Java Quick Reference

• static boolean isDigit(char ch)
• static boolean isLetter(char ch)

• int compareTo(T other)
  Returns a value < 0 if this is less than other.
  Returns a value = 0 if this is equal to other.
  Returns a value > 0 if this is greater than other.

• Double(double value)
• double doubleValue()

• Integer(int value)
• int intValue()

• static int abs(int x)
• static double abs(double x)
• static double pow(double base, double exponent)
• static double sqrt(double x)
• static double random()
  Returns a double in the range [0.0, 1.0)

• boolean equals(Object other)
• String toString()
• Object clone()

• char charAt(int index)
  Returns the character at the specified index.


• boolean equals(String anotherString)
  Returns true if this string has the exact same sequence of characters as the specified string. Otherwise, returns false.


• int indexOf(char ch)
  Returns the index of the first occurrence of ch. Returns -1 if not found.


• int indexOf(String str)
  Returns the index of the first occurrence of str. Returns -1 if not found.


• int length()
  Returns the length of this string.


• String replace(char oldChar, char newChar)
  Returns a new string resulting from replacing all occurrences of oldChar in this string with newChar.


• String substring(int beginIndex)
  Returns a new substring of this string from beginIndex to the end of this string.


• String substring(int beginIndex, int endIndex)
  Returns a new substring of this string from beginIndex up to but not including endIndex.


• String toLowerCase()
  Returns a new string that is a copy of this String converted to lower case.


• String toUpperCase()
  Returns a new string that is a copy of this String converted to upper case.

• StringBuilder()
• void append(String str)

• static void arraycopy(Object sourceArray, int sourceStartPos, Object destArray, int destStartPos, int numValues)
  Copies numValues values from sourceArray to destArray, starting at position sourceStartPos in sourceArray and position destStartPos in destArray.

• File(String fileName)

• FileWriter(String fileName) throws IOException

• PrintWriter(FileWriter fileWriter)
• void println(String s)
• void close()

• int size()


• boolean add(E obj)
  Appends obj to end of list; returns true.


• void add(int index, E obj)
  Inserts obj at position index. (0 ≤ index ≤ size), moving elements at position index and higher to the right (adds 1 to their indices) and adjusts size.


• E get(int index)


• E set(int index, E element)
  Replaces the element at position index with obj. Returns the element formerly at the specified position.


• E remove(int index)
  Removes the element from position index, moving elements at position index + 1 and higher to the left (subtracts 1 from their indices) and adjusts size. Returns the element formerly at the specified position.


• Iterator<E> iterator()

• static void sort(Object[] a)
• static void sort(Object[] a, Comparator c)

• static void sort(List list)
• static void sort(List list, Comparator c)

• int compare(Object o1, Object o2)

• Iterator<E> iterator()

• boolean hasNext()
• E next()
• void remove()
  Removes the last element that was returned by next.

• int size()
• boolean containsKey(Object key)


• V put(K key, V value)
  Associates key with value. Returns the value formerly associated with key or null if key was not present.


• V get(Object key)
  Returns the value associated with key or null if there is no associated value.


• V remove(Object key)
  Removes and returns the value associated with key. Returns null if there is no associated value.


• Set<K> keySet()
  

• boolean add(E obj)
  Enqueues obj at the end of the queue; returns true.


• E remove()
  Dequeues and returns the element at the front of the queue.


• E peek()
  Returns the element at the front of the queue.


• boolean isEmpty()

• Scanner(InputStream source)
• Scanner(File source) throws IOException
• Scanner(String text)


• String nextLine()
  Returns the next line of text. (Absorbs the newline but does not return it.)
• boolean hasNextLine()
  Returns true if there is another line in this scanner's input.


• String next()
  Returns the next complete symbol from this scanner.
• boolean hasNext()
  Returns true if this scanner has another symbol in its input.


• int nextInt()
  Returns the next symbol of the input as an integer.
  Throws InputMismatchException if the next symbol is not an integer.

• int size()
• boolean contains(Object obj)


• boolean add(E obj)
  If obj is not present in this set, adds obj and returns true. Otherwise, returns false.


• boolean remove(Object obj)
  If obj is present in this set, removes obj and returns true. Otherwise, returns false.


• Iterator<E> iterator()

• E push(E item)
  Pushes item onto the top of the stack; returns item.


• E pop()
  Removes and returns the element at the top of the stack.


• E peek()
  Returns the element at the top of the stack. Throws an exception if the stack is empty.


• boolean isEmpty()

• static String showInputDialog(String message)
• static void showMessageDialog(null, String message)

Using A Main Method

public class SomeClass
{
	public static void main(String[] args)
	{
		System.out.println("I run!");
	}
}

Printing To System.out

System.out.print("I print out!  ");
System.out.println("I'm on the same line!");
System.out.println("I'm on the next line!");

Reading From System.in

Scanner scanner = new Scanner(System.in);
...
String text = scanner.nextLine();

Reading From Files

Scanner fileIn = new Scanner(new File("somefile.txt"));
while(fileIn.hasNextLine())
{
	String line = fileIn.nextLine();
	...
}

Writing To Files

PrintWriter fileOut = new PrintWriter(new FileWriter("somefile.txt"));
...
fileOut.println("some text");
...
fileOut.close();

Using Strings

• Strings are objects that represent character sequences.
• Create new instances by enclosing text in quotes.
  String name = "Dave";
• Write \" to include a quotation mark in a string literal, \\ to include a backslash, or \n to include a newline character.
  "Say\n\"Hi\"" creates a string with 8 characters: S a y newline " H i "
• Different instances may contain the same characters, so don't compare with ==.
• Strings are immutable--no methods will change the sequence of characters.
• "Da" + "ve" will create the new string "Dave".

Using Arrays

• Arrays can hold either reference types or primitive types.
• Arrays can never change size.
• Arrays are not classes, and are therefore accessed using special syntax.

Using Exceptions

public void giveSnack(Food snack)
{
	friend.eat(snack);
}

• Make it someone else's problem by declaring that, like eat, the giveSnack method may also throw AllergicToPeanutsException.

  public void giveSnack(Food snack) throws AllergicToPeanutsException
  {
  	friend.eat(snack);
  }
  

  Your code will compile, but now anyone who calls your giveSnack method will need to handle the AllergicToPeanutsException.


• Use a try/catch block to tell Java what to do if it encounters an AllergicToPeanutsException.

  public void giveSnack(Food snack)
  {
  	try
  	{
  		friend.eat(snack);
  	}
  	catch(AllergicToPeanutsException e)
  	{
  		friend.starve();
  	}
  }
  

Using Interfaces

public interface Player
{
	int removeFrom(int total);
}

public class SimplePlayer implements Player
{
	public int removeFrom(int total)
	{
		return 1;
	}
}

Using Abstract Classes

public abstract class Player
{
	private String name;

	public Player(String n)
	{
		name = n;
	}

	public String getName()
	{
		return name;
	}

	public abstract int removeFrom(int total);
}

public class SimplePlayer extends Player
{
	public SimplePlayer(String name)
	{
		super(name);
	}

	public int removeFrom(int total)
	{
		return 1;
	}
}

Compile-Time Errors

Run-Time Errors