String.valueOf() / Integer.toString()

// Converts a number, boolean, or other type to a string.
String.valueOf(int value);
String.valueOf(double value);
String.valueOf(boolean value);
String.valueOf(char value);

// Converts an int to a string.
Integer.toString(int i);
Integer.toString(int i, int radix); // Converts using the specified radix.

// Converts a double to a string.
Double.toString(double d);

// Converts a string to an int.
Integer.parseInt(String s);

// Converts a string to a double.
Double.parseDouble(String s);

// Converts an int to a string.
int num = 42;
String str1 = String.valueOf(num);
String str2 = Integer.toString(num);
System.out.println(str1); // Outputs: 42
System.out.println(str2); // Outputs: 42

// Converts a double to a string.
double price = 9.99;
System.out.println(String.valueOf(price)); // Outputs: 9.99

// Converts a boolean to a string.
System.out.println(String.valueOf(true)); // Outputs: true

// Converts a string to an int.
String age = "25";
int ageNum = Integer.parseInt(age);
System.out.println(ageNum + 1); // Outputs: 26

// Converts a string to a double.
String weight = "65.5";
double w = Double.parseDouble(weight);
System.out.println(w * 2); // Outputs: 131.0

// Converts to binary and hexadecimal.
System.out.println(Integer.toString(255, 2));  // Outputs: 11111111 (binary)
System.out.println(Integer.toString(255, 16)); // Outputs: ff (hexadecimal)

// Parsing a non-numeric string throws an exception.
try {
    int x = Integer.parseInt("abc"); // Throws NumberFormatException.
} catch (NumberFormatException e) {
    System.out.println("Cannot convert to a number.");
}

String.valueOf() is a general-purpose method that can convert any type to a string. Internally, it calls each wrapper class's toString() method, so the behavior is nearly identical. If you only need to concatenate a number with a string, you can use the + operator (e.g., "Value: " + num) and the compiler will convert it automatically, but String.valueOf() makes the conversion more explicit and readable.

Integer.parseInt() and Double.parseDouble() throw a NumberFormatException if the string contains non-numeric characters. Always use a try-catch block to handle errors when converting user input.

To compare strings, use equals() / compareTo(). To convert a string to a character array, use toCharArray().