Files, Exceptions, and Abstractions: Lecture #7

Douglas Blank, Bryn Mawr College, CS206, Spring 2013

Similarities between Array and HashMap

Very similar in the abstract: they both have constant-time in setting and getting values

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• Both behave O(1) setting value
• Array[position] = value
• HashMap.put(key, value)
• Both behave O(1) getting value
• Array[position]
• HashMap.get(key)

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Differences between Array and HashMap

• Memory
• Arrays allocate fixed, full, contiguous memory for all possible array locations
• HashMaps allocate memory as needed, and can store arbitrary amount of items
• Arrays are typically dense: all positions have values
• HashMaps are typically sparse: keep collisions to a minimum
• Keys
• Arrays uses an integer as "key" (it is the location in the array)
• HashMaps can use any object as key, as it uses the object's .hashCode()

When something bad happens

• Beyond Syntax errors, beyond Logical errors, things can go wrong in a computer program
• We call these "exceptions"
• There are places in Java that well-defined exceptions can occur
• Accessing the Internet
• Reading in a file from your harddrive
• etc.
• Exceptions in Java must either be "caught" or "thrown"

Exceptions: rule of thumb

• Throw them and let some other code handle the issue
• At the "highest place" you can, catch the exception and give an error message

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Otherwise, you might be calling lower level code, and it just "eats" the problem without any indication that something is wrong.

Catch and Throw

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void someMethod() {

whereSomethingBadCanHappen();

}

Catch and Throw

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void someMethod() throws Exception {

whereSomethingBadCanHappen();

}

Catch and Throw

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void someMethod() {

try {

whereSomethingBadCanHappen();

} catch (Exception e) {

// recover

}

}

Input from a File

import java.io.*;

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public class TestIO {

public static int countLines(File f) {

}�public static void main(String[ ] args) {

}

}

Input from a File

Would like to read a line from a text file

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How to find the right Java class?

• Experience
• Textbook
• Internet Search

Input from a File: BufferedReader

1. To read a line at a time, we need a BufferedReader
2. BufferedReader needs an InputStreamReader
3. InputStreamReader needs an FileInputStream
4. FileInputStream needs a File
5. File needs a String

Input from a File

​

​

BufferedReader br = new BufferedReader(� new InputStreamReader(� new FileInputStream(� new File("test.txt"))));

​

br.readLine() // returns null when done

br.close()

​

Input from a File: throw

public static int countLines(File f) throws IOException {� BufferedReader br = new BufferedReader(� new InputStreamReader(� new FileInputStream(f)));

int count = 0;� String line = br.readLine();� while (line != null) {� count++;

line = br.readLine();� }� br.close();� return count;� } �

Input from a File: throw

import java.io.*;

​

public class TestIO {

...

public static void main(String[ ] args) throws IOException {

System.out.println(countLines(new File("Fileio.java")));

}

}

​

​

Input from a File: catch

public static int countLines(File f) {

int count = 0;� try {� BufferedReader br = new BufferedReader(� new InputStreamReader(� new FileInputStream(f)));

String line = br.readLine();� while (line != null) {� count++;

line = br.readLine();� }� br.close();

} catch (IOException e) {

}� return count;� } �

Input from a File: catch

import java.io.*;

​

public class TestIO {

...

public static void main(String[ ] args) {

System.out.println(countLines(new File("Fileio.java")));

}

}

​

​

Interactive Game

String line = br.readLine();

while (line != null) {

...

line = br.readLine();

}

Game File

state: Field

descr: You are in a beautiful field.

command: north

to: Dungeon

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state: Dungeon

descr: You are in the danky dungeon. � You smell onions.

command: south

to: Field

Interactive Game

String line = br.readLine();

while (line != null) {

if (line.StartsWith("state:")) {

} else if (line.StartsWith("descr:")) {

}

line = br.readLine();

}

​

Interactive Game

Let's write some code...