Student: ex01

private data members, public constructors, integers from command line arguments, main methods, built in toString method, simple compilation/execution with javac/java

Part of a series of tutorial articles about a Student class.

Code examples referred to on this page can be found here: https://github.com/ucsb-cs156/student-tutorial/tree/ex01.

(Note: updated for Java 17, 01/05/22)

Overview

In this first article, we will first explain what the Student class is supposed to do, i.e. provide an abstraction for a UCSB Student

We’ll then show some Java code with two classes: starter code for a Student class, and a driver class

The code will be presented in a repository of source code (“repo”) for short that is stored using the git version control system, a free and open source software package for version control.

This git repo is hosted as open source on github.com, a commerical company (acquired by Microsoft in 2018) that provides cloud-based hosting for git repositories, plus other web-based tools for software development.

This lesson will not include a full discussion of git/github—just the briefest overview of the minimal information needed to make use of the code in this repo.

Student.java: starter code for a Java class

The starting point Student class that has the minimum we need to get started, mostly in “stub” form:

• private instance variables for name and perm

• a default constructor, Student(), that takes no arguments, and creates an instance of class Student. We’ll assume that this creates an instance of Student with the name Sample Student and the perm 9999999

• a constructor that takes arguments for name and perm

• code to get values for those arguments from the command line, and pass them to the constructor.

• code that prints out these values using the toString() method of the class Object

We’ll see that the built in toString() method is not of much use. In the next example, we’ll override it to make something more useful. (Note on overriding: See https://ucsb-cs`56.github.io/topics/java_overriding_vs_overloading/ )

Main.java: a sample “driver” class

We’ll also provide Java code for a “driver” class with amain method—one that illustrates how to invoke the constructor for the Student class, and then invoke one or more of the methods of the object instance.

That main class will illustrate how to get simple integer parameters from the command line via the args parameter to main (the equivalent of argc/argv in C/C++).

Basic description of the Student class

The Student class provides an abstraction for a UCSB Student.

This tutorial will not try to motivate why we need a class for UCSB Students. We will assume that there is some need to represent these in a program. We’ll assume that once some Java code has gotten input from somewhere for the names and perms of a student, it needs to

• Construct objects that represent these students

• Do some computation over those objects (e.g. computing the average GPA, sorting a collection of students by GPA, etc.).

  We acknowledge that in this chapter of the tutorial, with fields only for name and perm, there aren’t many interesting computations we can do yet, but we’ll be adding additional fields as we proceed through the chapters in the tutorial.

We’ll assume, for now, that instances of the class are immutable, i.e. we provide a way to construct objects, and getters for name and perm, but no setters for name and perm. We may change this assumption at some point in the tutorial.

Accessing the code from the github repo

The example code that supports this lesson is in a repository (“repo” for short) at this link. Note that this link is showing the branch ex01, not the main branch of the repo.

• https://github.com/ucsb-cs156/student-tutorial/tree/ex01

A detailed discussion of git and github is a topic for a different tutorial; for this lesson, we’ll just go over the minimal information you need to get started with the code.

• You can browse the code, as well as downloading it all in a .zip file at the link shown above.

• On a system where git is installed (such as CSIL, you can use this command to create a copy of the repository that has a link back to the original so that you can pull updates at any time.

  • git clone https://github.com/ucsb-cs156/student-tutorial.git
  • git checkout ex01.git

That will create a subdirectory containing the contents of the repo; cd into it and you can start working with the code. You can also switch between branches using the git checkout command; this chapter of the tutorial uses the ex01 branch.

For more detail, consult these articles: git, github.

What are each of the files?

Here’s a brief over view of each of the files in the repo, and their purpose.

Simple compiling and executing with javac and java commands:

To compile and run with simple javac/java workflow:

• cd src/main/java
• javac Main.java
• java main

Example:

pconrad@Phillips-MacBook-Pro student-ex01 % ls
LICENSE		Main.class	README.md	Student.class	pom.xml		src		target
pconrad@Phillips-MacBook-Pro student-ex01 % cd src/main/java 
pconrad@Phillips-MacBook-Pro java % ls
Main.java	Student.java
pconrad@Phillips-MacBook-Pro java % javac Main.java
pconrad@Phillips-MacBook-Pro java % ls
Main.class	Main.java	Student.class	Student.java
pconrad@Phillips-MacBook-Pro java % java Main
Usage: java Main name perm
  perm should be a positive integer between 1 and 9999999
pconrad@Phillips-MacBook-Pro java % java Main "Phill Conrad" 1234567
s1 = Student@7344699f
s2 = Student@6b95977
pconrad@Phillips-MacBook-Pro java % 

A few things to note:

• When we compile Main.java, since it refers to class Student, that file automatically gets compiled to.
• The compiled code goes into a file called Main.class and Student.class
• We can only run a class that contains a main method.
• We do not specify the .class extension when running.
• We instead, specify the name of the class.

• The Student.class and Main.class files remain separate.

  • There is no “linking” phase as in C++/C that combines them into a single program
  • There is a way to combine multiple .class files into a single .jar file, but this is not the same as linking, as explained here
• When we write System.out.println("s1 = " + s1); we are trying to do string concatentation on two instances of the class java.lang.String. Since s1 is a reference to an instance of class String, it can’t be concatenated to a String. In this case, the JVM will try to invoke a toString() method on the object to convert it to a String.
• The toString() method that gets invoked here is actually part of the class java.lang.Object

Every class (directly or indirectly) extends Object

When we write public class Foo extends Bar { it signifies that Bar is the parent class of Foo, i.e. the Foo inherits from Bar.

If we write just public class Foo, unlike in C++ where that would signify that Foo has no parent class, in Java, any time we write this, it is the same as writing public class Foo extends java.lang.Object.

Notes:

• Classes that start with java.lang. are part of the Java Language.
• That includes java.lang.String and java.lang.Object, just to name a couple.
• We don’t have to write java.lang.Object; for any class that is part of the java.lang. package, we can leave off the java.lang. prefix, and just write Object.

One implication of this is that nearly every instance of every class in Java has a public String toString() method, because:

• If it doesn’t have its own, it inherits one.
• It may inherit one from its parent class, grandparent class, great-grandparent class, etc.
• But in the limit, if none of those classes have one, there will always be one in java.lang.Object.
• Ok, there are bizarre corner cases where someone deliberately tries to mess this up, say by declaring a private toString() method and making it inaccessible to us. But mostly, you can rely on there being one.

The thing is, the toString() method we inherit from java.lang.String is seldom what we want.

For example, in the output above, we see that the output of:

        System.out.println("s1 = " + s1); // implicit: s1.toString()
        System.out.println("s2 = " + s2); // implicit: s2.toString()

Is this:

s1 = Student@38cccef
s2 = Student@64b8f8f4

To fix this, we need to override the toString method. We’ll do this in the next article, Student: ex02

To compile and run with Maven

• cd into the main level of the repo (the one with pom.xml)
• mvn compile to compile (.class files end up under target/classes)
• mvn test-compile to compile the test code
• mvn test to run the tests
• mvn pacakge to produce a .jar file for the project and place it in the target directory
• mvn clean to eliminate the target directory so you can start fresh

To run, run mvn package first to produce a .jar file under target/

Then, to run without command line args:

java -cp target/student-1.0.0.jar Main

To run with command line arguments:

java -cp target/student-1.0.0.jar Main Phill 12345 

ucsb-cs156/student-ex01 % ls
LICENSE		Main.java	README.md	Student.java
ucsb-cs156/student-ex01 % javac Main.java
ucsb-cs156/student-ex01 % ls
LICENSE		Main.java	Student.class
Main.class	README.md	Student.java
ucsb-cs156/student-ex01 % java Main
Usage: java Main name perm
  perm should be a positive integer between 1 and 9999999
ucsb-cs156/student-ex01 % java Main Chris 1234567
s1 = Student@38cccef
s2 = Student@64b8f8f4
ucsb-cs156/student-ex01 %