Computational Methods Junior Honours : Checkpoint 3

Assessment: MarkingCP3.htm

Object assignment syntax

· Java assigns instances of objects (classes) differently from basic types like “int” and “double”.

int i, j;

i=2;

j=1;

i=j;

j=0;

// Results in i=1, j=0.

· However with classes, (see ComplexTester.java) the behaviour is less obvious. Assigning classes with an

An equals sign assigns by reference. This means that after A=B, the variables “A” and “B” refer to the same instance.

A unique instance can only be obtained using “A = new Complex” in some form.

This former has unintuitive side effects illustrated in ComplexTester, and should be avoided.

The latter should always be used unless you know what you are doing.

public static void GotchaObjectAssignment() throws IOException {

Complex A = new Complex(2,0);

Complex B = new Complex(1,0);

A = B;

B.set(0,1);

System.out.println("A "+A);

System.out.println("B "+B);

};

Yields A (0.0,1.0)

B (0.0,1.0)

public static void CorrectObjectAssignment() throws IOException {

Complex A = new Complex(2,0);

Complex B = new Complex(1,0);

A = new Complex(B);

B.set(0,1);

System.out.println("A "+A);

System.out.println("B "+B);

};

Yields A (1.0,0.0)

B (0.0,1.0)

· The reason for this is that Java “objects” such as “A” or “B” underneath are really “pointers” that refer to a bit of memory storing the object data

· A “new” chunk of memory is obtained whenever an object is assigned with a “new” constructor call, such as “A = new Complex();”

· If “A=B” is used, then both “A” and “B” refer to the same chunk of memory, and modifying “B” also modifies “A” because they now really refer to the same thing

· This is counter-intuitive and should be avoided like the plague.

Starting point: Complex class

Download two java files Complex.java, and ComplexTester.java
Download ComplexTester.input
Read them

Complex.java defines a public class for complex variables.
ComplexTester.java contains an example test code that uses the complex class

Compile them

javac Complex.java ComplexTester.java

Run the test

java ComplexTester

Understand how the output is generated.

EXPLAIN the two object assignment examples in ComplexTester to your demonstrator,
EXPLAIN why copy constructors should be used for object assignment.

Your task: Vector class

Create a file called Vector3d.java in a similar style to Complex.java. Write a class Vector3d with

1. Coordinates x,y,z

getter and setter methods - these are standard Java style conventions that are required for some of the optional assignments to work.

This is a good habit to get into because lots of Java tools assume this naming convention is used and break if you do not use it.

double getX()
double getY()
double getZ(),
void setX(double)
void setY(double)
void setZ(double)

3. A default constructor Vector3d() that assigns the coordinate to zero

4. A constructor Vector3d(xx,yy,zz) that initialises the coordinate to (xx,yy,zz)

5. norm2 & norm methods

6. toString() method (this enable Java to automatically print a Vector3d in methods like println)

static methods to perform add,sub,cross,dot operations on two Vector3d's:

8. a static method for mult and divide by double

It is probably best get Vector3d to compile (javac Vector3d.java) before moving on to writing your test code below.

Create a source code file called Vector3dTester.java (similar to ComplexTester.java) . This code should

Read from a file VectorTestInput the data for two Vector3d's.

2. Print to the screen the two Vector3d's, their magnitude, sum, dot product, cross product and the angle between them.

Compile this code (javac Vector3dTester.java)
Run this code (java Vector3dTester) for a few values of input vector. Check by hand that it is correct.

Harder: Choose appropriate vectors A, B, C, and make your program print output that verifies the following vector product identies

· |A.B| = |A| |B| cos q

· |AxB| = |A| |B| sin q

· A x B = - B x A

· A x (B+C) = ( A x B ) + ( A x C)

· A x ( B x C ) = (A.C) B – (A.B) C

· Verify that A.(BxC), as computed by your program, gives the volume a cuboid

Note that you will reuse the Vector3d class later in the course, so make sure it works well!

In order to work with later examples, the fields of your Vector3 class should be called x, y and z,
So that MyVector3.x, MyVector3.y and MyVector3.z (would be the components, assuming MyVector3 is an instance of the Vector3 class).

Improving the file reader:

1. Read this link and use StreamTokenizer::ttype to parse the input file in a way that detects errors

2. Use StreamTokenizer::whitespaceChars(c,c) to flag '(' ',' and ')' as ignored to allow the input file to contain "(x,y,z)" format.