### SCJP Sun® Certified Programmer for Java™ 6 Study Guide Chapter 10: Development
#### Certification Objectives Overview
In this section, we will delve into the advanced capabilities of the `java` and `javac` commands, particularly focusing on how to use packages and imports effectively. We will also explore how to determine runtime behavior for classes and command lines, use classes in JAR files, and utilize classpaths to compile code.
#### Using Packages and Imports
One of the key objectives in this chapter is to learn how to use packages and imports. Packages are essential in organizing classes and avoiding naming conflicts. In Java, you can declare a package using the `package` keyword followed by the name of the package. For example:
```java
package com.example.myapp;
```
To import a class from another package, you can use the `import` statement. If you want to import a single class, you would write:
```java
import com.example.another.MyClass;
```
If you need to import multiple classes from the same package, you can use the wildcard:
```java
import com.example.another.*;
```
#### Determining Runtime Behavior for Classes and Command Lines
Another important aspect covered in this chapter is determining the expected runtime behavior for classes and command lines. This involves understanding how the JVM (Java Virtual Machine) processes the classpath and resolves class names. When running a Java application, you typically specify the main class using the `java` command followed by the class name:
```bash
java com.example.myapp.MainClass
```
If the class is not found, you might need to adjust the classpath or ensure that the class is properly compiled and placed in the correct directory structure.
#### Using Classes in JAR Files
JAR (Java Archive) files are used to bundle classes, resources, and other metadata into a single file. To use a class from a JAR file, you need to include the JAR file in the classpath when compiling or running your application. For example:
```bash
javac -classpath /path/to/mylib.jar MainClass.java
java -classpath .:/path/to/mylib.jar com.example.myapp.MainClass
```
Here, `-classpath` specifies the classpath, and `./` represents the current directory where the compiled classes are located.
#### Utilizing Classpaths to Compile Code
Understanding classpaths is crucial for managing dependencies and resolving class files during compilation and execution. The `javac` command supports several options related to classpaths:
- **-classpath (or -cp)**: Specifies the search path for user-defined classes and packages. Multiple directories can be specified, separated by colons (`:`) on Unix systems or semicolons (`;`) on Windows.
- **-d**: Specifies the output directory where the compiled `.class` files should be placed. For example:
```bash
javac -d out/production/classes -classpath /path/to/mylib.jar MainClass.java
```
#### Two-Minute Drill: Q&A Self-Test
The "Two-Minute Drill" section provides a quick self-test with questions related to the topics covered in this chapter. This section helps reinforce your understanding of the material by testing your knowledge through practical examples and scenarios.
#### Summary
This chapter covers advanced topics related to the development process in Java, focusing on using packages and imports, determining runtime behavior, using classes in JAR files, and utilizing classpaths. By mastering these concepts, you will be better equipped to manage complex projects and resolve common issues related to class resolution and dependency management. Understanding these objectives is crucial for passing the SCJP certification exam and becoming a proficient Java developer.
