In Java, comments are used to add notes or explanations to code that are not executed by the computer. Comments are ignored by the compiler or interpreter, and do not affect the program's behavior.

There are two types of comments in Java:

1. Single-line comments: A single-line comment starts with two forward slashes // and continues to the end of the line. For example:

// This is a single-line comment

2. Multi-line comments: A multi-line comment starts with /* and ends with */. For example:

/*
This is a multi-line comment
that can span multiple lines
*/

Comments can be used to explain the purpose of a program, describe the function of a specific method or class, or to temporarily disable a block of code. Here are some examples:

// This program calculates the sum of two numbers
public class Calculator {
    // This method adds two integers
    public int add(int x, int y) {
        return x + y;
    }

    public static void main(String[] args) {
        /* This block of code is disabled for testing purposes
        Calculator calculator = new Calculator();
        int result = calculator.add(5, 7);
        System.out.println(result);
        */
    }
}

In this example, the single-line comment explains the purpose of the program, the method comment describes the function of the add method, and the multi-line comment temporarily disables the block of code in the main method.