strings 3.1.1 Basic String operations, String Comparsion 3.1.2 String methods : (charAt(), concat(), equals(), indexOf(), isEmpty(), join(), lastIndexOf(), length(),split(), substring(),trim()) 3.1.3 StringBuffer class and its con...

Unit 3 – Basic Concepts of Strings and Exceptions 3.1 Strings 3.1.1 Basic String operations, String Comparsion 3.1.2 String methods : (charAt(), concat(), equals(), indexOf(), isEmpty(), join(), lastIndexOf(), length(),split(), substring(),trim()) 3.1.3 StringBuffer class and its constructors. 3.1.4 StringBuffer methods : (append(),insert(), delete(), reverse(),capacity()) 3.2 Introduction to Exceptions: 3.2.1 Exception Types, User defined Exception 3.2.2 Throw, Throws 3.2.3 Try, Catch and Finally String class. (Ch : 8 → P – 200) Strings are a sequence of characters. Java implements strings as objects of type string. When you create a String object, you are creating a string that can not be changed. Here, each time you need an altered version of an existing string, a new string object is created that contains the modifications. The original string is left unchanged. Using this approach fixed, immutable string object is implemented. The most direct way to create a string is to write: String greeting = “Hello World!”; There are different other ways to create string object. For that different constructors are available in the String class. o To create empty string i.e. string with no characters: String s=new String( ); o To create a string with range of characters: char c[ ]={‘a’,’b’,’c’,’d’,’e’,’f’}; String s=new String(c); → abcdef o To create a string with range of characters: char c[ ]={‘a’,’b’,’c’,’d’,’e’,’f’}; String s=new String(c,2,3); → cde o To create a string by using another string object: String s1=”Hello”; String s2=new String(s1); → Hello Dr. Darshna Rajput Page 1  Unit 3 – Basic Concepts of Strings and Exceptions o To create a string with array of bytes: byte b[ ]={65,66,67,68,69,70}; String s=new String(b); → abcdef o To create a string with range of byte array: byte b[ ]={65,66,67,68,69,70}; String s=new String(b,2,3); → cde CharSequence. CharSequence is an interface that represents a sequence of characters. Mutability is not enforced by this interface. Therefore, both mutable and immutable classes implement this interface. Of course, an interface can't be instantiated directly; it needs an implementation to instantiate a variable: CharSequence rcf = "Hello"; Here, rcf is instantiated with a String. Instantiating other implementations: 1) CharSequence c1 = new StringBuffer("Hello"); OR StringBuffer sb=new StringBuffer("Hello"); CharSequence c1=sb; 2) CharSequence c2 = new StringBuilder("Hello"); Methods of String class. (Ch : 8 → P – 203) SN Methods with Description Example 1 int length( ) String s=”Hello”; int l; Returns the length of this string. l=s.length( ); →5 2 char charAt(int index) String s1=”Hello”; Returns the character at the char ch; specified index. ch=s1.charAt(1); → e Dr. Darshna Rajput Page 2  Unit 3 – Basic Concepts of Strings and Exceptions 3 String concat(String str) String s1=”Darshna “; String s2=”Rajput”; Concatenates the specified string to String s3; the end of this string. s3=s1.concat(s2); → Darshna Rajput 4 boolean equals(String str) String s1=”ABC”; String s2=”abc”; Compares this string to another boolean b; string object. It considers the case of b=s1.equals(s2); → false strings. 5 int indexOf(int ch/char ch/ String s1=”Kabhi Khushi Kabhi Gam”; String str) System.out.println(s1.indexOf(‘K’)); → 0 Returns the index within this string System.out.println(s1.indexOf(97)) ; →1 of the first occurrence of the System.out.println(s1.indexOf(“Khushi”));→6 specified character. 6 int indexOf(int ch/char String s1=”Kabhi Khushi Kabhi Gam”; ch/String str, int startIndex) System.out.println(s1.indexOf(‘K’,5)); → 6 Returns the index within this string System.out.println(s1.indexOf(97,5)) ;→ 14 of the first occurrence of the System.out.println(s1.indexOf(“Kabhi”,5)); specified character, starting the → 13 search at the specified index. 7 int lastIndexOf(int ch/char ch/ String s1=”Kabhi Khushi Kabhi Gam”; String str) System.out.println(s1.lastIndexOf(‘K’)); → 13 Returns the index within this string System.out.println(s1.lastIndexOf(97));→ 20 of the last occurrence of the System.out.println(s1.lastIndexOf(“Khushi”)); specified character. → 6 8 int lastIndexOf(int ch/char ch/ String s1=”Kabhi Khushi Kabhi Gam”; String str, int startIndex) System.out.println(s1.lastIndexOf(‘K’,10)); →6 Dr. Darshna Rajput Page 3  Unit 3 – Basic Concepts of Strings and Exceptions Returns the index within this string System.out.println(s1.lastIndexOf(97,15)); of the last occurrence of the →14 specified character, searching System.out.println(s1.lastIndexOf(“Kabhi”,5); backward starting at the specified → 0 index. 9 String substring(int beginIndex) String s1=”This is a Test”; System.out.println(s1.substring(10)); → Test Returns a new string that is a substring of this string. 10 String substring(int beginIndex, String s1=”This is a Test”; int endIndex) System.out.println(s1.substring(5,9)); → is a Returns a new string that is a substring of this string. 11 String trim( ) String s1=” Hello “; Returns a copy of the string, with System.out.println(s1.trim( )); → Hello leading and trailing whitespace omitted. 12 boolean isEmpty() // non-empty string String s1 = "Hello"; Checks whether a String is empty or // empty string not. This method returns true if the String s2 = ""; given string is empty, else it returns false. System.out.println(s1.isEmpty()); → false System.out.println(s2.isEmpty()); → true The isEmpty() method of String class is included in java string since JDK 1.6. Dr. Darshna Rajput Page 4  Unit 3 – Basic Concepts of Strings and Exceptions 13 static String join String s1=String.join (CharSequence delimiter, ("-","Welcome","to","DUIAS"); CharSequence... elements) System.out.println(s1); Returns a string joined with a given ➔ Welcome-to-DUIAS delimiter. In the String join() method, the delimiter is copied for each element. The join() method is included in the Java string since JDK 1.8. 14 String[] split(String sep) String s1="I Love India"; String words[]=s1.split(" "); Splits this string against given //splits the string based on whitespace regular expression and returns a for(int i=0;i