chapter 3 syntax, errors, and debugging fundamentals of java

Chapter 3Syntax, Errors, and Debugging

Fundamentals of Java

Fundamentals of Java 2

Objectives

Construct and use numeric and string literals. Name and use variables and constants. Create arithmetic expressions. Understand the precedence of different

arithmetic operators. Concatenate two strings or a number and a

string.

Fundamentals of Java 3

Objectives (cont.)

Know how and when to use comments in a program.

Tell the difference between syntax errors, run-time errors, and logic errors.

Insert output statements to debug a program.

Fundamentals of Java 4

Objectives (cont.)

Understand the difference between Cartesian coordinates and screen coordinates.

Work with color and text properties.

Fundamentals of Java 5

Vocabulary

Arithmetic expression Comments Coordinate system Exception Graphics context Literal

Fundamentals of Java 6

Vocabulary (cont.)

Logic error Origin Package Pseudocode Reserved words Run-time error

Fundamentals of Java 7

Vocabulary (cont.)

Screen coordinate system Semantics Syntax Virus

Fundamentals of Java 8

Language Elements

Every language, including Java has:– Vocabulary: Set of all of the words and

symbols in the language– Syntax: Rules for combining words into

sentences (statements)– Semantics: Rules for interpreting the meaning

of statements

Fundamentals of Java 9

Language Elements (cont.)

Table 3-1: Some Java vocabulary

Fundamentals of Java 10

Language Elements (cont.)

Programming vs. natural languages– Programming languages have small

vocabularies and simple syntax and semantics.– Programming language syntax must be

absolutely correct.– Programming language statements are

interpreted literally.Every detail must be present.

Fundamentals of Java 11

Basic Java Syntax and Semantics

Two categories of data types:– 1. Primitive data types: Numbers, characters,

and Booleans– 2. Objects

Syntax for manipulating primitive data types differs than for objects– Primitive data types are combined in expressions

with operators.– Objects are sent messages.

Fundamentals of Java 12

Basic Java Syntax and Semantics (cont.)

Objects must be instantiated before use.– Unlike primitives– String objects are a little different.

Six numeric data types– int and double are most commonly used

Also short, long, byte, and float– Each uses a different number of bytes for

storage.Each represents a different range of values.

Fundamentals of Java 13

Basic Java Syntax and Semantics (cont.)

Table 3-2: Some Java numeric data types

Fundamentals of Java 14

Basic Java Syntax and Semantics (cont.)

Literals: Items whose values do not change.– The number 5.0 or the string “Java”

Variable is a named location in memory. – Changing a variable’s value is equivalent to

replacing the value at the memory location.– A variable’s data type cannot change.

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Basic Java Syntax and Semantics (cont.)

Figure 3-1: Changing the value of a variable

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Basic Java Syntax and Semantics (cont.)

Variable declaration statement: Declares the identifier and data type for a variable– int age; (declares one int variable)– int a, b, c; (declares three int variables)– double d = 2.45; (declares and initializes

a variable) Constants are variables whose value cannot

change.– final double PI = 3.14;

Fundamentals of Java 17

Basic Java Syntax and Semantics (cont.)

Assignment statements:– <variable> = <expression>;– Value of expression assigned to variable

Arithmetic expressions:– Multiplication and division have higher

precedence than addition and subtraction.– Operators of same precedence evaluated from

left to right.– Parentheses are used to change evaluation order.

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Basic Java Syntax and Semantics (cont.)

Table 3-5: Common operators and their precedence

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Basic Java Syntax and Semantics (cont.)

The semantics of division (/) differ for integers and floating-point operators.– int / int yields an int.– double / double yields a double.

The modulus operator (%) yields a remainder.– 11 % 3 yields 2.

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Basic Java Syntax and Semantics (cont.)

Table 3-6: Examples of expressions and their values

Fundamentals of Java 21

Basic Java Syntax and Semantics (cont.)

Arithmetic overflow: Assigning a value to a variable that is outside of the ranges of values that the data type can represent

Mixed-mode arithmetic: Expressions involving integer and floating-point values– Lower-precision data types (int) temporarily

converted to high-precision data types (double)

Fundamentals of Java 22

Basic Java Syntax and Semantics (cont.)

Type casting: Temporarily converting one data type to another– Can type cast a single variable or an entire

expression– Place the desired data type within parentheses

before the variable or expression that will be cast to another data type.

int x = (int)(d + 1.6);

Fundamentals of Java 23

Basic Java Syntax and Semantics (cont.)

String concatenation: Append a String or value to another String– Use the + operator– String s = “string1” + “string2”;– String s2 = “String1” + intVariable1;

Escape character (\): Used in codes to represent characters that cannot be directly typed into a program– “\t” is a tab character

Fundamentals of Java 24

Basic Java Syntax and Semantics (cont.)

The String class’s length method gives the number of characters in a String.

Classes implement methods, and objects are instances of classes.– Objects can respond to a message only if their

class implements the method.Must implement a method with a matching

signature

Fundamentals of Java 25

Basic Java Syntax and Semantics (cont.)

Method signature:– Method name– Number and data types of method parameters

Method and variable names are user defined symbols.– Cannot use Java keywords (reserved words)

Packages: Used to organize related classes into a single unit for distribution

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Basic Java Syntax and Semantics (cont.)

Table 3-7: Java’s reserved words

Fundamentals of Java 27

Terminal I/O for Different Data Types

Table 3-8: Methods in class Scanner

Fundamentals of Java 28

Terminal I/O for Different Data Types (cont.)

Example 3-1: Tests three types of input data

Fundamentals of Java 29

Comments

Explanatory sentences inserted in a program Compiler ignores them Purpose is to make program more readable Two varieties:

– End of line comments: All text following a double slash (//) on a single line

– Multiline comments: All text occurring between a /* and a */

Fundamentals of Java 30

Comments (cont.)

Typical uses of comments:– Begin a program with a statement of its purpose– Explain the purpose of a variable declaration– Explain the purpose of a major segment of code– Explain the workings of complex or tricky

sections of code

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Programming Errors

Three types of programming errors:– Syntax errors: When a syntax rule is violated

Detected during compilationCompiler helps identify error

– Run-time errors: Occurs during executionDividing by 0Detected when program runsJVM indicates type of error and location

Fundamentals of Java 32

Programming Errors (cont.)

Three types of programming errors (cont.):– Logic errors (design errors or bugs):

Incorrect logic implemented in the programCode may be correct in every other way, but

does not do what it is supposed to do.Must thoroughly test and debug the program

when an error is found.– Desk checking: Examine code immediately after

it is written

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Debugging

One debugging method is to add extra lines of code to print values of selected variables at strategic points in the program.

Fundamentals of Java 34

Summary

Use the int data type for whole numbers and double for floating-point numbers.

Variable and method names consist of a letter followed by additional letters or digits.

Keywords cannot be used as names. Final variables behave as constants; their

values cannot change after they are declared.

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Summary (cont.)

Arithmetic expressions are evaluated according to precedence.

Some expressions yield different results for integer and floating-point operands.

Strings may be concatenated. The compiler catches syntax errors. The JVM catches run-time errors.

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Summary (cont.)

Logic errors, if caught, are detected by the programmer or user at run-time.

Can find and remove logic errors by inserting debugging output statements to view the values of variables.

The programmer can modify the color with which images are drawn and the properties of text fonts for a given graphics object.

Fundamentals of Java 37

Summary (cont.)

Java uses a screen coordinate system to locate the positions of pixels in a window or panel.– Origin is the upper-left corner of the drawing

area.– x and y axes increase to the right and

downward.