Arrays in Java

Arrays in Java are a fundamental data structure used to store multiple values of the same type in a single variable. They provide a fixed-size, ordered collection of elements and are an essential part of Java programming. This guide covers everything you need to know about arrays in Java.

1. Declaring and Initializing Arrays

Arrays can be declared and initialized in various ways.

• Declaration:

  int[] numbers;
  String[] names;

• Initialization:

  numbers = new int[5]; // an array of 5 integers
  names = new String[3]; // an array of 3 strings

• Combined Declaration and Initialization:

  int[] numbers = new int[5];
  String[] names = new String[3];

• Static Initialization:

  int[] numbers = {1, 2, 3, 4, 5};
  String[] names = {"Alice", "Bob", "Charlie"};

2. Accessing Array Elements

Array elements can be accessed using their index. The index is zero-based, meaning it starts from 0.

int firstNumber = numbers[0];
String firstName = names[0];

3. Modifying Array Elements

You can modify array elements by assigning new values to specific indices.

numbers[0] = 10;
names[0] = "David";

4. Array Length

The length of an array can be obtained using the length property.

int arrayLength = numbers.length;

5. Iterating Over Arrays

You can iterate over arrays using loops.

• For Loop:

  for (int i = 0; i < numbers.length; i++) {
      System.out.println(numbers[i]);
  }

• Enhanced For Loop:

  for (int number : numbers) {
      System.out.println(number);
  }

6. Multidimensional Arrays

Java supports multidimensional arrays (arrays of arrays).

• Declaration and Initialization:

  int[][] matrix = new int[3][3];
  int[][] predefinedMatrix = {
      {1, 2, 3},
      {4, 5, 6},
      {7, 8, 9}
  };

• Accessing and Modifying Elements:

  int element = matrix[0][0];
  matrix[0][0] = 10;

7. Array Methods

Java provides utility methods for arrays in the java.util.Arrays class.

• Sorting:

  Arrays.sort(numbers);

• Binary Search:

  int index = Arrays.binarySearch(numbers, 3);

• Copying:

  int[] copiedArray = Arrays.copyOf(numbers, numbers.length);

• Filling:

  Arrays.fill(numbers, 0);

• Comparing:

  boolean areEqual = Arrays.equals(numbers, copiedArray);

• String Representation:

  String arrayString = Arrays.toString(numbers);

8. Common Pitfalls

• Index Out of Bounds: Accessing an index outside the array's bounds throws an ArrayIndexOutOfBoundsException.

  int invalidElement = numbers[10]; // Throws ArrayIndexOutOfBoundsException

• Null Elements: An array of objects can contain null elements.

  String[] strings = new String[5];
  strings[0] = null; // Valid

• Fixed Size: Arrays have a fixed size; once initialized, their size cannot be changed. Use ArrayList for a dynamic array.

Strings in Java

Strings are an essential part of Java programming. The String class in Java is used to create and manipulate strings. This guide covers everything you need to know about strings in Java.

1. Creating Strings

Strings can be created in several ways:

• Using String Literals:

  String str1 = "Hello, World!";

• Using the new Keyword:

  String str2 = new String("Hello, World!");

• From a char Array:

  char[] chars = {'H', 'e', 'l', 'l', 'o'};
  String str3 = new String(chars);

2. String Immutability

Strings in Java are immutable, meaning once a String object is created, it cannot be changed. Any modification results in the creation of a new String object.

String original = "Hello";
String modified = original.concat(", World!");
System.out.println(original); // Outputs: Hello
System.out.println(modified); // Outputs: Hello, World!

3. Common String Methods

The String class provides many methods for string manipulation and analysis.

• Length of a String:

  int length = str1.length();

• Character at a Specific Index:

  char ch = str1.charAt(0);

• Substring:

  String substr = str1.substring(7, 12); // Outputs: World

• Concatenation:

  String str5 = str1.concat(" How are you?");

• Index of a Character or Substring:

  int index = str1.indexOf('W'); // Outputs: 7

• Comparison:

  boolean equals = str1.equals("Hello, World!"); // true
  boolean equalsIgnoreCase = str1.equalsIgnoreCase("hello, world!"); // true
  int compareTo = str1.compareTo("Hello, World!"); // 0 (lexicographically equal)

• Replacing Characters or Substrings:

  String replaced = str1.replace('l', 'p'); // Heppo, Worpd!
  String replacedSubstring = str1.replace("World", "Java"); // Hello, Java!

• Splitting a String:

  String[] parts = str1.split(", ");

• Trimming Whitespace:

  String trimmed = str1.trim();

• Converting to Upper/Lower Case:

  String upperCase = str1.toUpperCase();
  String lowerCase = str1.toLowerCase();

4. String Pool

Java optimizes memory usage of String objects by storing them in a special area called the string pool. When you create a string literal, the JVM checks the string pool to see if it already exists. If it does, it returns the reference to the existing string. Otherwise, it creates a new string and adds it to the pool.

String s1 = "Hello";
String s2 = "Hello";
boolean isSame = (s1 == s2); // true, both reference the same object in the string pool

String Formatting in Java

String formatting is an essential aspect of Java programming, allowing you to create well-structured and readable text. Java provides several ways to format strings, including String.format(), System.out.printf(), and simple string concatenation.

1. String.format() Method

The String.format() method allows you to create formatted strings using various format specifiers, similar to printf in C/C++.

Basic Syntax:

String formattedString = String.format(format, arguments);

Format Specifiers:

• %s: String
• %d: Decimal integer
• %f: Floating-point number
• %x: Hexadecimal integer
• %o: Octal integer
• %c: Character
• %%: Percent sign

Example:

int age = 25;
String name = "Alice";
double score = 95.5;

String formattedString = String.format("Name: %s, Age: %d, Score: %.2f", name, age, score);
System.out.println(formattedString); // Outputs: Name: Alice, Age: 25, Score: 95.50

Flags and Width:

• -: Left-justify
• +: Include a sign (+ or -)
• 0: Pad with zeros
• ,: Include grouping separators
• (width): Minimum number of characters to be printed

Example with Flags:

int number = 12345;
String formattedNumber = String.format("%,d", number);
System.out.println(formattedNumber); // Outputs: 12,345

double price = 123.456;
String formattedPrice = String.format("%010.2f", price);
System.out.println(formattedPrice); // Outputs: 0000123.46

2. System.out.printf() Method

The System.out.printf() method prints formatted output directly to the console. It uses the same format specifiers as String.format().

Example:

String name = "Alice";
int age = 25;
double score = 95.5;

System.out.printf("Name: %s, Age: %d, Score: %.2f%n", name, age, score);
// Outputs: Name: Alice, Age: 25, Score: 95.50

3. String Concatenation with +

For basic string formatting needs, simple string concatenation using the + operator can be used. Although it lacks the flexibility of String.format() and System.out.printf(), it is straightforward for simple tasks.

Example:

String name = "Alice";
int age = 25;
double score = 95.5;

String concatenatedString = "Name: " + name + ", Age: " + age + ", Score: " + score;
System.out.println(concatenatedString); // Outputs: Name: Alice, Age: 25, Score: 95.5

String Methods in Java

Character Retrieval

charAt(int index)

• Returns the character at the specified index.
• Syntax: str.charAt(index)

String s = "hello";
System.out.println(s.charAt(1));  // Output: 'e'

Comparison

compareTo(String anotherString)

• Compares two strings lexicographically.
• Syntax: str.compareTo(anotherString)

String s1 = "hello";
String s2 = "world";
System.out.println(s1.compareTo(s2));  // Output: -15

compareToIgnoreCase(String str)

• Compares two strings lexicographically, ignoring case differences.
• Syntax: str.compareToIgnoreCase(str)

String s1 = "hello";
String s2 = "HELLO";
System.out.println(s1.compareToIgnoreCase(s2));  // Output: 0

Concatenation

concat(String str)

• Concatenates the specified string to the end of this string.
• Syntax: str.concat(str)

String s1 = "hello";
String s2 = "world";
System.out.println(s1.concat(" ").concat(s2));  // Output: "hello world"

Search

contains(CharSequence s)

• Returns true if and only if this string contains the specified sequence of char values.
• Syntax: str.contains(s)

String s = "hello world";
System.out.println(s.contains("world"));  // Output: true

endsWith(String suffix)

• Tests if this string ends with the specified suffix.
• Syntax: str.endsWith(suffix)

String s = "hello";
System.out.println(s.endsWith("lo"));  // Output: true

indexOf(int ch)

• Returns the index within this string of the first occurrence of the specified character.
• Syntax: str.indexOf(ch)

String s = "hello";
System.out.println(s.indexOf('e'));  // Output: 1

indexOf(String str)

• Returns the index within this string of the first occurrence of the specified substring.
• Syntax: str.indexOf(str)

String s = "hello";
System.out.println(s.indexOf("ll"));  // Output: 2

indexOf(int ch, int fromIndex)

• Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.
• Syntax: str.indexOf(ch, fromIndex)

String s = "hello";
System.out.println(s.indexOf('l', 3));  // Output: 3

indexOf(String str, int fromIndex)

• Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.
• Syntax: str.indexOf(str, fromIndex)

String s = "hello";
System.out.println(s.indexOf("l", 3));  // Output: 3

lastIndexOf(int ch)

• Returns the index within this string of the last occurrence of the specified character.
• Syntax: str.lastIndexOf(ch)

String s = "hello";
System.out.println(s.lastIndexOf('l'));  // Output: 3

lastIndexOf(String str)

• Returns the index within this string of the last occurrence of the specified substring.
• Syntax: str.lastIndexOf(str)

String s = "hello";
System.out.println(s.lastIndexOf("l"));  // Output: 3

Equality

equals(Object anObject)

• Compares this string to the specified object.
• Syntax: str.equals(anObject)

String s1 = "hello";
String s2 = "hello";
System.out.println(s1.equals(s2));  // Output: true

equalsIgnoreCase(String anotherString)

• Compares this string to another string, ignoring case considerations.
• Syntax: str.equalsIgnoreCase(anotherString)

String s1 = "hello";
String s2 = "HELLO";
System.out.println(s1.equalsIgnoreCase(s2));  // Output: true

Length

length()

• Returns the length of this string.
• Syntax: str.length()

String s = "hello";
System.out.println(s.length());  // Output: 5

Replacement

replace(char oldChar, char newChar)

• Returns a string resulting from replacing all occurrences of oldChar in this string with newChar.
• Syntax: str.replace(oldChar, newChar)

String s = "hello";
System.out.println(s.replace('l', 'p'));  // Output: "heppo"

Splitting and Joining

split(String regex)

• Splits this string around matches of the given regular expression.
• Syntax: str.split(regex)

String s = "hello world";
String[] parts = s.split(" ");
for (String part : parts) {
    System.out.println(part);  // Outputs: "hello" and "world"
}

join(CharSequence delimiter, CharSequence... elements)

• Returns a new string composed of copies of the CharSequence elements joined together with a copy of the specified delimiter.
• Syntax: String.join(delimiter, elements)

String joinedString = String.join(", ", "one", "two", "three");
System.out.println(joinedString);  // Output: "one, two, three"

Substring

substring(int beginIndex)

• Returns a new string that is a substring of this string.
• Syntax: str.substring(beginIndex)

String s = "hello";
System.out.println(s.substring(2));  // Output: "llo"

substring(int beginIndex, int endIndex)

• Returns a new string that is a substring of this string, starting at beginIndex and ending at endIndex - 1.
• Syntax: str.substring(beginIndex, endIndex)

String s = "hello";
System.out.println(s.substring(1, 4));  // Output: "ell"

Case Conversion

toLowerCase()

• Converts all of the characters in this string to lowercase.
• Syntax: str.toLowerCase()

String s = "HELLO";
System.out.println(s.toLowerCase());  // Output: "hello"

toUpperCase()

• Converts all of the characters in this string to uppercase.
• Syntax: str.toUpperCase()

String s = "hello";
System.out.println(s.toUpperCase());  // Output: "HELLO"

Trimming

trim()

• Returns a copy of the string, with leading and trailing whitespace omitted.
• Syntax: str.trim()

String s = "  hello  ";
System.out.println(s.trim());  // Output: "hello"

Conversion

toCharArray()

• Converts this string to a new character array.
• Syntax: str.toCharArray()

String s = "hello";
char[] charArray = s.toCharArray();
for (char c : charArray) {
    System.out.print(c);  // Output: h e l l o
}

Static Methods

join(CharSequence delimiter, CharSequence... elements)

• Returns a new string composed of copies of the CharSequence elements joined together with a copy of the specified delimiter.
• Syntax: String.join(delimiter, elements)

String joinedString = String.join(", ", "one", "two", "three");
System.out.println(joinedString);  // Output: "one, two, three"

valueOf(char c)

• Returns the string representation of the passed character.
• Syntax: String.valueOf(c)

char ch = 'A';
String str = String.valueOf(ch);
System.out.println(str);  // Output: "A"

valueOf(char[] data)

• Returns the string representation of the character array.
• Syntax: String.valueOf(data)

char[] data = {'h', 'e', 'l', 'l', 'o'};
String str = String.valueOf(data);
System.out.println(str);  // Output: "hello"

Methods from Object Class

equals(Object anObject)

• Compares this string to the specified object.
• Syntax: str.equals(anObject)

String s1 = "hello";
String s2 = "hello";
System.out.println(s1.equals(s2));  // Output: true

toString()

• Returns a string representation of the object.
• Syntax: str.toString()

String s = "hello";
System.out.println(s.toString());  // Output: "hello"

Methods Due to Comparable Interface

compareTo(String anotherString)

• Compares two strings lexicographically.
• Syntax: str.compareTo(anotherString)

String s1 = "hello";
String s2 = "world";
System.out.println(s1.compareTo(s2));  // Output: -15

compareToIgnoreCase(String str)

• Compares two strings lexicographically, ignoring case differences.
• Syntax: str.compareToIgnoreCase(str)

String s1 = "hello";
String s2 = "HELLO";
System.out.println(s1.compareToIgnoreCase(s2));  // Output: 0

StringBuilder in Java

Overview

StringBuilder in Java is a mutable sequence of characters. It provides an efficient way to construct strings when frequent modifications, such as concatenations, are needed. Unlike String, which is immutable and creates a new object for every operation that modifies it, StringBuilder allows for in-place modifications, making it more efficient for tasks involving dynamic string manipulation.

Creation

StringBuilder()

• Constructs a string builder with no characters in it and an initial capacity of 16 characters.
• Syntax: StringBuilder sb = new StringBuilder();

StringBuilder sb = new StringBuilder();  // Empty StringBuilder with initial capacity of 16

StringBuilder(int capacity)

• Constructs a string builder with the specified initial capacity.
• Syntax: StringBuilder sb = new StringBuilder(capacity);

StringBuilder sb = new StringBuilder(20);  // StringBuilder with initial capacity of 20

StringBuilder(String str)

• Constructs a string builder initialized to the contents of the specified string.
• Syntax: StringBuilder sb = new StringBuilder(str);

String str = "hello";
StringBuilder sb = new StringBuilder(str);  // StringBuilder initialized with "hello"

Methods

Appending

append(String str)

• Appends the specified string to this character sequence.
• Syntax: sb.append(str)

StringBuilder sb = new StringBuilder("hello");
sb.append(" world");
System.out.println(sb.toString());  // Output: "hello world"

append(int i)

• Converts the integer to a string and appends it to this character sequence.
• Syntax: sb.append(i)

StringBuilder sb = new StringBuilder("value is ");
sb.append(100);
System.out.println(sb.toString());  // Output: "value is 100"

append(char c)

• Appends the specified character to this character sequence.
• Syntax: sb.append(c)

StringBuilder sb = new StringBuilder("hello");
sb.append('!');
System.out.println(sb.toString());  // Output: "hello!"

Insertion

insert(int offset, String str)

• Inserts the specified string into this character sequence at the specified position.
• Syntax: sb.insert(offset, str)

StringBuilder sb = new StringBuilder("hello");
sb.insert(3, "world");
System.out.println(sb.toString());  // Output: "helworldlo"

insert(int offset, char c)

• Inserts the specified character into this character sequence at the specified position.
• Syntax: sb.insert(offset, c)

StringBuilder sb = new StringBuilder("hello");
sb.insert(2, 'X');
System.out.println(sb.toString());  // Output: "heXllo"

Deletion

delete(int start, int end)

• Removes the characters in a substring of this sequence.
• Syntax: sb.delete(start, end)

StringBuilder sb = new StringBuilder("hello world");
sb.delete(5, 10);
System.out.println(sb.toString());  // Output: "hello"

Replacement

replace(int start, int end, String str)

• Replaces the characters in a substring of this sequence with characters in the specified string.
• Syntax: sb.replace(start, end, str)

StringBuilder sb = new StringBuilder("hello");
sb.replace(2, 5, "123");
System.out.println(sb.toString());  // Output: "he123"

Conversion to String

toString()

• Converts the contents of the StringBuilder to a String.
• Syntax: sb.toString()

StringBuilder sb = new StringBuilder("hello");
String str = sb.toString();
System.out.println(str);  // Output: "hello"

Other Methods

length()

• Returns the length (number of characters) of the StringBuilder.
• Syntax: sb.length()

StringBuilder sb = new StringBuilder("hello");
System.out.println(sb.length());  // Output: 5

capacity()

• Returns the current capacity of the StringBuilder.
• Syntax: sb.capacity()

StringBuilder sb = new StringBuilder(10);
System.out.println(sb.capacity());  // Output: 10

reverse()

• Reverses the characters in the StringBuilder.
• Syntax: sb.reverse()

StringBuilder sb = new StringBuilder("hello");
System.out.println(sb.reverse().toString());  // Output: "olleh"

StringBuffer in Java

Overview

StringBuffer in Java is similar to StringBuilder but provides synchronized methods, making it thread-safe for concurrent access. It is used when multiple threads are working on the same string concurrently and need synchronization. Like StringBuilder, StringBuffer also provides methods to manipulate mutable sequences of characters efficiently.

Creation

StringBuffer()

• Constructs a string buffer with no characters in it and an initial capacity of 16 characters.
• Syntax: StringBuffer sb = new StringBuffer();

StringBuffer sb = new StringBuffer();  // Empty StringBuffer with initial capacity of 16

StringBuffer(int capacity)

• Constructs a string buffer with the specified initial capacity.
• Syntax: StringBuffer sb = new StringBuffer(capacity);

StringBuffer sb = new StringBuffer(20);  // StringBuffer with initial capacity of 20

StringBuffer(String str)

• Constructs a string buffer initialized to the contents of the specified string.
• Syntax: StringBuffer sb = new StringBuffer(str);

String str = "hello";
StringBuffer sb = new StringBuffer(str);  // StringBuffer initialized with "hello"

Methods

Appending

append(String str)

• Appends the specified string to this character sequence.
• Syntax: sb.append(str)

StringBuffer sb = new StringBuffer("hello");
sb.append(" world");
System.out.println(sb.toString());  // Output: "hello world"

append(int i)

• Converts the integer to a string and appends it to this character sequence.
• Syntax: sb.append(i)

StringBuffer sb = new StringBuffer("value is ");
sb.append(100);
System.out.println(sb.toString());  // Output: "value is 100"

append(char c)

• Appends the specified character to this character sequence.
• Syntax: sb.append(c)

StringBuffer sb = new StringBuffer("hello");
sb.append('!');
System.out.println(sb.toString());  // Output: "hello!"

Insertion

insert(int offset, String str)

• Inserts the specified string into this character sequence at the specified position.
• Syntax: sb.insert(offset, str)

StringBuffer sb = new StringBuffer("hello");
sb.insert(3, "world");
System.out.println(sb.toString());  // Output: "helworldlo"

insert(int offset, char c)

• Inserts the specified character into this character sequence at the specified position.
• Syntax: sb.insert(offset, c)

StringBuffer sb = new StringBuffer("hello");
sb.insert(2, 'X');
System.out.println(sb.toString());  // Output: "heXllo"

Deletion

delete(int start, int end)

• Removes the characters in a substring of this sequence.
• Syntax: sb.delete(start, end)

StringBuffer sb = new StringBuffer("hello world");
sb.delete(5, 10);
System.out.println(sb.toString());  // Output: "hello"

Replacement

replace(int start, int end, String str)

• Replaces the characters in a substring of this sequence with characters in the specified string.
• Syntax: sb.replace(start, end, str)

StringBuffer sb = new StringBuffer("hello");
sb.replace(2, 5, "123");
System.out.println(sb.toString());  // Output: "he123"

Conversion to String

toString()

• Converts the contents of the StringBuffer to a String.
• Syntax: sb.toString()

StringBuffer sb = new StringBuffer("hello");
String str = sb.toString();
System.out.println(str);  // Output: "hello"

Other Methods

length()

• Returns the length (number of characters) of the StringBuffer.
• Syntax: sb.length()

StringBuffer sb = new StringBuffer("hello");
System.out.println(sb.length());  // Output: 5

capacity()

• Returns the current capacity of the StringBuffer.
• Syntax: sb.capacity()

StringBuffer sb = new StringBuffer(10);
System.out.println(sb.capacity());  // Output: 10

reverse()

• Reverses the characters in the StringBuffer.
• Syntax: sb.reverse()

StringBuffer sb = new StringBuffer("hello");
System.out.println(sb.reverse().toString());  // Output: "olleh"

Performance Considerations

• StringBuffer is synchronized, making it thread-safe for concurrent access.
• It is slightly slower than StringBuilder due to synchronization overhead.
• The initial capacity grows automatically as needed.