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Notes for Webster Programming Competition because I know JavaScript, not Java

Reading and Writing Files in Java

(Copied from http://www.vogella.com/tutorials/JavaIO/article.html)

Overview

Java provides a standard way of reading from and writing to files. Traditionally the java.io package was used, but in modern Java applications you use the java.nio.file API.

Java will read all input as a stream of bytes. The InputStream class is the superclass of all classes representing an input stream of bytes.

Reading a File in Java

To read a text file you can use the Files.readAllBytes method. The usage of this method is demonstrated in the following listing.

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;

// somewhere in your code
String content = new String(Files.readAllBytes(Paths.get(fileName)));

To read a text file line by line into a List of type String structure you can use the Files.readAllLines method.

List<String> lines = Files.readAllLines(Paths.get(fileName));

Files.readAllLines uses UTF-8 character encoding. It also ensures that file is closed after all bytes are read or in case an exception occurred.

Reading and Filtering Line by Line

The Files.lines method allows read a file line by line, offering a stream. This stream can be filtered and mapped. Files.lines does not close the file once its content is read, therefore it should be wrapped inside a try-with-resource statement.

In the following example unnecessary whitespace at the end of each line is removed and empty lines are filterer.

//read all lines and remove whitespace (trim)
//filter empty lines
//and print result to System.out

Files.lines(new File("input.txt").toPath())
        .map(s -> s.trim())
 .filter(s -> !s.isEmpty())
 .forEach(System.out::println);

The next example demonstrates how to filter out lines based on a certain regular expression.

Files.lines(new File("input.txt").toPath())
        .map(s -> s.trim())
 .filter(s -> !s.matches("yourregularexpression"))
 .forEach(System.out::println);

The next example extracts a line starting with "Bundle-Version:" from a file called "MANIFEST.MF" located in the "META-INF" folder. It removes the prefix and removes all leading and trailing whitespace.

package com.vogella.eclipse.ide.first;

import java.io.IOException;
import java.nio.file.Files;
import java.nio.file.Paths;
import java.util.Optional;
import java.util.stream.Stream;

public class ReadMANIFESTFile {
        public static void main(String[] args) throws IOException {
                String versionString = readStreamOfLinesUsingFiles();
                System.out.println(versionString);
        }

        private static String readStreamOfLinesUsingFiles() throws IOException {
                Stream<String> lines = Files.lines(Paths.get("META-INF", "MANIFEST.MF"));
                Optional<String> versionString = lines.filter(s -> s.contains("Bundle-Version:")).map(e-> e.substring(15).trim()).findFirst();

                lines.close();
                if (versionString.isPresent())
                {
                        return versionString.get();
                }
                return "";
        }
}

List of Common Java String Methods

(Copied from https://www.tutorialspoint.com/java/java_strings.htm)

Method Description
char charAt(int index) Returns the character at the specified index.
int compareTo(Object o) Compares this String to another Object.
int compareTo(String anotherString) Compares two strings lexicographically.
int compareToIgnoreCase(String str) Compares two strings lexicographically, ignoring case differences.
String concat(String str) Concatenates the specified string to the end of this string.
boolean contentEquals(StringBuffer sb) Returns true if and only if this String represents the same sequence of characters as the specified StringBuffer.
static String copyValueOf(char[] data) Returns a String that represents the character sequence in the array specified.
static String copyValueOf(char[] data, int offset, int count) Returns a String that represents the character sequence in the array specified.
boolean endsWith(String suffix) Tests if this string ends with the specified suffix.
boolean equals(Object anObject) Compares this string to the specified object.
boolean equalsIgnoreCase(String anotherString) Compares this String to another String, ignoring case considerations.
byte getBytes() Encodes this String into a sequence of bytes using the platform's default charset, storing the result into a new byte array.
byte[] getBytes(String charsetName) Encodes this String into a sequence of bytes using the named charset, storing the result into a new byte array.
void getChars(int srcBegin, int srcEnd, char[] dst, int dstBegin) Copies characters from this string into the destination character array.
int hashCode() Returns a hash code for this string.
int indexOf(int ch) Returns the index within this string of the first occurrence of the specified character.
int indexOf(int ch, int fromIndex) Returns the index within this string of the first occurrence of the specified character, starting the search at the specified index.
int indexOf(String str) Returns the index within this string of the first occurrence of the specified substring.
int indexOf(String str, int fromIndex) Returns the index within this string of the first occurrence of the specified substring, starting at the specified index.
String intern() Returns a canonical representation for the string object.
int lastIndexOf(int ch) Returns the index within this string of the last occurrence of the specified character.
int lastIndexOf(int ch, int fromIndex) Returns the index within this string of the last occurrence of the specified character, searching backward starting at the specified index.
int lastIndexOf(String str) Returns the index within this string of the rightmost occurrence of the specified substring.
int lastIndexOf(String str, int fromIndex) Returns the index within this string of the last occurrence of the specified substring, searching backward starting at the specified index.
int length() Returns the length of this string.
boolean matches(String regex) Tells whether or not this string matches the given regular expression.
boolean regionMatches(boolean ignoreCase, int toffset, String other, int ooffset, int len) Tests if two string regions are equal.
boolean regionMatches(int toffset, String other, int ooffset, int len) Tests if two string regions are equal.
String replace(char oldChar, char newChar) Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.
String replaceAll(String regex, String replacement Replaces each substring of this string that matches the given regular expression with the given replacement.
String replaceFirst(String regex, String replacement) Replaces the first substring of this string that matches the given regular expression with the given replacement.
String[] split(String regex) Splits this string around matches of the given regular expression.
String[] split(String regex, int limit) Splits this string around matches of the given regular expression.
boolean startsWith(String prefix) Tests if this string starts with the specified prefix.
boolean startsWith(String prefix, int toffset) Tests if this string starts with the specified prefix beginning a specified index.
CharSequence subSequence(int beginIndex, int endIndex) Returns a new character sequence that is a subsequence of this sequence.
String substring(int beginIndex) Returns a new string that is a substring of this string.
String substring(int beginIndex, int endIndex) Returns a new string that is a substring of this string.
char[] toCharArray() Converts this string to a new character array.
String toLowerCase() Converts all of the characters in this String to lower case using the rules of the default locale.
String toLowerCase(Locale locale) Converts all of the characters in this String to lower case using the rules of the given Locale.
String toString() This object (which is already a string!) is itself returned.
String toUpperCase() Converts all of the characters in this String to upper case using the rules of the default locale.
String toUpperCase(Locale locale) Converts all of the characters in this String to upper case using the rules of the given Locale.
String trim() Returns a copy of the string, with leading and trailing whitespace omitted.
static String valueOf(primitive data type x) Returns the string representation of the passed data type argument.
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