1 / 12

Lecture 3: Strings, Screen Output, and Debugging

Lecture 3: Strings, Screen Output, and Debugging. Yoni Fridman 7/2/01. Outline. Methods Math class methods Screen output Escape sequences Strings String methods String concatenation Debugging – Types of errors Fixing errors. Methods.

hop
Download Presentation

Lecture 3: Strings, Screen Output, and Debugging

An Image/Link below is provided (as is) to download presentation Download Policy: Content on the Website is provided to you AS IS for your information and personal use and may not be sold / licensed / shared on other websites without getting consent from its author. Content is provided to you AS IS for your information and personal use only. Download presentation by click this link. While downloading, if for some reason you are not able to download a presentation, the publisher may have deleted the file from their server. During download, if you can't get a presentation, the file might be deleted by the publisher.

E N D

Presentation Transcript

  1. Lecture 3: Strings, Screen Output, and Debugging Yoni Fridman 7/2/01

  2. Outline • Methods • Math class methods • Screen output • Escape sequences • Strings • String methods • String concatenation • Debugging – Types of errors • Fixing errors

  3. Methods • A method is a block of related statements that can be executed as a unit. • A large program is normally made up of many methods, each of which contains one algorithm. • To execute the block of statements, the method must be called. • This is done by writing the name of the method followed by a pair of parentheses. • Any arguments to the method go inside the parentheses. • Example: System.out.println(“Hello”); • System.out.println is the method name, “Hello” is the argument.

  4. Math Class Methods • Some methods return a value after they execute. • Methods are grouped into classes. • Let’s look at some methods in the Math class: • Math.pow(2.0, 3.0) returns 8.0. • Math.sqrt(4.0) returns 2.0. • Math.abs(-2.0) returns 2.0. • Math.max(1.0, 2.0) returns 2.0. • Math.min(1.0, 2.0) returns 1.0. • Math.round(1.7) returns 2. • <Code Example>

  5. Screen Output • As we’ve seen, println() is a method that outputs its argument to the screen. • The argument can be any expression of any type. • print() is a similar method, but unlike println(), it does not advance to the next line. • What about printing a blank line? Two ways: • Don’t pass any argument to the method – System.out.println(); • Use the escape sequence “\n” – System.out.println(“Hello\n”); • Make sure not to use a forward slash (/). • <Code Example>

  6. Escape Sequences • An escape sequence is a combination of characters that represent a single character. • The escape sequence \n represents the new-line character. • Java escape sequences start with the backslash (\). • \" represents a double quote ("). • Why is this necessary? • \\ represents a backslash (\). • Why is this necessary? • <Code Example>

  7. Strings • Since the argument to println() can be of any type, what type is “Hello”? • It’s not an int, a double, or a boolean. • “Hello” belongs to a special type called String, which has its own methods. • Strings are declared and initialized like any other type. • Example: String myName = “Yoni”; • Notice that the value must be in quotes. Why? • <Code example>

  8. n i Y o 0 1 2 3 String Methods • A string can be thought of as a numbered series of characters, starting from 0: • The string methods take advantage of this: • Assume myName stores the String “Yoni”. • myName.indexOf(“ni”) returns 2. • myName.indexOf(“Hi”) returns -1 (not found). • myName.length() returns 4. • myName.substring(1, 3) returns “on”. • myName.toLowerCase() returns “yoni”. • myName.toUpperCase() returns “YONI”.

  9. String Concatenation • Strings can also be concatenated (joined together). • Strings are concatenated using the + operator: • “Yoni” + “Fridman” gives the string “YoniFridman”. • “Yoni” + “ ” + “Fridman” gives the string “Yoni Fridman”. • myName + “ ” + “Fridman” gives the string “Yoni Fridman”. • What does myName += “Fridman” do? • Other types can also be concatenated to strings: • Example: “Two = ” + 2 gives the string “Two = 2”. • Beware: 1 + 2 + “Hello” gives the string “3Hello”. Why? • What does 2 + “” do? • <Code Example>

  10. Debugging – Types of Errors • Debugging is the process of finding bugs (errors) in a program and fixing them. • There are three types of errors: • Compile-time error: This is an error that the compiler finds and tells you about when you try to compile a program. • Run-time error: This is an error that the interpreter finds while a program is running, and that makes the program crash. • A run-time error is also known as an exception. • Incorrect behavior: This is an error that isn’t found by the compiler or by the interpreter. The program runs, but doesn’t give the results you intended.

  11. Fixing Errors • If the compiler finds a compile-time error: • It will give you an error message – read the message carefully before trying to fix the error. • <Code Example> • If the interpreter finds a run-time error: • It will also give you an error message, but these are less common. • If you’re getting incorrect behavior: • Use println() to track the values of relevant variables. • <Code Example> • Test your program carefully before turning it in. • Choose random test data, as well as boundary and special test data.

  12. Homework • Read: 4.1 (except for last two subsections), 4.2, 4.3, 4.4, 4.5, 8.2. • HW2 out today, due Friday. • Compatibility Calculator.

More Related